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Ask an Astrophysicist: Stars

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Physics of Stars

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Library of Past Questions and Answers

    Constellations and Stars
For questions on names and appearances of constellations, names of individual stars, unusual stars seen on the sky etc., please see our Night Sky page.
    How Stars Shine

Can you give me analogy to explain stars to young kids? (Submitted October 20, 1997)

The Question

I have a troop of Girls Scout ages 7, 8, 9, years old. In our Girl Scout book the girl have to learn what make the stars appear to be brighter or bigger than others, about the different colors that appear to the star. My husband knows some things about the stars and he is my co-leader but he doesn't know how to tell little girls about the stars without them walking way not knowing what he said. He need help telling it right to kids. Please help!

The Answer

I think what you are looking for is a familiar analogy, or something the troop can understand by experimenting.

There are two main reasons why one star could be brighter than another star. The first is that it produces MORE LIGHT. The second reason that a star can be brighter is if it is CLOSER to us. A good way to explain these two reasons to kids would be to compare the stars to light bulbs or flashlights. The scout leader can demonstrate using 2 light bulbs (say a 100 watt bulb and a 45 watt one), first putting them side by side, then moving the 45 watt bulb close and closer to the girls. Or the troop can gather in a darkened room (or go outside at night), each with her own flashlight (some are likely to be brighter than others), move about randomly and then guess who has the most powerful flashlight. They will be able to see for themselves that a dim flashlight right next to you can appear brighter than a powerful one further away.

Stars look different colors because they are different temperatures. Hotter stars are white or even blue and cooler stars are orange or red. The girls already have some experience with this but they just don't know it yet! Think about an electric stovetop. When it is turned on, the coils start to glow. At first they turn red, then orange and then brighter orange as they heat up. The center of a candle flame is even hotter than a stovetop (though it is a much smaller area, so it can't heat as much), and it is BLUE! The scout leader can show them a candle flame to demonstrate. It actually has different colors, all corresponding to different temperatures. The coolest parts are the ones near the outside (near the outside air) and the hottest parts are in the center of the flame.

By relating stars to this kind of down to earth examples, the girls should be able to come away with an understanding of what makes the stars look different; and better yet, the explanation will make sense, so hopefully it will stick!

Allie Cliffe and Koji Mukai
for Ask an Astrophysicist

Question ID: 971020d

Why do stars shine? (Middle School Level) (Submitted March 04, 1997)

The Question

Why does a star shine. Please answer at middle school level.

The Answer

Stars are giant balls of glowing gas. Stars shine because the gas inside them is so hot that a process called "nuclear fusion" takes place. Nuclear fusion is where 2 atoms come together (or "fuse") to form a different kind of atom; this process gives off a lot of energy that we can see as light.

You can find more about stars, what makes them shine, their life cycles, and more in our StarChild web site at:

I am sure that one of these 2 different levels of describing "what makes stars shine" will be what you need!

Question ID: 970304

Why do stars shine? (High School Level) (Submitted September 12, 1997)

The Question

How do stars create their energy?

The Answer

Stars create their energy through the process of nuclear fusion. Fusion is the process in which light atoms combine to form heavier atoms, giving off excess energy in the process. There are a number of different ways in fusion might take place in stars, depending on the temperature in the core of the star. Each of them is a multistep process in which other elements may be used or momentarily created. The simplest is the proton-proton chain, which occurs in stars having core temperatures less than 15 x 10^6 K. The proton- proton chain uses two steps to convert Hydrogen first into 3He, and then combines two 3He nuclei into He, and giving back 2 H. In the process, neutrinos and gamma-rays are emitted.

Other nuclear processes which occur in stars with higher core temperatures are the Carbon cycle and the Carbon-Nitrogen cycle. These cycles use Carbon or Carbon and Nitrogen to mediate the conversion of H into He. At even higher temperatures ( ~ 10^8 K), Helium fuses into Carbon via the "triple alpha" process.

Energy is given off because, e.g. the sum of the mass of 4 H nuclei is more than the mass of a He nucleus. The "excess" mass is converted into energy as part of the process.

Textbooks in astronomy or modern physics will have further details about these processes.

Jim Lochner
for Ask an Astrophysicist

Question ID: 970912b

Are there green stars? (Submitted April 08, 1997)

The Question

I have looked in and read many great books on astronomy but never found an answer to this question: Do green stars exist? If they don't why don't they? Is it theoretically possible to have a green star? And if they do exist how would they be classified(main sequence,supergiant,etc.)?

The Answer

Yes, stars of every color in the rainbow exist. In fact a star's color tells us something very important about it -- its temperature.

The visible spectrum goes as: Red, Orange, Yellow, Green, Blue and Violet with a red photon having less energy than Blue. So, blue stars are hot and red stars are cool (cool for stars that is).

The reason that people do not often mention green stars is simply that green is in the middle of the visible spectrum. Therefore a star that gives off a plurality of its light in the green (similar to our Sun), also gives off lots of red, orange, yellow, blue and violet light. When we see this mixture of colors it usually appears white or yellow.


Jonathan Keohane
-- for Imagine the Universe!

Question ID: 970408e

How are the temperatures of stars related to their colors? (Submitted May 14, 1998)

The Question

How do I relate the temperature of a main sequence star to its color?

The Answer

Stars produce energy primarily by nuclear reactions in their deep interiors. The nuclear reactions produce very high-energy particles and gamma-rays, but these can't escape easily through the outer layers of the star; they must scatter many times on their way out. This scattering process, plus the total amount of energy produced by the nuclear reactions, are what determines the spectrum of radiation escaping the star. This spectrum is pretty well described by a shape called a 'black body', which also applies to many terrestrial situations, such as an incandescent light bulb. For a black body the temperature and colors are related in a simple way. In order to figure it out, however, you have to decide what you mean by color. Astronomers have a couple of different ways of doing this, the most popular being the 'UBV' (ultraviolet, blue, visual) system. You can find a table showing how these colors are defined in terms of the wavelengths of light at:

Then, an approximate correspondence can be made between temperature and 'B-V', which is the difference between the B (blue) and V (visual) intensities:

T (degrees Kelvin)B-V

I hope this helps,

Tim Kallman
for "Ask an Astrophysicist"

Question ID: 980514a

Can a particular star change color? (Submitted October 06, 2003)

The Question

Can the stars change colour? I know that they can due to temperature change, but how would this change occur?

The Answer

The color of a star is linked to its temperature and its temperature is linked to what processes are taking place in and on the surface of the star. Stars evolve during their lives, going through different phases of elements burning. Say a star has finished burning its hydrogen and goes to the helium burning phase, this will change the temperature (hence the color) of the star.

Have a look at any book on the Hertzsprung-Russell diagram to get more information on the general evolution of the main sequence stars. For example, see:

We hope this helps,

Ilana and Barbara
for the "Ask an Astrophysicist" team.

Question ID: 031006a

What are the optimal wavelengths to view stars of differing spectral type? (Submitted March 13, 1997)

The Question

A while ago, I saw a curve which showed that 550 nm was the best wavelength for visible light for our Sun. On this curve, there was also information given showing that other stars had a wavelength for optimal viewing but in a different region of the electromagnetic spectrum. I remember seeing stars like Betelgeuse and Rigel on this curve, but I'm unable to remember which parts of the electromagnetic spectrum they matched up with and I can't locate this curve anywhere. What I'd like is to know which stars had it's light most visible in the ultraviolet and infrared regions. I'd appreciate it if you could let me know today, but I definitely understand if it's too much to ask.

The Answer

I don't know of a reference to a graph like the one you describe, but it is straightforward to predict the wavelength at which the spectrum of a star of a given type will reach a maximum. The Wien displacement law for black bodies has:

lambda_max = 2.898 x 10^7/T

where lambda_max is in angstroms, and T is in K. For the Sun, if it were a black body with T= T_eff = 5770, this implies that lambda_max=5000 angstroms. The observed max of the solar spectrum is a little bit short of this, I think, at about 4500 angstroms. For other stars, the predictions are as follows:

Spectral TypeTemperaturelambda_max
B810,000 K 3000 A

(B8 is Rigel's spectral type, and M2 is Betelgeuse)

Actual stellar distributions are more complex, and computer simulations to predict. Looking at Silva & Cornell 1992, ApJS, 81, 865, it looks like the actual peaks are at 4000 (B8), 4500 (G2), and 9000 (M2).

Hope that this helps.

Tim Kallman and Steve Drake
for the Ask an Astrophysicist team.

Question ID: 970313c

    The Locations and Motions of Stars

How do astronomers find the distances to stars? (Submitted April 15, 1997)

The Question

How do astronomers know for sure that stars are really as far away as they state?

The Answer

Thank you for writing the "Ask an Astrophysicist" service with your question. The methods astronomers use to measure distances to the stars is a piece of fundamental and active work in astronomy with important implications for how we understand the Universe around us.

Measuring the distances to stars is kind of like a house of cards: we use one method to get nearby stars, use a new method for further away stars which depends on our first measurements of nearby stars, then yet another method at further distances, and so on.

The first method astronomers use to measure distances to stars is called parallax. If you hold your finger in front of your face and close one eye and look with the other, then switch eyes, you'll see your finger seem to "shift " with respect to more distant objects behind it. The effect is called parallax.

Astronomers can measure parallax by measuring the position of a nearby star very carefully with respect to more distant stars behind it, then measuring those distances again six months later when the Earth is on the opposite side of its orbit. The shift is tiny... less than an arcsecond even for the nearest star (an arcsecond is 1/60 of an arcminute, which is 1/60 of a degree). In fact, I have heard (but only heard it once and never been able to find a reference to verify it, so label this as "interesting hearsay not necessarily to be believed ") that some of the early Greek astronomers specifically looked for parallax from the stars to work out whether the Earth orbited around the Sun. But their instruments could not measure the very small parallaxes nearby stars exhibit. Since they thought nearby stars were much closer than we now know, the fact they observed no parallax implied that the Earth did not orbit the Sun. Whether this is true or not, it was not until telescopes were invented that astronomers could measure parallaxes at all accurately.

Astronomers have been carefully measuring parallaxes for stars for centuries, and with remarkable precision. But it is painstakingly slow work with only a few thousand stars having well measured parallaxes. In 1989, the European Space Agency (ESA) launched a satellite called Hipparcos to accurately measure the parallax of some 120,000 stars (plus about another million or so stars with good, but lower precision). Hipparcos measurements increased the number of stars for which parallaxes are measured by a vast amount. Visit the Hipparcos web page for more details:

Parallaxes give us distances to stars up to perhaps a few thousand light years. Beyond that distance, parallaxes are so small than they cannot be measured with contemporary instruments. So astronomers use some more indirect methods beyond a few thousand light years. Rather than describe them in detail, let me point you to a good reference: George Abell's "Exploration of the Universe ". Your local library probably has a copy... and if it does not, almost any good astronomy textbook will also include this information.

