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Library of Past Questions and Answers

Astronomy (General)

The Question

I'd like to know exactly what interferometry is, how it works, and what its benefits are to astronomy.

The Answer

There are two important advantages of a telescope over, for example, a human eye. One is the collecting area (bigger telescopes scoop up more photons) and the resolution (how close things can be together and still be distinguished as separate). A telescope's resolution is inversely related to its physical size; basically, the further apart the two furthest points in the telescope are, the better the resolution (the smaller the distance between two objects still distinct can be). If you take a big telescope, like the Arecibo radio telescope, and start hacking away parts of it, you are taking away surface area and its collecting area goes down. BUT, as long as you leave the two parts that are the furthest apart, the telescope still has the same resolution.

This is what an interferometer does. It is a bunch of small telescopes that have the resolution of a big telescope (the size of the widest separation of two telescopes in the interferometer). Interferometers are great for observing fine detail, but because their collecting area is small, the sources observed have to be fairly bright.

In order to use the individual telescopes together, the light from each of them has to be added. This has to be done in a special way, however. Light is a wave and different parts of the wavefront will reach the different telescopes at the same time. This means that at one instant the trough of the wave could be arriving at one telescope while the crest is arriving at another. If these two were added they would cancel. To fix this, something must be added to make sure that the light wavefronts from the source arrives at each of the telescopes at the same time. This can be done after the data have been collected at the telescopes, or it can be done by adding called a "delay line" (a little extra path length) to the path that the light travels to each telescope. In this case, the light travels the same distance to each telescope, the wavefronts all arrive at the same time at the telescopes and they can be added together to make one image.

Traditionally interferometry has been used in radio astronomy. You can read about the interferometers that are part of the National Radio Astronomy Observatory at (this includes the VLA and the VLBA). With new technological advances (in computer hardware and in things like the CCD chips used in observations), optical interferometers are starting to come on line. One working optical interferometer is the Navy Prototype Optical Interferometer. To give you an idea of its resolution, its entire field of view is the size of one Hubble Space Telescope pixel. You can look at the NPOI home page at

A historical note: Interferometers were first used by Michaelson, who won the Nobel prize in 1907 for his work using an optical interferometer to measure very accurately the speed of light. The next use of interferometers was not until the 1970s, when the VLA came on line. You may wonder why there was such a long time lag between the two events. The answer is that Michaelson used his eye as a detector to see the interference, (the work that won him the Nobel prize used a VERY bright star!) and it was not until very recently that the technology has advanced to the point where suitable electronic detectors and the computers necessary to crunch the large amounts of data that an interferometer generates (remember the signals from all of the telescopes have to be stored and then combined with a computer, they cannot just be captured on photographic film) have been available.

If you are interested in some MORE information on optical interferometers, there is a pretty good (and fairly accessible) article in Physics Today (a magazine that may only be available in University libraries) by J. T. Armstrong. It is Vol. 48. page 42 (1995).

Hope this helps!

J. Allie Cliffe and Arsen R. Hajian
for Ask an Astrophysicist

Question ID: 971117j

The Question

All I wanted to ask you is that if we put a thermometer in Space with no other light or heat source around and absolutely no background radiation there, what would it read? Would the temperature be really cold or what?

The Answer

Yes, it would be really cold. Temperature measures the energy per "degree of freedom" (i.e. way something can move) of whatever molecules happen to be around. So, it it becomes so cold that the molecules stop all together, then this is the "absolute zero" temperature. On the Celsius Temperature Scale (i.e. water freezes at 0, and boils at 100) this takes place at -273 degrees C.

We usually use the kelvin temperature scale, where Zero Kelvin is this "absolute zero" temperature -- or -273 degrees C. Water freezes at +273 Kelvin and water boils at +373 Kelvin.

If we put a thermometer in darkest space, with absolutely nothing around, it would first have to cool off. This might take a very very long time. Once it cooled off, it would read 2.7 Kelvin. This is because of the "3 degree microwave background radiation." No matter where you go, you cannot escape it -- it is always there.

Jonathan Keohane
for Ask an Astrophysicist

Question ID: 980301b

The Question

I have searched everywhere, and cannot find the measurement of distance in space. What is it? Light years, kilometers, what?

The Answer

The common distance measures we use depend on what we are measuring.

1. For distances within our solar system, or other solar systems, the common unit is the 'Astronomical Unit' (A.U.)

1 A.U. = the average distance between the Earth and the Sun

2. For most everything else, stars, galaxies etc..., the distance unit is the parsec (pc). This is a convenient unit when measuring distances to stars by triangulation (what astronomers call parallax).

1 pc = 3.26 light years = about the distance to the nearest star

1 pc = 60 x 60 x 180/pi A.U. = 206265 A.U. --- by definition.

for distances within our galaxy or other galaxies it is kiloparsecs (kpc):

1 kpc = 1,000 pc

for distances between galaxies, and cosmology it is Megaparsecs (Mpc):

1 Mpc = 1,000,000 pc

3. The exception to these is when one is studying smaller object, such as a star or a planet. Then we might use kilometers. For dust grains, we might use microns (1/1,000,000 of a meter).

4. It is also common to compare objects. For example, if one is studying a star one might say "its radius is 5 solar radii", meaning it is 5 times the size of our sun. Similarly with galaxies, is it bigger or smaller than the Milky Way.

So, all in all, we use many units is astronomy. But, all in all, the parsec is the most common.

Note, astronomers only use light-years when talking to the general public or teaching classes.

Thanks for the question.

Good luck,

Jonathan Keohane
for Ask an Astrophysicist

Question ID: 980226c

The Question

I am 8 years old. Could you explain what a light year is and give an example that I can understand or relate to.

The Answer

A light year is the distance that light travels in one year. The speed of light is 186,287.5 miles per second. You can find out the number of seconds in a year by multiplying the number of seconds in a minute (60) by the number of minutes in an hour (60). Then multiply that by the number of hours in a day (24), and multiply that by the number of days in a year (approximately 365.25).

 So we've got 60 x 60 = 3600 seconds in an hour
 3600 x 24 = 86400 seconds in a day
 86400 x 365.25 = 31,557,600 seconds in a year.

So a light year is about (I've rounded off a bit) 5,878,786,100,000 miles. That's almost 6 trillion miles. The distance from the earth to the Sun is 93 million miles. The distance to the nearest star is 4.3 light years, and the distance to the Andromeda galaxy is 2 million light years.

The universe is a big place!

Jim Lochner
for Ask an Astrophysicist

Question ID: 980211a

The Question

I am a student and very interested in astronomy. I would like to ask about the different ways in astronomy that we can measure the distances? I know that we can measure some distances by using the parallax method. Are there other ways in astronomy which we can determine the distance? And what are they?

The Answer

You have asked a very good question, since finding distances to far away objects is a very important, and often very difficult, part of astronomy. There are only a few stars whose distances can be measured using parallax. Other methods used to find distances to objects too far away to use a direct method use some estimation of the real brightness of the object. If you know how bright something is and compare to how bright it appears to be you can get a distance (since objects further away are fainter in a calculatable way).

Within our solar system, the most accurate way is to bounce radar off the nearby planets. In this way we can get very precise distances for the Moon, Venus, and Mars. Also, since we know the relative distances of the planets from each other, we can find the absolute distances of the ones further out by determining the absolute distances of the nearest planets.

Progressing to larger distances, for stars that are too far away to use the parallax method, it is common to use pulsating stars called Cepheid variables. The good thing about these stars is that they have a well-defined relation between their intrinsic brightness and the period with which they vary. (Bright ones go through one cycle of period light variations slower than do faint ones.) This relation has been well calibrated for nearby Cepheids, therefore if we observe Cepheids in distant galaxies we can get their distances by timing their periods. You may have read in the newspapers a few years back about how Wendy Freedman at Mount Wilson used Hubble telescope observations of Cepheids in the Virgo cluster of galaxies to revise the extra-galactic distance scale.

In really far away galaxies where you cannot even distinguish individual stars, one has to rely on more indirect methods. For instance, based on the shape and color of a given galaxy, you can guess-timate its intrinsic brightness, and then from its observed brightness get the distance.

There is more discussion of distance estimates on our web site. This is our local search engine, within which you can type something like: "distance determination".

John Cannizzo
J. Allie Hajian
("Ask an Astrophysicist")

Question ID: 981102a

The Question

How do celestial bodies (planets, stars, etc.) begin to rotate? Does it happen when matter begins to form into a body?

The Answer

Most of the rotation of astronomical objects is left over from their formation process. If you are familiar with the conservation of angular momentum you will know that if a slowly rotating object contracts its rotation rate will increase, and this is probably the origin of the rotation of most solar system objects; the cloud of gas and dust which formed the solar system was rotating slowly before it contracted.

Rotational motion is described by the Euler equation, which can be written dL/dt=N, where L is the angular momentum, N is the torque, and d/dt is the time derivative (the instantaneous rate of change). For an object like an asteroid the torque is zero (since there are no external forces acting on it), so the angular momentum is constant. This sounds simple, but the problem lies in the meaning of the angular momentum. For a round object like a ball or a planet the angular momentum is given by the product of the rotation rate or angular velocity (in RPMs or other time units) and the moment of inertia. You should be able to find a definition for the moment of inertia in most introductory Physics textbooks.

I hope this helps,

Tim Kallman
for Ask an Astrophysicist

Question ID: 980721a

Related Areas of Physics

The Question

Do the laws of motion apply in space. Why is it that when in a space shuttle with no gravity, the people in the shuttle move with the shuttle?

The Answer

Yes, the laws of motion do apply in space. When the shuttle is launched it is maneuvered into a low Earth orbit-it is orbiting once around the Earth about every 90 minutes. It is following Newton's laws in that orbit around the Earth, just as all other Earth satellites, including the Moon, do. The period of the orbit squared is proportional to it's orbital semi-major axis (it's distance from the center of mass of the system) cubed. Geosynchronous (one orbit a day) satellites are therefore in a higher Earth orbit, while the Moon is still further from Earth. The shuttle and the astronauts are still gravitationally bound to the Earth, or they would not orbit. So it is not correct to think of the shuttle with "no gravity". The shuttle, the astronauts, and any equipment they have out are all orbiting the Earth in the same orbit, so the astronauts do not feel gravitationally bound to the shuttle, though they are still gravitationally bound to the Earth. This is why they float freely within the shuttle, and why they move with the shuttle.

Padi Boyd,
for the "Ask an Astrophysicist Team"

Question ID: 970419

The Question


  1. When a star (or light source) is moving away from you, does it emit blue or red light; and what about when it is moving towards you?
  2. With respect to the autumn and spring solstices: Do we get more "red" light in spring or autumn, more blue light .... etc?

We're just having a discussion at home and would like to clarify a point.

The Answer

  1. The fact that a star, or any light source, is moving toward or away from you does not affect what it emits - it affects what you (as the observer) perceive it emitting.

    The apparent shift of light toward the red when the emission source is moving away from us, or toward the blue when the emitter is moving toward us, is called the Doppler shift. Let us take a minute to try to explain what this is.

    Light consists of fluctuations, or waves, of the electromagnetic field. The wavelength (or distance from one wave crest to the next wave crest) of light is extremely small -- for visible light it ranges from four to seven ten millionths of a meter. The different wavelengths of light are what the human eye sees as different colors. The longest wavelengths appear in the red end of the spectrum and the shortest appear in the blue end. Now imagine a source of light at a constant distance from us, emitting waves of light at a constant wavelength. Obviously, the wavelength of the waves we receive will be the same constant wavelength at which they are emitted by the source. Suppose now that the source starts moving directly toward us. When the source emits the next wave crest, it will be nearer to us, so the distance we will see between the two wave crests arriving will appear to be smaller than when the star was stationary. This means that the wavelength of the waves we receive will be shorter (or shifted toward the blue end of the spectrum) than when the source was not moving. Similarly, if the source is moving away from us, the wavelength of the waves will appear slightly longer, or shifted toward the red end of the spectrum. The relationship between wavelength and speed is called the Doppler effect. We experience it every day -- like the engine sounds of a car approaching us having a higher pitch...and a lower pitch when the car moving away from us. Light and sound are both waves, so they both exhibit the Doppler effect.