The methods beyond a few thousand light years include:

Stellar motions: All stars are in motion, but only for nearby stars are these motions perceivable. Statistically, therefore, the stars that have larger motions are nearer. By measuring the motions of a large number of stars, we can estimate their average distance from their average motion.

Moving clusters: Clusters of stars travel together, such as the Pleiades or Hyades star clusters. Analyzing the apparent motion of the cluster can give us the distance to it.

Inverse-square law: The apparent brightness of a star depends both on its intrinsic brightness (its luminosity, or how bright it really is) and its distance from us. If we know the luminosity of a star (for instance, we have a measured parallax for one star of the same type and know that others of the same type will have similar luminosities), we can measure its apparent brightness (also called its apparent magnitude) and work out the distance using the inverse-square law. There are several variations on this, many of which are used to measure distances to stars in other galaxies.

Interstellar lines: The space between stars is not empty, but contains a sparse distribution of gas. Some times this leaves absorption lines in the spectrum we observe from stars beyond the interstellar gas. The further a star is, the more absorption will be observed since the light has passed through more of the interstellar medium.

Period-luminosity relation: Some stars are regular pulsators. The physics of their pulsations is such that the period of one oscillation is related to the luminosity of the star. If we measure the period of such a star, we calculate its luminosity. From this, and its apparent magnitude, we can calculate the distance. See:

Jesse Allen and Padi Boyd
for "Ask an Astrophysicist "

Question ID: 970415c

What is the nearest star other than the Sun? (Submitted February 25, 2002)

The Question

What is the nearest star to the Earth besides the Sun?

The Answer

The nearest star to Earth is Proxima Centauri (Alpha Centauri C). You will find a list of other nearby stars at:


Hans Krimm for "Ask an Astrophysicist"

Question ID: 020225a

How many known stars are there within 100 light years of Earth? (Submitted January 23, 1998)

The Question

Could you please give me an estimate of the amount of stars located within 100 light years from earth?

The Answer

As a function of distance, the Gliese star catalog lists the following number of stars. A parsec is 3.26 light years.

Distance Number Density

Distance from Sun in parsecsnumber of starsstars/cubic parsec

The fall-off in density is probably due to the fact that many stars are too faint to be cataloged at a distance more than 5 parsecs. (At still larger distances, the density of stars does vary as you move outside of the local spiral arm of our galaxy and into the less-populated regions above and below the disk.)

Using the 0.120 stars/cubic parsec number, and using a volume for a distance 100 light-years = 100/3.26 = 30.7 parsecs

Number = density * volume
	= 0.120 stars/cubic parsec  * 4/3 pi (30.7 parsecs)^3
	= 14,600 stars

Most of these stars are completely unknown.

David Palmer
for Ask an Astrophysicist

Question ID: 980123d

How many solar-type stars are within 50 light years of the Sun? (Submitted June 09, 1998)

The Question

I'm doing a research paper on solar type stars within 50 light years of the sun. Can you give me a reference on the web?

The Answer

The astronomy frequently-asked-questions (FAQ) list

Subject: G.05 Where can I get stellar data (especially distances)?
Author: Steve Willner 

The Strasbourg Astronomical Data Center maintains a large inventory of
astronomical catalogs, including star catalogs.  Access at  

The HIPPARCOS catalog,, represents a gigantic
improvement both in systematic accuracy and in precision over previous
catalogs, but it is limited to fairly bright stars (magnitude limit
around 11).  Keep in mind that all astronomical data have
uncertainties.  Distances can be especially problematic, and it is
vital to know what the uncertainties are.

One large (3803 stars) compilation of nearby stars can be found at
An excerpt from the "ReadMe" file follows:

   Preliminary Version of the Third Catalogue of Nearby Stars
       (Astron. Rechen-Institut, Heidelberg (1991))

        The present version of the CNS3 contains all known stars within
    25 parsecs of the Sun. It depends mainly on a preliminary version
    (Spring 1989) of the new General Catalogue of Trigonometric
    Parallaxes (YPC) prepared by Dr. William F. van Altena (Yale
        The catalogue contains every star with trigonometric parallax
    greater than or equal to 0.0390 arcsec, even though it may be
    evident from photometry or for other reasons that the star has a
    larger distance. For red dwarf stars, new color-magnitude
    calibrations for broad-band colors were carried out and applied.
    For white dwarfs, the recipes of McCook and Sion in ApJS, 65, 603
    (1987) were applied. Stroemgren photometry was used (not yet
    systematically) for early-type stars and for late dwarfs, the
    latter supplied by E. H. Olsen from Copenhagen Observatory
    (private communication).
        Contrary to the CNS2 (Gliese 1969) trigonometric parallaxes
    and photometric or spectroscopic parallaxes were not combined.
    The resulting parallax in the present version is always the
    trigonometric parallax---if the relative error of the
    trigonometric parallax is smaller than 14 percent. The resulting
    parallax is the photometric or spectroscopic parallax only if no
    trigonometric parallax is available or if the standard error of
    the trigonometric parallax is considerably larger.

If you'd like to use the astronomical data, say, to calculate
distances between stars, a useful reference is

Since the catalog was compiled (1991) the Hipparcos mission has provided better distances to these stars. However, this catalog will probably be sufficient for your needs.

David Palmer
for Ask an Astrophysicist

Question ID: 980609b

Are stars only found in galaxies or globular clusters? (Submitted September 27, 1998)

The Question

I read in a German news magazine (Der Spiegel) that there appear to be a number of stars located outside of galaxies. How is this possible if stars are only born in galaxies or globular clusters?

The Answer

One often observes galaxies which are interacting with each other via tidal forces. This happens when two galaxies pass near to each other, or undergo a glancing collision. The effect of such interactions is to draw out long streamers of stars and gas from the main body of the galaxy. This process casts the material off into intergalactic space at high enough speeds so that it never returns to the parent galaxy.

Thus, one would expect galaxy-galaxy interactions to supply the intergalactic medium with stars and gas. Therefore it is not surprising to find stars in intergalactic space, where they do not appear to have an association with any nearby galaxy.

John Cannizzo C. Allie Hajian
for "Ask an Astrophysicist"

Question ID: 980927a

How far is the furthest star visible from Earth (Submitted March 29, 1998)

The Question

It's probably a really easy question (its just for curiosity's sake), but how far away, in light years, is the furthest star visible from the earth? (by any means possible). The name of this star is not important. Do you think there are stars in the outer regions of the universe that we can not see? What are your theories why this is so?

The Answer

The intrinsically brightest star [a supernova] is thought to be about a million times brighter than our own Sun. In astronomy, the brightness of stars is expressed in "magnitudes". This is a logarithmic scale, that works as follows. Our Sun has an intrinsic or absolute magnitude of about 5. This is the apparent magnitude our Sun would have if it were 32.6 light years away. A star 100 times brighter would have a magnitude of 0; a star 10000 times brighter would have a magnitude of -5; a star 1000000 (i.e. a million) times brighter would have a magnitude of -10.

With the Hubble telescope, using an exposure time of several hours, one can see stars to about 30th magnitude. This is about 10 billion times fainter than our Sun, if it were 32.6 light years away. The brightness of any object falls off as the square of the distance from the observer, so the Hubble telescope could just see our Sun if it were 3.26 million light years away. If you were to replace our Sun with a star a million times brighter, it could be seen about a thousand times further away, i.e., about 3 billion light years.

In answer to your last question, since this estimate is only for the very brightest stars, and since the distance I obtained is still less than the size of the visible Universe (about 15 billion light years), there are surely many faint stars at great distances which we cannot see.

J.K. Cannizzo
for Ask an Astrophysicist

Question ID: 980329a

How long does it take for the light from stars to be visible here on Earth? (Submitted July 10, 1997)

The Question

How long does it take for the light from stars to be visible here on Earth?

The Answer

That's an interesting question. Light travels at 300,000 kilometers per second or 186,000 miles per second. The time it takes for light from stars to reach us is the distance to the star divided by this speed. The nearest star to us is the Sun and it takes about 8.3 minutes for its light to reach us here on Earth.

Other stars are so much farther away that it is convenient to express the distance to them in units of the distance traveled by light in one year. This unit is called a light year. The next closest star to us is Proxima Centauri. This star is 4.3 light years away which means that light from it takes 4.3 years to reach us. Our galaxy is about 100,000 light years across. This means that it can take tens of thousands of years for light from some stars in our galaxy to reach us. For stars that we can see in nearby galaxies it can take millions of years. The farthest objects we can see are quasars. They are so distant that the light we see from them today left billions of years ago.

So when we look up at the stars we are looking back in time. This is useful for astronomers because when we look at very distant objects we can see what the Universe was like a long time ago.

Damian Audley
for the Ask an Astrophysicist team

Question ID: 970710c

What keeps stars from crashing into one another? (Submitted March 14, 1997)

The Question

What keeps the stars from crashing into each other?

The Answer

There is a very short answer to your question, and that is that space is very large, and there is lots of room for stars, moons, and planets to move around without colliding with each other. Often, when two objects look close together on the sky, one of them is much further away than the other. Therefore, they are not really close together at all. This is true for many of the stars in the constellations that we are familiar with, and it is true for stars and planets which look close to our Moon. The nearest stars are light years away, while the Moon is about a billion times nearer. Collisions between stars are believed to happen, but they must be very infrequent. Collisions inside our solar system happen fairly often between planets and comets or meteors. Each "shooting star" is an example of such a collision, and 2 years ago a fairly large comet collided with Jupiter.

I hope this helps!

Tim Kallman
for the Ask an Astrophysicist team

Question ID: 970314b1

Are stars fixed or stationary? (Submitted April 11, 1998)

The Question

My question is, are stars fixed or stationary?

The Answer

All stars move, and for a variety of reasons. One reason is that all stars in our Galaxy are revolving about the center of the Galaxy. Our Sun makes one such revolution every 2 hundred million years or so. They may also move, if an explosion gives them an extra "kick." Or a star may be in a binary system, orbiting about another star. Those are just several examples.

There are two major ways of observing the motion of stars. One method, for stars close by, is to actually observe the movement in the sky , against a fixed background of stars which are known not to move much over long periods of time. Note, that if the star was heading straight in our direction, we wouldn't observe any motion at all. This is comparable to observing an airplane at night which is heading into your line of sight. You will just see it as a light getting brighter, but it won't actually seem to be moving with respect to distant background objects. So, though this is a good method, its somewhat limited. Another method is to observe the Doppler shifting of spectral lines. Lines in the spectrum of a star will shift slightly into the blue region of the spectrum if the star is moving towards us, and red if its moving away. If the object happens to be moving perpendicular to us, and is too far away to use the first method, then we probably cant get much information on the stars velocity. Fortunately, few objects move exactly in our line of sight or exactly perpendicular, so we can usually get some velocity information for a star, if we look at it enough.

Steve Bloom
for Ask an Astrophysicist

Question ID: 980411c

What are the different methods to weigh a star? (Submitted May 27, 1997)

The Question

What are the different methods to weigh a star? Is it possible to use the Doppler shift?