    The Doppler effect is named after the Austrian scientist Christian Johann Doppler. He first predicted it must occur in a scientific paper he wrote in 1842.

  2. The summer solstice occurs about June 22 and the winter solstice around December 22. These 2 events are just special points in the orbit of the Earth around the Sun. On the summer solstice, the Sun shines most directly upon the northern hemisphere. On the winter solstice, the Sun's rays shine most directly on the southern hemisphere. This happens, of course, because the Earth is tilted 23.5 degrees on its rotational axis. So the plane in which the Earth revolves around the Sun is not coincident with the plane of the equator. The result is that the northern hemisphere is tilted toward the Sun in in June and away from the Sun in December.

    Given the orbit that the Earth takes around the Sun, we are moving slightly away from the Sun starting in January and we start moving slightly toward the Sun in July. However, given the tiny velocity with which these movements take place (relative to the speed of light), any Doppler shift this induces is imperceptible.

Question ID: 961029a

The Question

I have been searching all over the place to find something about gravity wells of planets, stars and moons. I am in Grade 12 and would like some information accordingly.

The Answer

I assume that by "gravity wells" you are talking about the "gravitational potential" well associated with any object that possesses mass.

First, I need to make sure you understand what a "well" is in the world of physics. In the world of physics, a "potential" can be thought of as an energy field. There are 2 basic types of potentials, "barriers" and "wells". They get their names from what they look like when you plot them.

Now, applying this "barrier" and "well" idea to gravity, the shape is a well (assuming a constant gravitational field). So we talk about "gravitational potential wells". The equation in such circumstances is that the gravitational potential energy is equal to the one object's mass times the acceleration due to gravity of the 2nd object times the distance they are apart.

In the case of two stars attracting each other or a particle being attracted by a neutron star, the gravitational field is not constant. It falls off as the square of the distance between the 2 objects. So the resulting equations get more complicated. Nevertheless, gravity is a wonderful thing...any two objects which possess mass will always exert a gravitational attraction on one another and this "potential" source of energy can be converted into other kinds of energy...such as heat, light, and sound. Try this: hold a book a couple of feet from the floor. Let go of it. Congratulations, you have just converted gravitational potential energy which the book possessed by being in the gravitational field of the Earth into sound!

Conversion of gravitational potential energy into other forms of energy has some practical applications. When nasa launches an interplanetary satellite, it often takes advantage of a "sling-shot" effect to increase the satellite's speed. This is called a gravitational assist. In fact, the satellite galileo actually came back to the Earth twice for gravitational assists on its way to Jupiter. You can see a diagram of this at

Hope this helps.

Andy Ptak and Laura Whitlock
for Ask an Astrophysicist

Question ID: 970326b

The Question

What is plasma (the fourth state of matter) used for? Where can it be found on Earth? What would the texture be?

The Answer

Plasmas are common and have many uses. The flame of a fire is a plasma, and you know what the uses of fire are, and you know what its 'texture' is like. The mass in the Sun and most other stars is in the form of plasma. There are instruments, called inductively-coupled plasma spectrometers, which inject a sample into a plasma and can determine the sample's chemical composition at the level of parts per billion.

Andy Ptak
or the Ask an Astrophysicist

Question ID: 971007b

The Question

I recently learned that the universe is governed by four forces. I know that one of them is gravity. Could you please tell me the other three if you know what they are.

The Answer

You are right-- there are four known forces:

1. Gravity - This force acts between all mass in the universe and it has infinite range.

2. Electromagnetic - This acts between electrically charged particles. Electricity, magnetism, and light are all produced by this force and it also has infinite range.

3. The Strong Force - This force binds neutrons and protons together in the cores of atoms and is a short range force.

4. Weak Force - This causes Beta decay (the conversion of a neutron to a proton, an electron and an antineutrino) and various particles (the "strange" ones) are formed by strong interactions but decay via weak interactions (that's what's strange about "strangeness"). Like the strong force, the weak force is also short range.

The weak and electromagnetic interactions have been unified under electroweak theory (Glashow, Weinberg, and Salaam were awarded the Nobel Prize for this in 1979). Grand unification theories attempt to treat both strong and electroweak interactions under the same mathematical structure; attempts to include gravitation in this picture have not yet been successful.

Hope this helps,

Jeff Silvis and Mark Kowitt
For Ask an Astrophysicist

Question ID: 980127c

The Question

I was wondering, how many elements are known to exist to the present date? (on the periodic table)

The Answer

The current total is now around 112. The ones up through atomic number 92 (uranium) are naturally occurring, whereas the "transuranic" elements are synthesized in experiments wherein heavy nuclei are made to interact with each other.

There are a couple of informative articles in the magazine Scientific American which describe this work, which was carried out in the early days primarily in Berkeley, but now is done mainly in Darmstadt, Germany. Here are the references:

-The Synthetic Elements, Seaborg & Bloom, April 1969
-Creating Superheavy Elements, Armbruster & Muenzenberg, May 1989

A website which contains the current periodic table of the elements is

By clicking on the symbol for each element, you can learn more about that particular element.

J.K. Cannizzo
for Ask an Astrophysicist

Question ID: 980326a

The Question

Can light only be seen when it is reflected off particles? Is space dark because it is a vacuum and there are no particles for a light wave to reflect off of? How is it possible for light to travel through a vacuum? I am also puzzled whether light is emitted by photons, waves, or both?

The Answer

What an interesting set of questions. I'm going to explain light in terms of the visible spectrum and our eyes (our sensors of light), but it holds true for the entire electro-magnetic (em) spectrum.

Your eye has specialized cells (rods and cones) that detect the intensity (brightness) and color of visible light photons. When one of these photons enters your eye, these cells convert its energy into a nerve signal that registers in your brain.

So to see an object it must either:

1) Emit photons towards your eye;
(the Sun, a candle flame, a light bulb, a TV).

2) Deflect photons towards your eye;
(the Moon, a dog, a plant, a telephone).

As to the reason space is dark, you're right! It's because there is a vacuum in space, and no particles to reflect the Sun's light from space and into our eyes.

In respect to how light travels in a vacuum, I recommend that you check out Imagine the Universe! at:,

which explains how light moves, and has a link to a definition of the particle/wave duality of light:

Speaking of which, light is neither a wave nor a particle, but has aspects of both. It can be considered to consist of particle-like packets of wave-energy called photons. The particle and wave interpretations are not in conflict. Rather they are useful when considering different properties of light. For instance, the scattering of X-rays as they pass through a metal foil is easy to understand using a particle model, while the diffraction patterns produced when light is passed through narrow slits are easier to understand in terms of overlapping waves. Wave-particle duality is an outcome of the quantum mechanical nature of matter, under which the universe on the sub- atomic scale is not made up of hard objects with precise positions, but rather of entities with some spread of possible locations.

For more details, you might want to look for an elementary textbook on quantum mechanics.

Keep wondering about things,

Michael Arida and Paul Butterworth
for Ask an Astrophysicist

Question ID: 970529c

The Question

Which method is used for actually detecting neutrinos?

The Answer

Most neutrino detectors use the light produced when a neutrino interacts with an electron or nucleus. Look at the home pages listed on

David Palmer and Samar Safi-Harb
for Ask an Astrophysicist

Question ID: 990209a

Astronomy Resources

The Question

I would like to know how I can receive email about the latest events and news notes. I have a general interest in astronomy. Where do you buy astronomy calendars and pictures?

The Answer

There are many ways to receive email about the latest events and news in astronomy.

You can get the latest news and information about nasa by subscribing to NASA News Briefs. You can request to be put on the mailing list by sending email to

In addition, there are a few other astronomy related organizations that advertise free email subscriptions. (We grabbed this information off the Usenet group


The September issue of SpaceViews, an online publication of space science, technology, and policy, is now available. This month's issue includes:

  • Reports on Planetfest '97 and the AAS Division for Planetary Sciences Conference;
  • MoonLink: bringing space missions to schools;
  • The latest on Mir, Mars Pathfinder, and other space news;
  • Book reviews;
  • And more news, articles, and special features!

The September issue is available on the Web at

Visit the SpaceViews Web site at:

for more information on the publication, including how to get a free e-mail subscription. Any questions or comments about SpaceViews can be directed to the editor, Jeff Foust, at

S&T Weekly News Bulletin and Sky at a Glance

In response to numerous requests, and in cooperation with the Astronomical League and the American Association of Amateur Astronomers, S&T's Weekly News Bulletin and Sky at a Glance are available via electronic mailing list too. For a free subscription, send e-mail to and put the word "join" on the first line of the body of the message. To unsubscribe, send e-mail to and put the word "unjoin" on the first line of the body of the message.

(S&T stands for Sky and Telescope)

Further, one of our web sites that is updated daily and which you may find interesting is the Astronomy Picture of the Day:

For getting astronomy merchandise, check your nearest planetarium or science center. A place we particularly like in Ontario is the Ontario Science Center in Toronto. (Yes, some of us have visited it.) It's one of the coolest hands-on science centers around and it has calendars and pictures for sale.

You might also check out some of the online astronomy catalogs available, e.g. from Sky Publishing (the folks who publish Sky and Telescope), the Astronomical Society of the Pacific, and Astronomy Magazine:


Astronomical Society of the Pacific:

Finally, the University of York and the University of Toronto,

both have good programs in astronomy, and resources that you may be interested in.

So there's lots of information and resources out there !

Best Wishes,

Jim Lochner, Mike Arida, Maggie Masetti, and Leonard Garcia
for Ask an Astrophysicist

Question ID: 970910c

The Question

My son is 9 years old and very much interested in stars, planets etc. I have visited your excellent site made for children. Could you please advise me on where I can get some videos about learning Astronomy for young children?

The Answer

In response to your question, I asked a number of colleagues for their video suggestions, talked to staff at the Public Affairs Office and the Teacher Resource Center at nasa Goddard Space Flight Center, and searched the Internet. The results have been quite disappointing. There seem to be few good widely-available astronomy videos for children.

Sky Publishing (the publishers of the excellent popular magazine "Sky and Telescope") have a section of their on-line catalog devoted to "Start Right in Astronomy". I think it says a lot that the only video in the section is Carl Sagan's "Cosmos" series - which is still very good, but was made many years ago. Go to:

The Astronomical Society of the Pacific also has a catalog you might want to look at. (A colleague recommends against their video 'Astronomy 101' however, because it contains some basic errors). Their catalog is available via their website:

Another colleague recommends the Bell Science Series, available from:

Rhino Home Video
2225 Colorado Avenue
Santa Monica, CA 90404

Most videos produced by the BBC or derived from the Nova TV series (often the same thing) are of very high quality and might be interesting to many older children.

The situation for videos produced by NASA seems particularly embarrassing. Many excellent videos have been made, but I've just made a number of calls that suggest that record-keeping has been poor and nobody feels that it is their responsibility to make them readily accessible. If you are near a NASA Center, you might want to visit the Teacher Resource Center located there. TRC staff are often happy to help interested parents. If not, try calling the Public Affairs Office at the nearest Center.

My guess is that the reason there isn't a lot more good material more widely available is that two groups necessary to make high quality science videos - filmmakers and scientists - each thinks the other would be too much trouble to work with!

The situation for non-video introductory material is much better. If you go to the Learning Center site associated with Starchild, you will find many links to other sites on the Internet which may be helpful. (Be aware however that because it's so easy to build a web site a lot of terrible material exists elsewhere). The Learning Center URL is:

By far the best resources however are your local bookstore and library. The average quality of introductory material in print is high and if you look around enough you are sure to find many good books.

I hope this response has been of some help.

Best wishes,

Paul Butterworth
for the "Ask an Astrophysicist" team

Question ID: 970401a

The Question

I am a 6th grader and I know a lot of the constellations by name and enjoy looking at the stars and sky at night. This past weekend while camping our Girl Scout Troop went star gazing in an open field. It was really dark and really beautiful.