The Answer

Yes, you are correct, the most accurate method of "weighing" a star is by use of the Doppler shift. Let me explain.

To find the mass of the Sun we take advantage of the planets orbiting it. Each planet requires a centripetal force to keep it from flying away; this force is supplied by the Sun's gravity. Setting these two forces equal to each other we can write an equation (also in introductory physics books):

Force of Gravity = Centripetal Force

(G x MSun x Mplanet) / (r x r) = (Mplanet x V x V) / r

G = Newton's Constant of Gravity
MSun = The Mass of the Sun
Mplanet = The Mass of the Planet (like the Earth)
V = The Velocity the planet that is orbiting
r = The distance from the planet to the Sun (1 A.U. for the Earth)

simplifying the equation we see the the mass of the planet cancels out (using algebra) so:

MSun = (V x V x r) / G

Now, we will apply this same idea to other stars. However there is one big catch -- we cannot see planets orbiting other stars. So what do we do? We look at binary star systems. In a binary star system, there are two stars orbiting each other.

We measure the velocity of each star (V) using the Doppler shift that you referred to, and the distance between each star the their common center of mass (r) (which we find by taking pictures of the stars through a telescope). So we can find the total mass of the stars using the same equation as above.

I hope this explains your question.

Thank you very much for asking.


Jonathan Keohane
for Ask an Astrophysicist

Question ID: 970527a

Are there other ways to measure the mass of bodies in space rather than by gravitational means? (Submitted June 09, 1997)

The Question

I wish to know how we go about calculating the mass of our Sun and other stars. Are there other ways to measure bodies in space rather than by gravitational means? Do we estimate the mass of our Sun by measuring it against the combined mass of all the planets in our solar system? I'm interested in the equations and procedures.

The Answer

You've asked about one of the fundamental issues in astronomy, namely determining the mass of objects such as the Sun and other stars. The short answer is that there is no other way to *directly* measure the mass of the Sun or any other star than by observing the gravitational effects of one object on another.

You can estimate the mass of the Sun one of two ways. Both use Newton's Laws of motion. The first way uses Newton's revision of Kepler's third law, which states that the period squared or any body orbiting the Sun is proportional to its average distance from the Sun cubed. Newton generalized this for all gravitating systems. In the case of the Sun, the equation we can use is:

Mass_of_Sun=((4*pi2)/G) (a3/P2), where pi=3.14159, G is a fundamental constant, a is the radius of the Earth's orbit about the Sun, and P is the orbital period of the Earth about the Sun.

Another method uses Newton's second law and gravitation. In this case, one starts with F=m*a (where F is the force on an object, m is the mass of the object and a is the acceleration of the object due to the force). Since the gravitational force can be expressed as

F = G(MSun)(Mearth)/(R2)

where MSun and Mearth are the masses of the Sun and Earth, and R is the distance between the two, and the acceleration for a circular orbit is equal to the velocity2/R, Newton's second law can be rewritten in this case to give


In both cases, putting in the values for Earth's orbital velocity, distance from Sun and the value for G gives a value of about 2 x 1030 kg for the mass of the Sun. (That's 2 with 30 zeroes after it.)

Since you are obviously very interested in astronomy, I encourage you to locate a textbook on introductory astronomy at the college level. It will have discussions of mass determination, and many of your other questions at about the same mathematical level as this answer. Some textbooks are not very mathematical at all. Try 'Astronomy The Cosmic Perspective' by M. Zeilik and J. Gaustad, or just look around your school or community library for astronomy texts. In any such text, you will find the values of the Earth's orbital parameters, and can go about determining the mass of the Sun on your own!


Padi Boyd
for the Ask an Astrophysicist

Question ID: 970609f

    Our Sun as a Star

How does the Sun compare to other stars, is it average? (Submitted January 08, 1999)

The Question

Is the Sun one of the hottest stars in space?

The Answer

No, the Sun is pretty middle of the road in terms of both size and temperature. The hottest stars are blue-white in color, and have surface temperatures of greater than 50,000 degrees Kelvin. The coolest are red and have surface temperatures of less than 4000 degrees Kelvin. The Sun is yellow and has a surface temperature of about 6000 degrees Kelvin. All stars are much hotter at their centers, however.

Thanks for your questions

Eric Christian
for Ask an Astrophysicist

Question ID: 990108a

How long does it take a photon to leave the Sun? (Submitted April 14, 1998)

The Question

Why does it take so long for a photon to escape from the sun? How would the amount of interactions affect this time?

The Answer

The core of the Sun is extremely hot, many millions of degrees, so electrons are stripped from their atoms. This means that there are a lot of free electrons whizzing around in the Sun. With the densities that are typical for the core of the Sun (a few gm/cm3) there will be many interactions with photons. It is the density and the effective cross section of an electron , and the average velocity of an electron (usually determined from the temperature) which determine the amount of time for an interaction (or, actually, between interactions). The size of the Sun, and the average distance a photon travels between interactions ("mean free path") determines the number of interactions (its actually proportional to the square of (radius of sun)/ (mean free path). Once we plug in the right numbers for the Sun, I get a number of about 100,000 years. I think the more "official" number is about 1 million years.

Steve Bloom
for Ask an Astrophysicist

Question ID: 980414a

How do we know the core temperature of the Sun? (Submitted December 16, 1998)

The Question

I am a science teacher in a middle school in Istanbul, some of my students asked me how a scientist can measure the temperature of the core of the sun? Since I am not sure, and the archives don't really say, I thought I would give this a try. The only thing I can think of is that by measuring the amount and different kinds of radiation that come from the core scientist can then extrapolate the approximate temperature?

The Answer

That is a really good question, and one which is surprisingly simple on the surface (or to a first order approximation).

First lets look at a pot of water at room temperature. (I know this seems strange, but bear with me on this.) If you leave it alone, after a few days, if nothing starts growing, the water level will go down as water evaporates off of the top, until finally you are left with an empty pot. Right?! At room temperature, the water isn't boiling, but every now and again a water molecule can "jump" into the air. How fast this happens is determined by hydrostatic equilibrium. (i.e. a balance between what is pushing one way with what is pushing the other way). In the case of the pot of water, it is how much pressure (from gravity pulling down on the atmosphere) is pushing down on the water, compared with how much the water molecules, which have some energy and want to move, are pushing up on the air.

The actual definition of boiling point is when the atmospheric pressure = the vapor pressure of the liquid. This is why at high elevations (like Mt. Ararat) the boiling point of water is much lower than at sea level (Istanbul). At higher elevations there is less air pressure to push on the water, so it can boil (escape) at a lower temperature. The boiling point of water is about 90 degrees Celsius at 3,000m instead of 100 degrees at sea level.

So gravity pushes on the atmosphere, and the weight of all that air pushes on the water to prevent it from boiling, UNLESS you add energy.

Well, the Sun works the same way. If the Sun were not burning, gravity would compress all the gas down into a much smaller space. Since the Sun is bigger than just a ball of gas held down by gravity, we know (along with other things, like the fact that it is glowing VERY brightly) that there must be some source of energy in there. (In the Sun's case the only thing that can produce enough energy over such a long period of time is nuclear fusion.) If we can measure the size (radius) of the Sun, and have a good estimate of its mass, using some physics we can calculate the temperature in the center needed to hold the rest of the Sun up.

The reason that we can't just look at the radiation (light) that comes from the center of the Sun to measure its temperature is that the Sun is so dense that even the most energetic gamma-rays can only travel a few centimeters in the center before being absorbed by another atom, and then re-emitted a little while later. (This is called Compton scattering).

If you light a fire, the farther away from the fire you get, the cooler it feels. This doesn't mean that less energy is given off, but that the same amount of energy that is sent from the fire is spread out over a larger area, and doesn't feel as hot. (This is called the Inverse Square Law) The surface of the Sun glows at about 5,000 degrees C, not because it has nuclear fusion going on, but because it absorbs the energy from the center and then sends it on to us. A good animation showing this can be found at:

The X-rays we see from the Sun come from the Corona, a region of VERY hot (about 1,000,000 degrees) gas above the surface of the Sun, but which we can only see during a total eclipse. (There will be a total Eclipse visible in Turkey this summer.)

The only thing that we can detect that comes directly from the center of the Sun are neutrinos. These small particles are so non-reactive that over 99.9% of them go right through the earth without touching (affecting) anything. The small fraction that we do detect gives us information on the nuclear processes in the center of the sun.

More mathematical details are below:

Here is the full equation which equates the pressure of the gas to the volume it takes up:

dP/dr = -rho GM(r)/r^2

P is the pressure at radius r, rho is the density, usually denoted with the Greek letter. G is the gravitational constant. M(r) is the mass INTERIOR to radius r, since this is the material which pulls down on a particle at radius r.

If we calculate pressure using the simple estimate

P = nkT

where n is the number density particles, k is the Stefan-Boltzmann constant, and T is the temperature in degrees Kelvin, we get the relation:

T_c ~ (G M_sun / R_sun) (m_p/k)

Here k is the Stefan-Boltzmann constant and m_p is a proton mass, M_sun and R_sun are the solar mass and radius.

This gives T_c ~ (6.67e-8*2e33/7e10) (1.67e-24/1.38e-16) ~ 2.3e7 K

Which is actually quite close to more accurately derived numbers. Just knowing the solar radius and Mass and that the Sun is supported in hydrostatic equilibrium basically sets the interior temperature. More detailed modeling uses more complex and accurate microphysics and fits the observed radius, mass and solar luminosity to derive the run of temperature in the sun.


Mike Arida, Tod Strohmayer and Andy Ptak
for "Ask an Astrophysicist"

Question ID: 981216a

When will the Sun become a red giant star? (Submitted June 04, 2004)

The Question

How long until the Sun becomes a red giant?

The Answer

The Sun will become a red giant in about 5 billion years, which is slightly more time than it has already been a star. There's a lot of nice information about the Sun at

Hope that helps.

-Kevin and Dirk,
for "Ask an Astrophysicist"

Question ID: 040604a

    Giant Stars

What is a red giant? (Submitted October 16, 1997)

The Question

Can you please tell me in plain language what a Red Giant is?

The Answer

I'll give you a short answer and a longer one. The short answer is that towards the end of a star's life, the temperature near the core rises and this causes the size of the star to expand. This is the fate of the Sun in about 5 billion years. You might want to mark your calendar!

The long answer is that stars convert hydrogen to helium to produce light (and other radiation). As time progresses, the heavier helium sinks to the center of the star, with a shell of hydrogen around this helium center core. The hydrogen is depleted so it no longer generates enough energy and pressure to support the outer layers of the star. As the star collapses, the pressure and temperature rise until it is high enough for helium to fuse into carbon, i.e. helium burning begins. To radiate the energy produced by the helium burning, the star expands into a Red Giant.

Jeff Silvis
For Ask an Astrophysicist

Question ID: 971016

Are there any clear pictures of red or blue giants? (Submitted March 03, 1997)

The Question

Are there any clear and real photos of any red or blue giants? If there are, where can I find them?

The Answer

Stars are so small on the sky that they appear as points in optical telescopes. There is only one case that we are aware of where a resolved picture of a star was taken with the Hubble Space Telescope (resolved) means that the resolution of the telescope was high enough to see structure or any spatial extent). The star was Betelgeuse, a red supergiant which is the "largest" star in the sky (other than the Sun, of course) in terms of its angular size. By this we mean how big the star appears in the telescope, as opposed to its physical size. The paper on this includes pictures and can be found in the Astrophysical Journal, volume 463:L29L32, 1996 May 20. This periodical is, unfortunately, usually only found at Universities or Research Institutions.

Here is are links to the image of Betelgeuse and its caption. and

Steve Drake and Andy Ptak
for Ask an Astrophysicist

Question ID: 970303a

Are there stars larger than red giants? Are they candidates for black holes? (Submitted June 16, 1997)

The Question

I have just became aware of the enormity of Betelgeuse. Do we know of stars that are larger than this? If so, approximately how many are of this scale are larger? Does a star have to be of this size to be a candidate for a black hole?

The Answer

Betelgeuse (also known as alpha ori) is a very large star, an M supergiant. This is because it has evolved far from the state in which stars spend most of their lives, known as the main sequence. For stars on the main sequence, which includes our Sun, there is simple proportionality between size and mass, and also a simple scaling for luminosity. For evolved stars the situation is less simple. Betelgeuse is more than 1000 times larger than the Sun, and 50000 times as luminous, but only about 20 times as massive. Most of the light from Betelgeuse comes out in the infrared, however, which is very different from the Sun. One consequence of the advanced evolutionary state of Betelgeuse is that it probably was much more massive when it was on the main sequence, and has already lost a significant fraction of its mass (probably more than half) in a stellar wind.

There are many stars that are as massive as Betelgeuse is now, and probably many that are as massive as Betelgeuse was when it was on the main sequence. Of the 100,000,000,000 (100 billion=10^11) or so stars in our galaxy, it is estimated that approximately 1% have main sequence masses greater than 30 times that of the Sun, which is where Betelgeuse may have started out. A very crude estimate is that such stars spend 1% of their lives as supergiants, which would suggest 10,000,000 stars similar to Betelgeuse in our galaxy.

In spite of this fact, there are very few stars which are visible to the naked eye which are as large as Betelgeuse. This is simply a consequence of the fact that we can distinguish bright stars in only a small fraction of the galaxy. Another one is Mira, in the constellation Cetus. Mira is probably larger than Betelgeuse, so large that it is thought that the outer layers of the star are barely held together by gravity. Mira is known to pulsate and eject its outer layers, probably in large part because of its weak gravity. Possibly the most massive known star is eta carina, which may have been 150 times as massive as the Sun when it first formed, and may be 50 - 60 times as massive as the Sun currently. In the 1830s eta carina underwent a tremendous outburst during which it became a brilliant naked eye object and ejected an amount of gas with mass approximately equal to the mass of the Sun.

It is likely that the minimum main sequence mass for a star which will eventually make a black hole is 8 - 10 times the mass of our Sun. This is quite a bit less than Betelgeuse had when it was on the main sequence, and there are many such stars in our galaxy.

I hope this helps!

Tim Kallman for Ask an Astrophysicist

Question ID: 970616b

How do red giant stars lose mass? (Submitted January 01, 1998)

The Question

At a recent visit to the Pacific Science Center in Seattle Washington, I attended a Planetarium presentation in which we learned about the life cycle of a star. The guide explained how a star will expand as the gases burn up and change (I might not be describing this very well). I asked whether the weight of the star changes as it expands. The guide did not answer my question -- I don't think he knew. Also: does the Sun get lighter (in weight) as it burns up?

The Answer

What you're describing is the change of a star from the 'main sequence' to the 'red giant' stage. (See, for example,

for further explanation.)

A star will lose mass in two different ways:

(1) Nuclear fusion --- when 4 hydrogen nuclei are combined to form a single He nucleus, about 0.3% of the original mass is converted into energy. However, this is an extremely slow process and much less important than:

(2) A star expels matter in the form of a "stellar wind". Although it happens to all stars to some extent (including our Sun), and can be spectacular for some stars at certain stages of their life cycle, it's not particularly strong for a star changing from a main sequence star to become a red giant.

Best wishes,

Koji Mukai

for "Ask an Astrophysicist"

p.s. We use the term mass to describe what I think you're asking about. You weigh much less if you are standing on the Moon instead of here on Earth, but your mass would be the same --- so the mass is a much more fundamental quantity than the weight.

Question ID: 980101a

What is a yellow giant? (Submitted January 05, 1998)

The Question

In a text book I have come across yellow giants. Which stars pass through the yellow giant stage? I know of the red giants. Are there other 'giants' too? What exactly is a yellow giant?

The Answer

Yes, there are many types of giant stars, blue-white, white, yellow, orange, and red. Yellow giants are a phase of stars with masses heavier than the Sun, but it is phase that doesn't last very long, so there aren't many of them. If you look at a standard Hertzsprung-Russell diagram (which plots spectral class or color vs. Absolute magnitude. See there are blue-white and white giant stars on the main sequence, and red giant stars near the end of life of many stars. As heavy stars move off the main sequence towards their red giant stage, they can move through a yellow giant phase.

Thanks for your question.

Eric Christian
for Ask an Astrophysicist

Question ID: 980105a

What is the largest known star and how big is it? (Submitted April 21, 1998)

The Question

What is the largest known star and how big is it in relation to our Sun.

The Answer

The largest known star (in terms of mass and brightness) is in the Pistol Nebula. It is believed to be 100 times as massive as our Sun, and 10,000,000 times as bright.

David Palmer for Ask an Astrophysicist

Question ID: 980421a

Are there any stars larger than the Solar System in volume? (Submitted February 09, 2002)

The Question

Are there any stars larger than our solar system?

The Answer

Thank you for your question. If we consider the size of the solar system as the distance of the Sun to the furthest planet, Pluto, then the solar system is roughly eight thousand times larger than the radius of the Sun. The largest stars we know about are called red supergiant stars (there is a relatively nearby one called Betelgeuse). The largest red supergiant we know of is in the binary system VV Cephei, and is close to four thousand solar radii. This is large enough to encompass the orbits of Mercury, Venus, Earth, Mars, and Jupiter; but makes it not quite halfway out to Pluto.

-- Michael Loewenstein and Amy Fredericks for "Ask an Astrophysicist"

Question ID: 020209a

What are R,N,C & S stars? (Submitted January 25, 2007)

The Question

While self-teaching in astronomy (amateur stargazer class), I've come accross spectra marked R,N,C & S.(The reference is in an online version of Hipparcos). While I've become well-aquainted with the usual OBAFGKM system, these leave me a little puzzled. After a look on the web, I've found about 7,000 files that describe the details (lots of research papers), but not the basics (such as where they are on the HR diagram or even their names such as "an R star is better known as a Jones Star") without which the later information isn't much help!

The Answer

Thanks for your question. The R, N, C & S classification for stars are reserved for a special class of stars known as carbon stars, stars with a rather high carbon content. Most carbon stars are variable stars and many are thought to be in the red giant/AGB phase on the H-R diagram. A very good description can be found at the following website:

Hope this helps,
Georgia & Mike
For "Ask a Astrophysicist"

Question ID: 070125a

    White Dwarf Stars

How long does it take a Red Giant star to become a White Dwarf? (Submitted May 12, 2005)

The Question

I am interested in the time it takes a red giant to become a white dwarf? Specifically, I am thinking about the Sirius mystery concerning the ancient records of Sirius as a red star. Although most articles I have found say that it is impossible, under our current stellar evolutionary model, for it to have evolved into a white dwarf, no literature that I have found explains exactly the time needed for it to have become a white dwarf.

The Answer

The answer to your question likely depends on the mass of the star in question. However, if we consider the Sun, then according to a timeline posted by John Baez, a mathematical physicist at the University of California, it will take about 400 million years for the Sun to go from red giant to white dwarf.

You can see his timelines at the following web page:

We hope this helps!

Barbara & Veronica
For the Ask a High Energy Astronomer team

Question ID: 050512a

Can you recommend an Internet site with information about white dwarfs? (Submitted October 28, 1997)

The Question

Could you recommend an Internet site for the most current information on white dwarfs?

The Answer

There is (we think) a good general discussion of white dwarfs on our Imagine the Universe site, at

Unless you have a more specific question, we think this is a good place to start.

Tim Kallman
for Ask an Astrophysicist

Question ID: 971028b

What is the surface temperature of a white dwarf star? (Submitted May 12, 1997)

The Question

What is the surface temperature of a white dwarf star?

The Answer

As you may know, a white dwarf is the cinder of a star which used to be like the Sun. At the end of its life, such a star expels much of its atmosphere, and the nuclear fusion stops. The hot core, about the size of the Earth but much denser, becomes exposed: this is the white dwarf.

When a star has just become a white dwarf, it is hotter than 100,000 K (about 180,000 F). It then gradually cools --- after many billions of years, it can become cooler than the Sun (which is about 6,000 K). So there is no particular temperature associated with the white dwarfs.

They are called 'white dwarfs', but not all are actually white; the first few that were discovered are white, with temperatures near 10,000 K. These are still the easiest to discover, so perhaps you can say that 10,000 K is the typical temperature of white dwarfs that we know of.

Blue (say 50,000 K) 'white dwarfs' are rare because they cool quickly; yellow and red (cooler than, say 6,000 K) ones are dim and very hard to discover, but there may be many if we look hard enough. Some astronomers look for these cool white dwarfs and estimate how long they have been cooling, so they can say something about the age of the Universe.

Koji Mukai and Jim Lochner
for Ask an Astrophysicist

Question ID: 970512e

What is a black dwarf? (Submitted October 02, 1997)

The Question

How are black dwarfs and neutron stars similar?

The Answer

A 'black dwarf' is a white dwarf that has cooled down enough that it no longer emits light. See the Imagine the Universe science pages ( for the differences between white dwarfs, neutron stars, and black holes.

A white dwarf is formed when a star has burned all of its original hydrogen and helium fuel to elements such as carbon, nitrogen and oxygen. If the star doesn't have enough mass, the pressure at its center is too low to burn these elements further, and so it no longer produces heat. It is, however, still hot from the earlier burning stages, so it still glows for a while until it cools down. It takes tens to hundreds of billions of years for it to cool down entirely, and the Universe hasn't been around that long--the oldest stars are between 10 and 20 billion years old. Therefore there are no black dwarfs yet, but there will be in the future.

David Palmer
for Ask an Astrophysicist

Question ID: 971002b

Can you explain the concept of degenerate gas pressure? (Submitted November 18, 1997)

The Question

Considering that a white dwarf is highly dense, with small radius, and yet very hot, is it possible that inside it could be liquid and not gas? If not please explain degeneracy of a gas, I'm reading Chandrasekhar, and having hard time understanding his book, but I'm fascinated by astronomy, and astrophysics.