I really like to look at stuff on the Internet about the universe. I need some help in understanding how I might find specific information on the Sun, planets, or stars.

For example, I was wondering: "How hot is the sun?"; "What star might be hotter than the sun?"; and "What star might be cooler than the sun?". Is there someplace that I can find just facts such as temperature, size, distance from earth of stars?

The Answer

You are asking how to find answers to questions on astronomy, using the Internet. I will tell you how I would find out these answers.

First of all, the Internet is not always the best place to learn things. If you can go to a library and look through their books on the Sun and stars, you will probably find a book which explains most of what you want to know. If I wanted to answer the questions you asked, the easiest thing for me to do would be to grab some books from my shelf and say: "The temperature of the surface of the Sun is 5770 kelvin which is 5,500 Celsius (or 10,000 F). Stars which appear bluish are hotter than the Sun, stars which appear reddish are cooler than the Sun. For example: Rigel, Vega and Sirius are blue (hot), Arcturus, Aldebaran, and Betelgeuse are red (cool)." These numbers and examples come out of an astronomy textbook.

But sometimes you can't get to a good library, sometimes the Internet is more practical or convenient. If I were looking for information on starfish instead of stars, all the astronomy books in my office wouldn't be much help. There is an encyclopedia in an office two floors down, and that might be the fastest way to learn things. But if I wanted to know a specific obscure question, such as 'are there were any poisonous starfish?', the encyclopedia probably wouldn't help me. I'd either go to the library, ask a biologist, or go to the net where, with enough digging, I would find that the 'Crown of Thorns' is the only known venomous starfish.

It is easiest to find things with search engines if you know the exact words or phrases commonly used. Searching for "list of stars" will be less useful than searching for "star catalog", and searching for "star colors" is less useful than "spectral classification". And you will not try to find "Hertzsprung-Russell diagram" unless you already know what it is. If you do not know the exact words that people use, you can consult a glossary such as

There are many places to learn things on the Internet, if you can find them. You found one: 'Imagine the Universe'. We also have a site called Starchild (

The Astrophysics Data System is a good place for astronomy: It has star catalogs and other information. It also has a copy of the 'Handbook of space Astronomy & Astrophysics' by Martin V. Zombeck This is one of the (paper) books on my shelf that I often refer to. Most of the pages in it are very hard for anybody but an astronomer to understand. The book includes a list (a 'star catalog') of the brightest stars in the sky at and the following pages. The catalog includes distances (labeled 'd' and given in units called 'parsecs', each of which is 3.26 light years) and a quantity called 'B-V', which tells how blue a star is: numbers below 0.65 are bluer and hotter than the Sun, numbers above 0.65 are redder and cooler.

If you look at a Hertzsprung-Russell diagram you will often find them marked with both the spectral classification and the temperature. If you look in the 'spec' column of the table, you will see entries like 'A0 p'--the first letter means that the spectral class is 'A', and so the temperature is about 10,000 Kelvin.

As you can see, it is easiest to use the web to find out about astronomy if you are already an astronomer. But I hope my reply has been helpful.

David Palmer
for Ask an Astrophysicist

Question ID: 970930b

School Projects

The Question

Do you have any neat astronomy science fair project ideas? Do you recommend any books to find some?

The Answer

There are so many science fair ideas that it is impossible to provide a list in a reasonable amount of space. Besides, the best kind of science fair ideas (or any other ideas for that matter) are the ones you come up with yourself. If you are having trouble, there are several books on the subject, and there are quite a few web sites which are relevant, for example:

However, I would encourage you not to simply copy from these sources, but to think about interesting variations on them.

I hope this helps!

Tim Kallman for the Ask an Astrophysicist team

Question ID: 970509e

The Question

I am in 8th grade. I have to do a science project and I am really looking forward doing it. I really want to do something that has to do with physics especially gravitation, inertia, or relativity but I must be able to test it. Can you please help me get a topic that might take my science project all the way to state science fair?

The Answer

I don't know whether this will get you to the state Science Fair, but you could do something based on this common lecture demonstration.


  • 2 lengths of sewing thread
  • 500g (1 lb) weight
  • a tall overhang to tie the string from

Tie one string from the overhang (like a hangman's gallows) onto the weight.

tie second string from weight toward floor.

            |    |
   string 1 |    |
            |    |
   weight  {-}   |
            |    |
   string 2 |    |
            |    |
            |    |

Now if you pull the bottom string slowly the top string will break (your force + the force of gravity on the weight). If you jerk the bottom string down quickly it will break (the inertia of the weight will not give the impulse force enough time to put tension on the top string)

David Palmer and Michael Arida
for Ask an Astrophysicist

Question ID: 971117d

The Question

My son, as a requirement for his 7th grade science class, is to have a working model science fair project. While going through some magazines he came upon the idea to build a "Trash Bag Hot-Air Balloon", which would show and demonstrate one type of lighter than air vehicle.

While this seemed to be a great idea for his project, his teacher is not too enthused as "everyone knows how they work." As a result of his comments we have been trying to find the explanation of the scientific principles that are being demonstrated in this project and, hopefully, prove his project more "science project" worthy to his teacher.

Is there any help, information, or advice you can possibly offer that will help him with this project?

Thank You for your time.

The Answer

Our expertise is in designing and building detectors to collect X-rays and gamma-rays from astrophysical objects, and then to interpret the data. It might seem like hot air balloons would be a little outside of our area of interest, but actually, since the radiation we are most interested in observing is absorbed by the Earth's atmosphere, many of our high energy astrophysics experiments are flown on balloons. This way they can get above a substantial fraction of the absorbing atmosphere. The balloons used for scientific payloads are helium filled, but the principle of employing a balloon filled with a lighter gas to get and remain airborne is basically the same.

The basic physics behind hot air balloon travel is the effect of increased temperature on the motions of molecules of a gas, and thereby on the density of the gas. In order to understand this, you'll need a little algebra, and one of the basic ideas of thermodynamics, called the Ideal Gas Law.