The Answer

The section on white dwarfs in the Imagine site (at is useful and describes degenerate pressure in the section 'What's Inside a White Dwarf'. It describes what is basically taking place and also the principles of degenerate pressure (both electron and neutron), which will be the most relevant for understanding Chandrasekhar.

Thanks for your questions.

Eric Christian and J. Allie Cliffe
for Ask an Astrophysicist

Question ID: 971118a

Can a white dwarf really have a diamond core? (Submitted February 28, 2003)

The Question

Recently I learned that white dwarf stars have a core that can become a diamond over millions of years because of the pressure exerted on it by gravity. Is this true? Are there white dwarfs in the universe that are literally orbiting in galaxies as dead stars with a diamond core?

The Answer

Yes, it's true that white dwarf stars will eventually form a core of crystallized carbon, much like terrestrial diamonds, with a little bit of oxygen impurity. When a star like our sun extinguishes its fuel, it will leave behind a hot core at a temperature of 100,000 degrees with a surface gravity 100,000 times that of Earth. In this resulting white dwarf the heaviest elements (carbon and oxygen) sink to the center, while hydrogen and helium rise to form an atmosphere around the cooling star. By measuring pulsations of a nearby cooling white dwarf, astronomers were able to determine that the interior of the star had partly solidified. It takes a few billion years to cool, but white dwarfs do essentially form giant diamonds in their interiors.

for 'Ask and Astrophysicist'

Question ID: 110223a

    ...and Other Types of Stars

What are the different types of stars? (Submitted January 02, 1999)

The Question

I was wondering what the types of stars were? I only know a few, such as white dwarf, and red giant.

The Answer

About 90 percent of the stars lie in the H-R diagram, marked by the main sequence:

A star shines by radiating energy from the nuclear reactions. Once a star runs out of its fuel, it dies and becomes compact. The lifetime of a star depends on its mass. High-mass stars evolve faster and die sooner than low-mass stars. The most massive stars may live only a few million years, and the lowest mass stars can live 100's of billions of years.

Low-mass stars die quietly, while high-mass stars die in tremendous explosions (supernovae explosions). A star like our Sun would die and end as a white dwarf:

Stars about five to ten times more massive than our Sun would explode and form a neutron star. More massive stars turn into black holes:

Samar Safi-Harb & David Palmer
for Ask an Astrophysicist

Question ID: 990102a

What are brown dwarfs? (Submitted October 18, 1998)

The Question

What are Brown Dwarfs?

The Answer

Brown dwarfs are small-size objects, believed to result from condensations of fragments of molecular clouds. A brown dwarf has a small mass, too low to ignite nuclear fusion. Such a star, would be small, not much larger than Jupiter, and warm from its contraction. It would emit copious infrared radiation, thus the name "brown dwarf".

You can check the following site on brown dwarfs:

Eric Christian for for Ask an Astrophysicist

Question ID: 981018a

What kind of stars are the Cepheid variables? (Submitted April 25, 1998)

The Question

Does anyone actually know what exactly Cepheid variables are and what causes the fluctuation in absolute magnitude? Also, why is the period of Population I Cepheid variables proportional to their distance from our solar system - is this a relativistic effect?

The Answer

The Cepheids are pulsating variables: The radius and the surface temperature of a Cepheid change periodically so the overall brightness varies.

The temperatures of the Cepheids are in the 6000-8000K range, which is when hydrogen atoms become ionized, and this causes the opacity of the atmosphere to change. When the opacity is high, meaning that the radiation has a hard time getting out, in a particular region of the atmosphere, the supply of energy from stellar interior gets trapped there. So the temperature and the pressure increase in this region. This eventually causes expansion, which causes atmosphere to become more transparent...

The periods of Cepheids are not related to their distances. The are linked with the true brightness (and hence called the Period-Luminosity relationship). We can easily measure the apparent brightness and the period of the pulsation of a Cepheid variable. Since the apparent brightness depends on the true brightness and distance, we can use the measurements to infer the distance to this Cepheid.

Hope this helps. Best wishes,

Koji Mukai
for Ask an Astrophysicist

Question ID: 980425c

What are Wolf-Rayet stars? (Submitted June 03, 1998)

The Question

Recently my teacher asked us if we could try to find any info on the Wolf-Rayet stars. She said that she read it in an article that had to do with stars. I've checked around, but I can't find anything on this topic. Do you know of any places I can check to see if I can help her? I've read about Wolf (345?), which is a star, but nothing on Wolf-Rayet stars. I appreciate the time you take to try and answer the many questions.

The Answer

Wolf-Rayet refers to a type of star, sometimes abbreviated "WR star". WR stars are hot, luminous, and contain atmospheres whose thickness is comparable to the size of the star. (Most stellar atmospheres are proportionally much thinner.) WR stars are also losing mass in the form of a wind at a high rate of between about 10-6 and 10-5 of a solar mass per year. By comparison, our Sun loses about 10-14 solar masses per year in its solar wind (a solar mass is about 2 X 1033 grams). The strong winds from WR stars are due to the fact the radiation pressure (the force of light pushing on the gases) in the atmosphere is quite strong.

You should be able to learn more about WR stars in a mid-level college astronomy text. (The topic may be too specialized for an introductory text.)

One of our local Wolf-Rayet experts, Dr. Michael Corcoran, offers the following more detailed information:

Wolf-Rayet stars (named for their discoverers) are very large, massive stars (stars which are about 20 times bigger than the sun) nearly at the end of their stellar lives. As these stars age, material which the stars have cooked up in their central nuclear furnaces (like carbon and oxygen) gradually reaches the surface of the star. When enough material reaches the surface, it absorbs so much of the intense light from the star that an enormously strong wind starts to blow from the star's surface. This wind becomes so thick that it totally obscures the star - so when we look at a Wolf-Rayet star, we're really just seeing this thick wind. The amount of material which the wind carries away is very large - typically, a mass equivalent to that of the entire earth is lost from the star each year. The mass loss is so large that it significantly shortens the star's life, and as you can imagine has important effects on the space surrounding the star too. We think that very massive stars become Wolf-Rayet stars just before they explode as supernova (though no one has yet seen such a star explode).

J.K. Cannizzo
(for "Ask an Astrophysicist")

Question ID: 980603a

Were massive stars more common in the early Universe? (Submitted March 06, 2005)

The Question

I read that gamma-ray bursts are caused by hypernova explosions of Wolf-Rayet stars in the distant universe. Assuming this is correct, wouldn't these massive stars seem to be more prevalent in a younger Universe? Under what conditions might these stars form, and how long would formation take? If formed in the earliest Universe, wouldn't these stars be less likely to contain initial impurities of heavier elements?

The Answer

You can get started by reading the archived answer on our site:

There are more details at

Our page tells you that the Wolf-Rayet stars are massive stars near the end of their lives. Astronomers are not completely sure how these stars form, but it is possible that most extremely massive stars (more than forty times the mass of the sun) become WR stars for a short time near the ends of their lives. The mean lifetime of WR stars is estimated at only around 100,000 years and the formation time (from an existing star) is comparable. The reason there were probably more WR stars earlier in the universe is because it is believed that there were more massive stars early on. Stars early in the universe would initially contain fewer heavier elements than stars today, but heavy elements are produced inside WR stars and then distributed throughout their galaxy by the strong winds that WR stars generate. Thus WR stars are an important contributor to the enrichment of galaxies by heavy elements.

Hans Krimm for "Ask an Astrophysicist"

Question ID: 050306a

Are there sunspots on non-solar type stars? (Submitted November 01, 2005)

The Question

Do sunspots exist on bright stars, brown dwarfs, and non-solar type stars?

I have a difficulty in finding this information. In forums, they say they don't, while artworks show that they do.

It is known that G stars through M stars have sunspots.

Do W, O, B, A, F stars, Carbon Stars, and S stars have sunspots? Do Brown Dwarfs, or late - M, L, and T stars have sunspots?

The Answer

This is an interesting question that is still much debated among astronomers and one that generated a lot of interest around here (which is part of the reason why this reply took so long). Basically starspots are generated by the interaction of the stellar differential rotation (different parts rotating at different rates) with the surface magnetic field. Because the field is frozen into the photosphere, as the star differentially rotates the field becomes twisted and the magnetic pressure increases locally, which causes a decrease in the local temperature and an apparently dark spot. For this to happen you need a magnetic field, which requires an active stellar dynamo, which usually requires a convective stellar envelope around a radiative core.

Stars more massive than a few solar masses (O, B, and A) have convective cores and radiative envelopes, so that they should have very weak or non-existent magnetic fields and few if any spots. Similarly, very low mass stars (like brown dwarfs) are thought to be nearly entirely convective, so that the dynamo effect should be fairly weak, generating a weak field and few spots. Of course Jupiter has spots, so that probably brown dwarfs do have spots, but these spots are not the magnetically-generated variety. WR stars don't have observable spots since you can't see down to the photosphere because of their dense winds. Not sure whether C stars or S stars have been shown to have spots...

What happens in the hottest stars and in the coolest stars like brown dwarfs is particularly being debated right now.

In brown dwarfs, variability has been detected in a fraction of them (these observations are hard because brown dwarfs are so faint), and for various observational and theoretical reasons, this variability is generally believed to be due to 'weather', i.e., local variations in the atmospheric opacity (what you and I would call clouds), rather than starspots, although in a few cases (e.g., 2MASS J1155395-372735, Koen 2003 Monthly Notices of the Royal Astronomical Society, vol 346, page 473) the latter may be a viable explanation.

In hot stars, one of the big paradigm shifts in this field is the recognition that magnetic fields can play a role in their outer atmospheres. Since these star lack convective outer envelopes it's a bit of a puzzle how the magnetic fields are produced and persist. They are likely global in nature e.g., mostly dipolar and quadrupolar, and don't seem to produce localized cool starspots as are seen in the cooler stars with convective envelopes.

The Carbon and S stars are usually quite variable anyway, and their rotation periods would typically be years, so it would be hard to 'see' starpots on them (and because of their slow rotation their dynamos would be very weak anyway). They also likely have a pattern of large-scale hot and cool convective cells on their surfaces, such as have been claimed to exist on the M supergiant Betelgeuse which would also confuse things...


Hans Krimm, Mike Corcoran, and Steve Drake
for Ask an Astrophysicist

Question ID: 051101a

Why do some Red Dwarf stars have such huge flares? (Submitted December 15, 2008)

The Question

I read the news online that a red dwarf named EV Lacertae 16 light years away has released solar flares so bright in x-rays, so powerful, it would have been visible to the naked-eye if it was on the night sky instead of the day sky. They also said it was also so bright, it caused instruments on board Swift to automatically shut down. How can a star so small, so much less energetic than the Sun release that much energy? If the Sun relatively emitted the same amount of energy as that red dwarf did, it would probably be deadly for life on the day side on the Earth. Maybe the Sun has emitted a similar deadly amount of energy sometime ago and caused extinctions. The red dwarf is 15 times younger than the Sun.

The Answer

Thank you for your question! The key point here is the last sentence of what you wrote - EV Lac is much younger than the Sun. Because of that, it is still rotating rapidly, which creates a strong magnetic field in the star's atmosphere (much stronger than our Sun's). Flares are consequences of magnetic fields in stars. In particular, when magnetic fields get "twisted" by stellar rotation, material in the star's outer atmosphere can be ejected in a flare. Because EV Lac is rotating much more quickly than the Sun, and consequently has a stronger magnetic field, the flares it experiences are much stronger and brighter than the Sun's, despite its relatively small mass. However, the Sun could have experienced such powerful flares a few billion years ago, when it was much younger. You can learn more about this topic here:

Nick Sterling and Jason Link,
for "Ask an Astrophysicist"

Question ID: 081215a

    The Life Cycles of Stars

How are stars formed, and how do they end up? (Submitted April 22, 1997)

The Question

Are all stars formed from nebulae? If not where do they come from? What happens to a star after they are burned out? Do all stars end up a black hole or supernova? What determines their final destination? Where will our Sun end up when its fuel is spent? What are dwarf stars?

The Answer

Yes, all stars are formed from nebulae (the plural of nebula). Nebula is a term for a cloud of gas, and stars form from gas. Stars more massive than ~ 6 solar masses are expected to supernova, stars less massive than this (like our Sun, of course) become white dwarfs. After a supernova, there may be nothing left, or there could be a remnant: either a neutron star or a blackhole. If the remnant is more massive than around 3 solar masses it will probably end up as a blackhole. Stars are smallest when they are burning hydrogen into helium, which is what stars do during most of their lifetimes. Stars in this stage are sometimes called dwarfs. There are also two other kinds of "dwarfs": white dwarfs are burned-out stars mentioned above (the Learning Center has more info on these), and brown dwarfs are stars which never accumulated enough mass to start burning hydrogen.

Andy Ptak
for the Ask an Astrophysicist Team

Question ID: 970422f

How are large stars able to form? (Submitted March 4, 2013)

The Question

In star formation, gases coalse by gravitational forces. As the pressure increases the temperture reaches the point at which nuclear fusion begins and the star is born. The outward pressure of the fusion holds balance with gravity delaying further collapse as well the solar forces remaining away from the vicinity of the star. My question is how can very massive stars form since the gravitational force seems to cause fusion of hydrogen into helium thus preventing further coalseing of gases?

The Answer

It's a good question --- the answer lies in the finer details of the timeline of star formation.

Nuclear fusion starts very late in the process of star formation. This is because the high temperature and high pressure required for nuclear fusion can be achieved only after the star has already formed. The mass of the proto-star cloud is determined much earlier in the process. When fusion starts, it does indeed stop any further growth of the protostar.

You can read a college undergraduate level summary at:

Hope this helps,

Koji & Alexandre
for "Ask an Astrophysicist"

Question ID: 130404a

Inside a nebula, how big are the immense balls of dust and gas before they become stars? (Submitted April 26, 1999)

The Question

Inside a nebula, how big are the immense balls of dust and gas before they become stars?

The Answer

That is a very good question.

The "proplyds" or protoplanetary disks, as these systems are called, seem to be a few (about 5-8) times larger than our solar system.

For more information take a look at:

Hope this helps,
Mike Arida
for Ask an Astrophysicist

Question ID: 990426a

How can astronomers determine the age of a particular star? (Submitted March 19, 1998)

The Question

How can scientists figure out how old a specific star is?

The Answer

Can you look at a stranger on the street, and guess how old that person is? And how do you do that?

You can do it because you have a good idea of how a person grows and changes with age, and you look for certain physical characteristics of that person.

Astronomers can do something similar with stars. When a star like the Sun is born, it is shrouded in a cloud of dust and gas (planets can form from this cloud). Then it reaches the 'main sequence', where it spends most of its life. The Sun is probably halfway through the main sequence. Then it will turn itself into a red giant.

So if an astronomer sees a star in a cloud of dust and gas, she will guess that it's a young star. When she sees a red giant, she knows that the star is approaching the end of its life. You can tell between a main sequence and a giant, for example, by measuring the spectrum of the star.

There is one important complication: We know that, more massive a star is, the faster it burns up its fuel and the faster it grows. If there is a star 25 times as massive as the Sun, it can't be very old (which may still mean that the star is 3 million years old!).

This is the kind of thinking an astronomer has to use to estimate the age of a star. Of course, this isn't perfect. For one thing, our model of how a star ages may be wrong (it's not likely to be completely wrong, but it does need occasional fine tuning), just like your idea of how a person looks at age 30 could be a little bit off.

Hope this helps.

Best wishes,
Koji Mukai
for Ask an Astrophysicist

Question ID: 980319d

How long do stars live? (Submitted November 10, 1997)

The Question

How long do stars live? I am 7 years old.

The Answer

Stars live different lengths of time, depending on how big they are. A star like our Sun lives for about 10 billion years, while a star which weighs twenty times as much lives only 10 million years, about a thousandth as long.

David Palmer
for Ask an Astrophysicist

Question ID: 971110j

What is the range of the lifetimes of red dwarf stars? (Submitted February 22, 2012)

The Question

How long is the longest and shortest length of life for red dwarf stars?

The Answer

Thanks for your question. Red dwarf stars have lifetimes exceeding the present age of the Universe, burning for about 100 billion years (for red dwarf stars with about 1/4 of the mass of the Sun) to 10 trillion years (stars with 1/10 of the mass of the Sun).

Hope that answers your question!

-Antara & Bret
for "Ask an Astrophysicist"

Question ID: 120222a

What are the life cycles of the stars? (Submitted February 25, 1997)

The Question

I would just like to know what the life cycles of the stars are. Where they came from, and their life cycles, and an explanation of the Hertzsprung-Russell diagram.

The Answer

We primarily study the compact remnants of stars, not the evolution of stars. However, we can give a brief overview. Stars form from dense gas, usually in molecular clouds. After a star forms, it burns hydrogen into helium. It does this until the hydrogen begins to run out and then further stages of burning occur, i.e., helium burns into heavier elements. If the star is less massive than several times the mass of the Sun, it will eventually become a white dwarf. If it is more massive than this it will first implode and then explode in a supernova explosion. While the star is still burning hydrogen it is on the Main Sequence of the H-R diagram, which is a plot of temperature versus luminosity for stars. When the star is done burning hydrogen, it enters other regions of the H-R diagram such as the "Horizontal Branch", "Giant Branch" and "Asymptotic Giant Branch". For more info, check out


Andy Ptak
for the Ask an Astrophysicist team

Question ID: 970225a

What is the chemical composition of stars? (Submitted November 12, 1996)

The Question

I was looking on the Internet for information on the composition of stars and was not finding any information. I looked at a site you put together and thought perhaps you might know where I can find some good information. If you have the opportunity I would appreciate any help you might have.

The Answer

The area you are interested in, the composition of stars, is a very important and interesting field for astronomers. It touches on many related questions, such as how do stars form? How do stars shine? Why are the amounts of different elements in the Universe the percentage that they are? It is not easy to find WWW references simply on "chemical composition of stars", but if you pick one of the related areas of interest to you then it is possible to find some information about this on the WWW.

Stars begin their life when an ordinary dense cloud of interstellar matter becomes unstable and begins to collapse. The composition of such a cloud of matter determines the composition of the star which results from the collapse. Let's start by looking at the chemical composition of the star we know the most about, our Sun.

Astronomers study the spectrum of the Sun to determine it's chemical composition. In the visible region alone, from 4000 to 7000 angstroms (10-10 meters), there are thousands of absorption lines in the solar spectrum. These lines have been cataloged, and tell us that there are 67 chemical elements identified in the Sun. There are probably even more elements in the Sun that are present in such a small amount that our instruments can't detect them. Here is a table of the 10 most common elements in the Sun:

ElementAbundance (% of totalAbundance
number of atoms)(% of total mass)
Hydrogen 91.271.0
Helium 8.7 27.1
Oxygen 0.078 0.97
Carbon 0.043 0.40
Nitrogen 0.0088 0.096
Silicon 0.0045 0.099
Magnesium 0.0038 0.076
Neon 0.0035 0.058
Iron 0.0030 0.14
Sulfur 0.0015 0.040

You see that hydrogen is by far the most abundant element in the Sun, followed by helium. Those two together make up 99.9 percent by number of the total atoms in the Sun! This is also what we find in the composition of the Universe as a whole.

When other stars are studied spectroscopically it is found that most stars are composed of around 70 percent hydrogen and 28 percent helium by mass, very similar to what we see in the Sun. The fraction of all other elements, the "heavier" elements, is small and varies considerably from 2 or 3 percent by mass in Sun-like stars to 0.1 to 0.01 percent by mass in stars found in globular clusters. We call those stars with very little heavy elements "population II stars" and those with Sun-like heavy element abundances "population I stars". Theories of stellar evolution state that the population I stars are a later generation of stars, that formed after some enrichment of gas clouds between stars had already taken place. That is because stars "burn" lighter elements into heavier ones during their lives (scientists call this process "nucleosynthesis"). Right now, the Sun is burning hydrogen into helium at it's center, or "core". This is the chain of nuclear fusion that powers the Sun. The net effect is that four hydrogen nuclei combine to create one helium nucleus, some gamma-ray radiation and two neutrinos. The gamma-ray photons slowly lose energy as they pass through the solar interior, and the energy eventually escapes in the form of visible light. The neutrinos escape unhindered into space at the speed of light, and the helium stays in the core. Other stars, which have used up all the hydrogen fuel in their cores, burn helium into beryllium and carbon. Massive stars that evolve beyond this point then burn carbon into heavier elements, and so on. This process is called nucleosynthesis.

During the later parts of their lives, stars can shed material into the surrounding space, depositing heavy elements. The most dramatic way this is done is through a supernova explosion. In fact, since the earliest moments of the Universe, during the Big Bang, heavy elements have only been produced as a by-product of stellar evolution! That's what astronomers mean when they say "we are all star-stuff."

Why is there so much hydrogen and some helium to begin with? This is tied to our theories of the Big Bang. If the Universe started in conditions of extremely high temperature, then the matter would organize in a way that there were the most particles, or more simple elements like hydrogen and helium.

So, now after my long-winded introduction, here are the WWW sites I can recommend:

I hope this helps in your search for information on the chemical composition of stars using the WWW.

Padi Boyd and the Ask an Astrophysicist team

Note: Mistakes in an earlier version of this page (in abundance by number of Iron and Sulfer, and abundance by mass of Iron) have been corrected on 2014 July 29. See also a table of Solar abundances according to various studies, which lists abundances by number relative to that of hydrogen, showing that this is still an active research area, with noticeable disagreements among different authors.