A hot air balloon stays afloat in the cooler air surrounding it due to the buoyant force on it. This is the same force that acts on you when you are in a pool of water. You have probably noticed it is much easier to lift someone if you are both in a pool of water, and this is due to the partial support the water is offering: the buoyant force. This force was studied by the Greeks before 200 B.C. and can be understood by Archimedes' principle: any body completely or partially submerged in a fluid is buoyed up by a force equal to the weight of the fluid displaced by the body. If B represents the buoyant force and W the weight of the displaced fluid, then b=W.

We need to consider the sum of the forces acting on the balloon, which is totally submerged in the air around it. The buoyant force B acts upward on the balloon, and gravity acts downward. The weight of the balloon, w, is the same as the gravitational force downward. Since these two forces act in opposite directions (buoyant force up, gravity down), the total force on the balloon is F(total)=B-w.

Now we need to represent each force in terms of things we can measure. These are: the density of the fluid and the volume of the balloon. Remember that density is defined as the mass of an object divided by its volume. The weight of the displaced air equals the buoyant force, and weight is always equal to the mass times the acceleration due to gravity (w=Mg). The mass of the displaced air, M, is just the density of the displaced air multiplied by the volume of the balloon (that's the volume being displaced). So, if d=density of the cool, surrounding air, then w=D*V*g, where V is the volume of the balloon. And since b=W, b=D*V*g. For the weight of the balloon, w=mg. Assuming the basket attached to the balloon can be ignored for now, we need only get the mass of the air inside the balloon. That will be equal to the density of the air inside, d, times the volume. w=d*V*g. Then F(total)=B-w=D*V*g-d*V*g=(D-d)*V*g. When this is a positive number, the force is in the upward direction. That occurs when the density of the air inside the balloon, d is less than the density of the surrounding, cooler air, D.

How can the density of the air inside become less that the air outside? Since the gas inside is the same as outside the balloon, we can use the Ideal Gas Law to study what happens to the density as the temperature is increased. One way of stating this law is: for a gas with a constant molecular weight, the pressure is proportional to product of the density and the temperature (p=K*D*T). Here, K is just a constant, and T is the temperature. That means for a gas at constant temperature, the density is given by: d=P/(K*T). As the temperature increases, the density decreases. At this point the story is almost complete. If you and your son are still interested in following this line for a science project, then I will leave it to you to discover what is happening to the molecules of gas inside the balloon that causes the density to drop. You can find a discussion of the ideas of buoyancy, and how gases are affected by temperature, in any introductory high school or college physics book. The books will have lots of pictures and worked-out examples, and will give you a more detailed description than the one here.

I spoke with a high school physics teacher this weekend about your question, and she told me that there is a kit available to students for making a hot air balloon. Perhaps your son's teacher was concerned that if he used a kit to build the balloon he might not learn as much as if he started a project from scratch. I would encourage you to talk to the teacher some more, and find out what they want the kids to get out of their project. If your son comes away from this project with a better understanding of the forces acting on a balloon, and on the effects of heat on a gas, then it sounds like a good learning experience. It the teacher is aiming for something different in the class, perhaps you and your son can build the balloon on your own in your spare time!

I hope this helps. Good luck with the balloon and the science fair.

Padi Boyd
for Imagine the Universe!

Question ID: 970106a

History of Astronomy & Astrophysics

The Question

Why did ancient civilizations, like Mayans and Greeks, have such advanced Astronomy? How did they build their observatories?

The Answer

Have you ever gone to a completely dark site, far, far away from any cities or even a small village, on a clear, dark night, and watched the stars? Many city dwellers today don't know how overwhelmingly beautiful the night sky can be, when there are no street lights around. Yet, there are many who enjoy stargazing as a hobby. The ancient peoples had a much better view of the night sky than we usually do, and they had little entertainment during the night (no MTV, no electric guitars, no Blockbusters, no Simpsons, no Friends, no NFL, no World Series), so they were well motivated to gaze at the sky.

Any civilizations with advanced enough astronomy gained practical advantages, too. A calendar, based on the regular patterns of the Sun, the Moon, and the stars, is essential if you want to be successful at agriculture in temperate climates. Astronomy gives you navigation, too -- not only at sea (the Polynesians being the most amazing example), but being able to tell which way is north at night may well be an advantage if you wanted to hunt or wage a battle at night.

It is hard to compare the Greeks and the Mayans because they lived several thousand years apart. The Ancient Greek society seemed to consider knowledge and learning to be an important part of their culture - thus studying astronomy was a natural extension of that.

I would guess that astronomy was an important part of the Mayan religion, which is why they studied it. You'll have to research that part yourself. There are a lot of good resources on the web...I'll help you out with the first few.

Here are a couple of links on Mayan Astronomy:

This one includes more general information about the Mayan culture.

I don't believe either culture used telescopes, but instead used their eyes and other aids (like pyramids in the case of the Mayans) to study the different positions of planets and the motions of the sky above them. You might want to check out some general history of astronomy sites to see when the telescope was invented, and what other cultures were doing (i.e. the Britons with Stonehenge...)

Maggie Masetti & Koji Mukai
for Ask an Astrophysicist

(Links updated in 2012 November)

Question ID: 980917b

The Question

What tools did the ancient astronomer use ? Who were some ancient astronomers ? What is the History of Astronomy?

The Answer

The history of astronomy is the study of humankind's early attempts to understand the skies. All people have looked up and wondered about the Sun, Moon, planets, stars, and their complex ballet of motion. Interpretations vary widely among cultures, but often the sky is considered as the abode of gods, where humans can never touch. The consideration of stars and planets as physical objects that obey knowable laws started in the Middle East (and somewhat in China) and has spread into cultures that are the intellectual heirs of the Greeks. A fairly modern view of the heavens only started in the early 1600's when galileo first turned the newly invented telescope to the heavens and saw worlds in their own right. With the newtonian revolution in physics, it was realized that stars were just Suns, and all obeyed the same Laws of Physics as hold here on the Earth. In the 1900's, the detailed study of everything up in the sky has become a major pursuit which is growing exponentially. The history of astronomy looks at all these perceptions and advances.

There are many ancient astronomers from many cultures all around the world, many of whom have their name lost over the ages. For example, we do not know who or when the planets were recognized as being different from stars. In some sense, most ancient people were 'astronomers' since all lived under non-light-polluted dark skies and everyone wonders what is up there. The names of the Egyptian, Mayan, and Chaldean astronomers are all lost, even if we know of some of their results. The best known astronomers are those associated with the development of the modern scientific results. For example, Hipparchus (Greek ~3 century BC) discovered the precession of the equinoxes, ptolemy (Greek in Alexandria ~100 AD) systematized the geocentric system of planets, copernicus (Polish, 1500s) proposed the heliocentric system, kepler (Czech?, ~1600) came up with detailed laws for planetary motion, Galileo (Italian early 1600s) made great discoveries with his telescope, Newton (English, late 1600s) discovered the basic laws of Physics that allow us to understand the cosmos, and Edwin hubble (American, died ~1940) who discovered that the Universe is expanding.

The history of astronomy is a very long one and astronomy has been pursued by all cultures, so there is a very wide range of tools. Before the discovery of the telescope, the only observing devices that people could use was the human eye, perhaps aided by any of a variety of sighting devices. Thus, the Chinese used armillary spheres, Tycho brahe (Danish late 1500's) used long sighting 'tubes', neolithic farmers made Stonehenge to point to midsummer sunrise, and Ptolemy noted planet positions with respect to stars. After the discovery of the telescope, there was a steady push to larger-and-larger telescopes. Starting around the 1800's, various instruments, like micrometers and spectrometers, were constructed to give very detailed measures of the light coming from stars. Starting around 1900, the photographic plate and then the CCD camera, have revolutionized astronomy due to their great sensitivity.

To answer your three questions in detail, it could take a year of study or more, depending on your desired depth of answer. We cannot provide you with a whole class in the history of astronomy. Fortunately, there are many resources that you can use. One of the best, is to go to your local library and check out books there. This is a time honored and effective means for learning much. On the web, here are some addresses that will allow you to branch out widely:

Maybe the best thing for you to do, is to every night go outside and to look up. The beauty of the sky is what makes it so fascinating.

Brad Schaefer
Yale University

Question ID: 980215e

The Question

I am interested in finding out who was the first to discover that the earth and the rest of the planets revolve around the sun.

The Answer

The Greek astronomer Aristarchus, who lived in the first half of the third century BC, is credited as being the earliest known person to suggest that the earth revolves around the sun. Aristotle, who lived in the 4th century BC, had considered such an idea. But he rejected that idea because he thought that the motion of the earth around the Sun would cause a regular shifting in the positions of the stars. This shifting is called parallax, and Aristotle didn't see this occur. However, Aristotle was unaware of the enormous distances to the stars, which make such motion unobservable without telescopes.

In the modern era, Nicholas copernicus is credited as setting the heliocentric model of the solar system on a firm footing. He wrote about this in 1543. In 1609 Johannes kepler used the very accurate observations of Mars made by Tycho brahe (in the 1590's) to demonstrate that the position of Mars could be accurately predicted using sun-centered solar system suggested by Copernicus.

Jim Lochner
for Ask an Astrophysicist

Question ID: 980212a

The Question

My question for you is this: Throughout the better part of the 20th century people have always believed that the earth remained stationary.

I would like to know who the first person was to prove that the earth does in fact spin around its axis, and by what means did he/she use to come to this conclusion. Was it a tangible experiment?

The Answer

Thank you for contacting our Ask an Astrophysicist service. It has been generally accepted that the earth is not stationary for hundreds of years.

I suppose the first tangible proof that the earth rotates was provided by Jean-Bernard-Leon Foucault in the nineteenth century. He found that if a long pendulum with a heavy bob was set swinging, the plane in which it was swinging would appear to rotate.

The plane of a Foucault pendulum only rotates with a period equal to the Earth's sidereal period (23hrs 56min) at the poles. At lower latitudes, the period is (23h 56m)/sin(latitude), and at the equator, the plane doesn't rotate at all.

There is no reason why the pendulum's plane should rotate. It just appears to rotate because the ground under it is rotating. If you do a web search on "Foucault pendulum" or "Foucault's Pendulum" you should find lots of links.

Variations in gravity due to centrifugal force have been measurable for some time. There is also the subtle Sagnac effect (now the principle of operation of optical gyroscopes) that causes a phase difference in light moving in opposite directions along the components of a path parallel to the direction of rotation. Specs for the Canterbury Ring Laser based on this principle are described in

Some commentators attempt to find some contradiction of Einstein's predictions here; but in fact it is a general relativistic effect due to the rotation frame of reference, which is precisely why it is evidence in situ that the Earth rotates.

We won't discuss what astronauts have seen, because that might be a matter of perspective... but the equatorial bulge of the Earth, and the measured Doppler shifts from opposite sides of spiral galaxies and various disks viewed edge on certainly provide strong evidence that other things in the universe rotate. We could in principle measure such a Doppler shift of the Earth from space, using lasers... at least from the Moon.

Damian Audley, Mark Kowitt, Eric Christian, John Cannizzo and Kevin Boyce
for Ask an Astrophysicist

Question ID: 980218d

The Question

I am studying to become an elementary teacher, and I am interested in how the distances of the planets and stars were discovered. I read the question asked about stars, but I am interested in how these methods were discovered. In other words, what is the history? It seems difficult for me to imagine how we can estimate these distances when they are so far away. It is especially interesting how scientists first judged these distances before we sent any space craft or satellites into space. Could you please tell me how I might begin to research this question?

The Answer

The people we call Ancient Greeks (some of whom lived in e.g., Egypt) did a lot of this stuff a few centuries B.C., to the limits of naked eye observation. Eratosthenes measured the size of Earth, to good accuracy. Hipparchus and Aristarchus measured the distances to the Moon and to the Sun respectively (the Sun distance was off by a factor of ~20).

The ancient Greeks knew about parallax, and from the fact that the stars didn't move over the course of the year, they determined that the Earth did not move--unless of course the stars were so ridiculously far away that the movement would not be seen (Aristotle, I believe).