Question ID: 961112a

Are we really made of stardust? [Follow-up to above] (Submitted September 21, 2005)

The Question

Within one of your answers is a statement that I've heard before, but... never really fully understood. You wrote 'we are all star-stuff.' Do you mean, that people are made out of the same things (elements) that stars are made of? Are people made out of star-dust?

Thank you for a wonderful site. I appreciate your answers, and your time.

P.S. If... we all once were a star... A (very) young childs next question might be. 'Mom...? What do Star's really think about? Maybe the words really were...'Twinkle, twinkle little star. How I wonder... Who you are?'

The Answer

The statement that we are all "star stuff," coined by the late astronomer Carl Sagan (not sure if this was before or after Joni Mitchell sang "we are stardust; we are golden. we are billion year old carbon"), is meant to imply more than that we are made of the same elements that stars are made of. Beyond that, the elements themselves (carbon, nitrogen, oxygen, etc.) were synthesized, cooked up as it were, in the nuclear furnaces that are the deep interior of stars. These elements are then released at the end of a star's lifetime when it explodes, and subsequently incorporated into a new generation of stars -- and into the planets that form around the stars, and the lifeforms that originate on the planets.

-- Michael Loewenstein and Amy Fredericks for "Ask an Astrophysicist"

Question ID: 050921a

What fraction of hydrogen in the universe has ever been in the stars (Submitted April 16, 2010)

The Question

I have been watching 'The Universe' series from the History Channel. After watching the episode on stellar evolution, I began to wonder. How much on the hydrogen in our own sun has never been in a star before, and how much is 'recycled' hydrogen from stars that previously lost their hydrogen by a supernova explosion? Or, how much of the hydrogen in the universe has never been in a star?

The Answer

This is an interesting question. It is known that most of the hydrogen now in the galaxy is within stars. One estimate I found (Gene Smith's Astonomy Tutorial: is that only about 5% of hydrogen in the galaxy is in the form of interstellar gas. That's within a galaxy. Outside of a galaxy, but within a galaxy cluster, the density of hydrogen is about a thousand times less, but clusters are millions of times larger than galaxies, so there is likely much more hydrogen in intergalactic gas than there is in stars.

Now, knowing how much of this hydrogen has never been in stars is a tougher problem. Within a galaxy it is likely that most of the gas is "recycled." We can infer this from the abundance of heavy elements in interstellar gas. In intergalactic space, it was thought that the gas was largely primordial, but astronomers have detected signs of iron in intergalactic gas, telling them that at least some of the gas was ejected by supernovae.


Hans Krimm and Kevin Boyce
for Ask an Astrophysicist

Question ID: 100416a

Is hydrogen necessary to form a star? (Submitted October 28, 2001)

The Question

Is hydrogen necessary for the creation of stars. Can stars be born out of the fusion of heavier elements? What happens when molecular hydrogen in the Universe is exhausted or get to be very rare ?

The Answer

Thank you for your question. In principle, since any element lighter than iron will produce energy through fusion one could make stars out of helium, or carbon, etc. However, the heavier the element (hydrogen being the lightest) the higher the central temperature -- and therefore the higher the total stellar mass -- required for fusion to proceed. This means that the lower limit for an object to be a star would increase greatly from its present value of about one-tenth the mass of out Sun.

There is a certain amount of recycling that occurs in that stars will shed some fraction of their hydrogen during their lifetimes, and this material can eventually form new stars. But this recycling is not 100% efficient, so that in the far future the available hydrogen will indeed run out and star formation will essentially cease to occur.

-- Michael Loewenstein and Amy Fredericks for "Ask an Astrophysicist"

Question ID: 011028a

Does the chemical composition of a star have an effect on its luminosity and life span? (Submitted October 20, 1997)

The Question

Does the chemical composition of a star have an effect on its luminosity and its life span? If so, why?

The Answer

The chemical composition of a star affects its evolution in ways that most high-energy astronomers rarely worry about, although low-energy astronomers who work with ordinary stars have more experience with this.

Metals, (elements heavier than Helium), tend to be more opaque than hydrogen and helium. This is because, being more complex, they have more absorption lines in their spectrum, and these lines represent opportunities for an atom to stop photons. (This is extremely oversimplified.)

This makes high-metallicity atmospheres more opaque, so the photons are trapped longer in the interior of the star, increasing the temperature of the core and fluffing the star up to a larger size for any given total energy production.

One example of this is that Cepheid variables have a different period-luminosity relationship for Population I (Sunlike metallicity) and Population II (lower metallicity) stars. This was the cause of one of the sudden changes in our understanding of the size of the Universe, when it was realized that the Magellanic Clouds (mostly Pop II) were at different distance than the calculated values based on Cepheids in our neighborhood (Pop I).

Another possible example is that Supernova 1987A in the Large Magellanic Cloud was a blue supergiant before it went off. It may be that if the 1987A precursor were Pop I instead of Pop II, it would have been a red supergiant at the end of its life. Red supergiant supernovae tend to be brighter than blue supergiant supernovae, due to their greater initial size (and thus radiating area). Since the red ones are so much more visible than the blue ones, most people thought that this type of supernova (Type II) occurred only in red supergiants. When I was a lad (~1982), I learned that Type II supernovae were mostly found among Pop I stars--an observational bias that was only generally recognized after 1987A.

David Palmer
for Ask an Astrophysicist

Question ID: 971020f

What part do neutrinos play in the life cycles of stars? (Submitted January 20, 1998)

The Question

I have heard that neutrinos are responsible for a large amount of energy transfer out of stellar cores. I have also heard that neutrinos sometimes "carry" photons with them. How does this phenomena occur? I have a B.S. in Physics with some formal exposure to astronomy and particle physics.

The Answer

In our Sun, 98% of the energy is produced by the chain of reactions which produce a helium nucleus from 4 protons. This process produces a couple of neutrinos with typical energies of a few hundred keV. Total predicted neutrino energy flux is about 10-20 W/m^2 at the distance of Earth, or about 1% of the sunlight flux. (These numbers are loosely derived from tables and graphs in ). Note that theory and experimental measurements seem to disagree with each other by a factor of ~3 in this field.

However, some stars are much more neutrino-oriented. In the final stages before the core collapse leading to a supernova, a massive star burns 1.4 solar masses of silicon into iron in about 2 DAYS. The power produced is 10,000,000 as much as it was during its hydrogen-burning main-sequence life (when it was an immensely bright blue or red supergiant), but 99.99998% of that power is ghosted away as neutrinos. The core collapse supernova itself releases 99% of its energy as neutrinos.

I have never heard of neutrinos carrying photons with them.

David Palmer
for Ask an Astrophysicist

Question ID: 980120a

What is the cause of the Helium Flash in evolved stars? (Submitted April 09, 1999)

The Question

What exactly causes helium flashes in stars at the point when they begin to burn helium rather than hydrogen?

The Answer

A helium flash occurs because the core of the star is in what is known as a "degenerate" state. This means that the core has contracted so much that the pressure of the electron shells of the atoms making up the core prevent the core from contracting further. Under normal gas conditions (i.e. NOT a degenerate state), an increase in the temperature of the core would cause an increase in core pressure resulting in the core expanding and the temperature then dropping. This state is known as hydrostatic equilibrium. With a degenerate core, the temperature increases but the pressure doesn't. This extra energy ignites the helium creating run-away nuclear reactions. This is what is referred to as a "helium flash." For more details, check out:

Hope this helps.

Allie Hajian & Sean Scully
for Ask an Astrophysicist

Question ID: 990409a

Why don't large mass stars become degenerate like some low mass stars? (Submitted May 26, 2010)

The Question

During stellar evolution, for small and medium stars (until 8-10 solar), during the white dwarf formation process, the star doesn't have enough mass to increase the core temperature to start carbon fusion. After the white dwarf formation process, the core matter is in a degenerate state. But in huge stars, the mass is high enough to allow the core to reach a higher temperature, starting carbon fusion, and forming other chemical elements (until iron formation, when a supernova is born). For large stars, the core is not under a degenerate state (I suppose, because the carbon burning is not a runway reaction).

So, why in the white dwarf formation process does the core matter reach a degenerate state, but it doesn't in huge mass stars? Is it because in large mass stars the carbon fusion occurs in a plasma state core?

The Answer

For a stellar core to be in a degenerate state, the density has to be extremely high, and the temperature cannot be too high. If the core exceeds a certain critical temperature (which also depends on the chemical composition), it cannot become or remain degenerate.

Now, the cores of more massive stars have higher temperatures, because you need higher thermal pressure to balance the gravity of all the matters above. When you work out the numbers, it turns out that the cores of massive stars (more than 8 times the mass of the Sun) are always hot enough not to become degenerate.

For somewhat more details, check out:

Hope this helps,

Koji & Georgia
for "Ask an Astrophysicist"

Question ID: 100526a

Do neutrinos come from only supernovae, or from all stars? (Submitted January 07, 2009)

The Question

Do red dwarf stars send out neutrinos, or do only supernovae?

The Answer

All stars, red dwarfs included, release neutrinos when they convert hydrogen to helium.

Amy C. Fredericks and Michael Loewenstein
for Ask an Astrophysicist

Question ID: 090107a

    Clusters of Stars

What is a star cluster? (Submitted February 02, 1998)

The Question

What are star clusters made of? What forms a star cluster?

The Answer

Star clusters are groups of stars which are close together in space, rather than just accidentally lined up one behind the other. There are several different kinds. Among the most spectacular are huge balls of many thousands of stars, called globular clusters, which formed early in the history of galaxies, when they were rich in star-forming gas clouds. Other much smaller clusters, called open clusters, are visible where groups of stars have formed nearby more recently and are now drifting apart.

Paul Butterworth
for the Ask an Astrophysicist team

Question ID: 980202d

Is it possible to determine the age of a star cluster? (Submitted March 11, 1999)

The Question

Is it possible to determine the age of a star cluster?

The Answer

Yes, it's actually much easier to determine the age of a star cluster than of an isolated star.

A star cluster includes many stars that were created at about the same time, but of different masses (sizes), all at the same distance (more or less). By observing the brightness and color of each star, astronomers can construct a color-magnitude, or Hertzsprung-Russell (HR) diagram:

Stars like the Sun spend most of their life on the 'main sequence', a dense concentration of stars on a narrow belt going from the upper left to the lower right of the HR diagram. The mass of the star determines where on the main sequence it is located; the mass of the star also determines how soon the star will move away from the main sequence. By looking at the HR diagram of a cluster of stars, in particular where the main sequence ends, you can estimate the age of the cluster fairly accurately --- much more so than for single stars.

Best wishes,

Koji Mukai & Maggie Masetti
for Ask an Astrophysicist

Question ID: 990311a

Were the Pleiades formed in the Orion Nebula? (Submitted June 16, 1997)

The Question

I thought, if I am not mistaken, that the Pleiades open cluster was formed in the Orion nebula, and then drifted away to their present location. Is this true, or is it a false memory?

The Answer

I have not heard of the suggestion that the Pleiades originated in Orion. The two associations are both quite distant, and in different directions (angular separation about 80 degrees on the sky), and a quick calculation shows that they would have to be receding from each other at about 10 km/s in order to reach their current positions. This is not a large velocity, but the scenario would also require that the Pleiades maintain its integrity over the course of its lifetime, and this seems somewhat unlikely on the face of it.

A few more facts: The Pleiades is a loose cluster of approximately 100 stars with an average age estimated at 78 million years. The stars in the Pleiades are approximately 125 parsecs or 407.5 light years from our solar system. These are very young stars, much younger than our own Sun, estimated at 5 billion years old, much younger even than our own planet, Earth. These are very hot, bright stars of spectral type B, much hotter and about 10 times more massive than our Sun, spectral type G. They have not yet moved away from the interstellar gas cloud, or nebula, from which they formed. Remnants of this nebula can readily be seen in photographs of the group. Studies of the proper motions of these stars, or their movement through space, have shown that they are in the process of dispersion.

I hope this helps,

Tim Kallman for the Ask an Astrophysicist Team

Question ID: 970616a

If Globular Clusters rotate, why are they still round? (Submitted June 18, 2009)

The Question

What is known (or presumed) about the movement of stars within a globular cluster of stars, such as M13 in the constellation Hercules? Is the whole cluster rotating in the same direction, or are the stars randomly revolving around the center? Will the cluster eventually flatten out?

The Answer

Thank you for your question. Like other stellar spheroids (the halo and bulge of our galaxy, elliptical galaxies), globular clusters are supported against collapse via the mutual gravitational attraction of their constituent stars by random motions. This acts as a sort of pressure that opposes gravity, with the average velocity analogous to the temperature of a gas. This is in contrast to spiral galaxy disks that are rotationally supported. Globular clusters may also have a small net rotation, but at a speed that is significantly smaller than the random velocities -- otherwise globular clusters wouldn't be globular, but flattened.

Michael Loewenstein and Amy Fredericks
for `Ask an Astrophysicist'

Question ID: 090618

    Specific Stars

Can you tell us what the closest star to the Sun, Alpha Centauri, is made of, what its measurements are, and if it has any special assets? (Submitted July 17, 1997)

The Question

We would like to find out some information on the star, Alpha Centauri. We are 10 years old, but already have followed up a great interest in astronomy. We would especially like to know what Alpha Centauri is made of, what its measurements are and if it has any special assets.

The Answer

Here are some facts about Alpha Centauri:

Visible only from latitudes south of about 25 degrees N, the star we call Alpha Centauri lies 4.35 light-years from the Sun. But it is actually a triple star system. The two brightest components Alpha Centauri A and B form a binary. They orbit each other in 80 years with a mean separation of 23 astronomical units (1 astronomical unit = 1 AU = distance between the Sun and Earth). The third member of the system Alpha Centauri C lies 13,000 AU from A and B, or 400 times the distance between the Sun and Neptune. This is so far that it is not known whether Alpha Centauri C is really bound to A and B, or if it will have left the system in some million years. Alpha Centauri C lies measurably closer to us than the other two: It is only 4.22 light-years away, and it is the nearest individual star to the Sun. Because of this proximity, Alpha Centauri C is also called Proxima (Centauri).

Alpha Centauri A is a yellow star with a spectral type of G2, the same as the Sun's. Therefore its temperature and color also match those of the Sun. Alpha Centauri B is an orange star with a spectral type of K1. Whereas Alpha Centauri A and B are stars like the Sun, Proxima is a dim red dwarf with a spectral type of M5 - much fainter, cooler, and smaller than the Sun. Proxima is so faint that astronomers did not discover it until 1915.

Want more info?

As regards the size of the system, alpha Cen A is a double star with a period of 79.9 yrs. In terms of how the size of the system compares with ours, that is not a well defined question. If you were to use the size of our solar system as defined by the planetary orbits, and compare this with the size of alpha Cen A as defined by the binary orbit, then out solar system would be larger. The size of an orbit varies as the 2/3 power of the orbital period, so, going out to the orbit of Pluto which has an orbital period around the Sun of 248 yrs, the relative size of the orbit compared to a Cen A is (248/80)^{2/3}, or about twice as big. If, for our solar system, you were to include the orbits of comets, this ratio would be even larger. On the other hand, the a Cen system may also contain smaller bodies such as planets and comets which orbit far out from the central stars, so the sizes of the of our two solar systems may be rather similar.

There is every reason to believe that other stars have planets. It is not clear how stable the planetary orbits would be, however, in a double or triple star system. They could not have nice, nearly circular orbits like the planets in our solar system. To be stable, the orbits would have to be in some kind of "resonance" with the stars, in order to be prevented from being expelled from the system. I would find it difficult myself to accept stable planets in a multiple star system, unless the planets' orbits were large compared to the separation of the stars. In that case, however, the light from the stars would be pretty faint, and the planets might be too cold for life.

Tim Kallman and John Cannizzo
for Ask an Astrophysicist

Question ID: 970717b

I need some information about Deneb, Rigel, Betelgeuse, and Antares. (Submitted December 15, 1996)

The Question

I need some information on stars. What I need is the age and if it is binary or trinary or alone. The stars that I am interested in are 1. Deneb, 2. Rigel, 3. Betelgeuse, and 4. Antares. I have looked in our local library and could not find the information. I am in the 6th grade. Thank you very much.

The Answer

The stars you ask about are some of the brightest stars in the sky, so there is lots of information on them. The place I look for information like this is:

The Peterson Field Guide Series #15
Stars and Planets
by D.H. Menzel and J.M. Pasachoff
Copyright 1983, by the authors $12.95

You should be able to find it in your local library.

Most of the information you need is in table 1 on page 8 of the book mentioned above. For other information you might need the tables in back of the book or the sky charts.

However, ages of stars are harder to measure, because it is not something we can directly observe. We can measure directly a star's brightness. From its color we can measure its temperature (blue is hot and red is cold). Close stars appear to move very slightly in the sky throughout the year, because the Earth is going around the Sun. From this we can measure the star's distance. By observing binary stars' orbits, we can also measure their mass.

So, the book I mentioned will tell you much of this information, but not the age.

A few things to be aware of to understand the tables in the book:

  1. Brightness is measured in Magnitudes (the smaller the magnitude the brighter the star). This is how bright it looks to your eye. On a dark night in a remote area we can see with the naked eye stars of 6th magnitude or brighter. In a city, we can see stars of 1st, 2nd or 3rd magnitude or brighter. A star can be bright either because it is near by, or because it is really bright. All of the stars you mention are much brighter than the Sun (Sirius is 24 times brighter -- table A4). If someone lived on a planet near those stars, they would not be able to see the Sun with their naked eye.
  2. Temperature is measured in degrees Kelvin (= degrees Celsius + 273), or by color or by "spectral type". The spectra types are, from hottest to coolest, O B A F G K M. These have historically been remembered by, "Oh Be A Fine Girl/Guy Kiss Me." Later three types of strange cool stars were added as R, N and S for Right Now Smack. See table A-3 for a conversion between each of these.
  3. Mass is measured in "solar masses", so the Sun has a mass of 1 solar mass.
  4. Distance is often measured in light years (how long it takes light to travel in one year -- or about a Million years by jet airliner).
Now, back to your question of age. The more massive a star is the brighter it is, thus the faster it runs out of fuel. So, in general, the more massive a star is the shorter its life-span. So a star like Rigel must be quite young, as compared to our Sun (which is 4 billion years old).

In general, the exact age of one star is quite hard to measure. If there is a cluster of stars, you can assume they are all the same age, so we can find the age of all the stars. Even this is sometime hard to do too, because the exact age depends on many things, including the exact chemicals that make up the stars.

I hope this helps.

Jonathan Keohane
for Imagine the Universe!

Question ID: 961215b

Can you tell me about Polaris? (Submitted April 10, 1997)

The Question

Is Polaris bigger or smaller than our Sun? (I'm 8 years of age).

The Answer

Polaris ("The North Star") is a unique star because it is directly overhead at the North Pole. This makes it very useful, because it is always in the North. Not only that, but the farther North you go, the higher up in the sky it appears. If you are ever lost at sea, you can always tell which way is north, and what your latitude is, just by looking at Polaris.

On the other hand, as a star goes it is quite typical. It is not the brightest star in the sky, but plenty bright enough to see at night.

As far as how it compares to the Sun (which is type G2 V), it is only slightly hotter, but much brighter, larger and heavier than the Sun. Here is the relevant information on it and stars of similar type (F8 Ib).

Temperature = about the same as the Sun

Wattage = about 10,000 x brighter than the Sun [Astronomers call this "luminosity"] Our Sun gives off: 400,000,000,000,000,000,000,000,000 Watts

Size = 100 x our Sun [in Radius, so 1,000,000 times bigger in volume] Our Sun is 100 x the size of the Earth

Mass = 10 x our Sun our Sun is 300,000 times heavier than the Earth

Thank you for your question.


Jonathan Keohane and Mike Arida

-- for Imagine the Universe!

Question ID: 970410c

What are the evolutionary changes happening to Polaris? (Submitted November 12, 1998)

The Question

I read that Polaris may be reclassified as a different type of star soon. Is that true, and if so, what is the new classification?

The Answer

Polaris is a known Cepheid variable star with a period of 4 days, however, during the last decade, the amplitude of its brightening and fading has decreased precipitously to less than 0.1 magnitude as compared to the 1.5 magnitude swings that usually characterize Cepheid variable stars with this same 4 day period. An extrapolation suggested that it would stop pulsing sometime earlier this year. However, it did not stop.

Also, the Canadian magazine of astronomy & stargazing "SkyNews" had an article in its March/April 1998 issue titled:

"The Polaris Beat" The North Star, the nearest Cepheid variable star, hasn't been behaving itself, and Canadian astronomers are trying to find out why.

You have to subscribe to read the article, though...

See also:

I hope this helps.

Karen Smale, Mike Arida, David Palmer, and Tim Kallman
for Ask an Astrophysicist

Question ID: 981112b

Can you tell me about Vega? (Submitted April 06, 1998)

The Question

I'm a junior in High School and I am trying to find information about the star Vega. I know the basics but I just want to know more about this star.

The Answer

There are two good web sites for getting information about specific stars:

For more detailed information about particular stars (e.g. a star's distance, magnitude, temperature, etc.), see

Vega is of particular interest because it was one of the first stars for which a disk of dust was discovered surrounding a star. An infrared excess was discovered about the star in 1983 when the Infrared Astronomical Satellite (IRAS) observed it. This is indicative of dust surrounding the star, and planets might form from this disk. Vega is also important because it is used a standard calibration star in optical astronomy. It is used to calibrate the color scale for stars.

Jim Lochner, Steve Bloom and Gail Rohrbach
for Ask an Astrophysicist

Question ID: 980406d

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