Once people started flitting around the world in boats, navigation became important--which made it useful to determine the size of the Solar system to make calculations more accurate. Measuring the transit of a planet across the Sun's disk from multiple places on Earth was an accurate way of measuring the parallax distance. Some of Captain Cook's voyages were for this purpose.

A good college-level text on astronomy will give you a simplified history of the development of 'the cosmological distance ladder' which allows us to use a chain of different techniques, each calibrated against the previous one, to determine the distance to distant galaxies and quasars.

David Palmer
for Ask an Astrophysicist

Question ID: 980420a

Astronomy, Society & Religion

The Question

I am doing research into technology and humans as part of a course on the Art of Human Computer Interface Design. If you could find the time I would appreciate your response to the following question. Is Technology advancing at too rapidly to be safe and what can we do to control it?

The Answer

I can only give you my personal opinion on this matter --- what follows below does not necessarily reflect the official positions of NASA.

No, I don't think that technology is advancing too rapidly to be safe.

For one thing, there is the clear-cut benefit of technology, that it protects us against natural disasters such as storms, famines, and diseases.

For another, one benefit of science and technology is that we can see any dangers more clearly, and those dangers that we notice can be publicized far more widely than ever before. I'm no expert on the field of risk assessment, but all the articles I've seen (in newspapers, and "Scientific American" type magazines) agree that the public are often misinformed about the comparative risks of various natural and man-made disasters. This is at least partly a psychological effect --- an airplane crash that kills 150 people is more likely to leave an impression than the daily tolls of car accidents, even though, statistically speaking, driving is much more dangerous than flying.

However, it is true that technology makes us powerful, and that any misuse (intentional or by mistake) has the potential to harm us to an ever increasing degree. Nevertheless, I remain cautiously optimistic. The reason for this is that the vast majority of scientists and engineers want to make the world safer, and they are becoming more and more aware of potential pitfalls of a technologically advanced civilization. So I don't complain when people attempt to assess the dangers of technology --- the more people think about risks, the safer we are. In fact, I myself often have questions about the safety of specific applications of technology; overall, though, I consider the advances in technology to have been, and will likely be, very beneficial.

I would argue that a good index of the safety of the technological civilization is the life expectancy. Despite some real dangers from technology gone wrong (e.g., air pollution), the life expectancy in all first world countries has been rising steadily. As long as this trend continues, I would argue that the technology is advancing at the right rate.

Best Wishes,

Dr. Koji Mukai,
An astrophysicist and a contributing member of Imagine the Universe!

Question ID: 970323

The Question

What is the big bang theory? What do you believe?

The Answer

The big bang theory is the theory that the universe started from a single point, and has been expanding ever since.

This has been well-established by observations, such as the apparent movement of galaxies away from us, and the cosmic microwave background radiation believed to be the leftover light from the big bang.

The evidence for a big bang having taken place about 15 to 20 billion years ago is overwhelming, so I naturally believe that it is the case.

However, if your real question is "why did the big bang happen in the first place?" then that ceases to be an astronomical question, but a religious one.

Some astronomers, who are religious, argue that the big bang theory confirms the existence of God and the basic elements of the creation story as told in the Bible. First came light, then the heavens, then the Earth ...

However, many other scientists do not. Scientists, like people in most any profession, have a vast diversity of religious beliefs. Some of us attend houses of worship, others do not. Some of us consider ourselves very religious, others consider ourselves staunch atheists. Just because we study astronomy does not mean we have any more agreement as to the ``why'' questions than anyone else.

On the other hand, it is safe to say that as scientists we can agree on an approach to learning about nature and the universe. This approach is one of using observations to test theories. And when a theory has been tested as much as the big bang theory, with each test reconfirming its validity, then we believe that it likely true -- at least more true than those theories which have failed the observational tests.

Good luck on your quest for the truth.

Jonathan Keohane
for Ask an Astrophysicist

Question ID: 971108a

The Question

I am puzzled between my beliefs and religion. I do not know what to tell my child about the creation of the universe. She seems really interested in knowing how all that we know exists.

I personally believe that no one knows for sure how the Universe was created or how we were created. Why are we here, a place in the Universe, this infinite Universe. Where did we come from?

The Answer

This is a pretty big question! I admire both of you for struggling with it.

Our work -- like those of scientists everywhere -- is concerned with the 'whats' and 'hows' of the Universe, rather than whether or not there is a 'why'. While it is not our role to discuss beliefs or religion, we can help you by telling you what astronomers have learned about the creation of the Universe. The scientific method (based on testing and modifying explanations until they agree with observations - and then making more observations to be explained!) has been astoundingly effective in investigating the history of the Universe and showing how one event followed another in a way understandable and predictable from a small number of physical principles (such as Newton's Laws of Motion and Einstein's Theory of Relativity).

We now have a very good picture of how the Universe has evolved since the so-called Big bang (some 15 billion years or so ago) to the present. Even twenty years ago, Nobel Prize winner Steven Weinberg was able to write a popular book "The First Three Minutes" which describes in some detail the particle interactions likely to have occurred during the first 180 seconds of the Universe!

Many non-scientific groups in human history have also thought they had a good picture of the Universe, but the crucial difference was that their explanations were either not tested or not testable. The scientific view is tested by many thousands of scientists every day and wrong ideas cannot survive very long. Of course there are still many mysteries remaining, but most of them concern quite fine details of galactic and stellar evolution.

Even the origin and evolution of life are much better understood than is usually realized. The mechanisms causing the simple life forms present on the earth more than 3 billion years ago to diversify into the tremendous biological variety we see today are well known. Many of the chemical steps required to produce the first life forms from simple and abundant molecules have been reproduced in the laboratory. Others have not, but the evidence for life in the oldest rocks we have examined (and the recently discovered similar evidence for ancient life on Mars) suggests that life may begin readily when the right materials and conditions are together for enough time.

To explore the work of our laboratory go to:

Note that the StarChild part of our site has been written for children between the ages of 4 and 14.

For other discussions concerning the origin and evolution of the Universe, books by Hawking ("A Brief History of Time" and others), Gribbin ("In the Beginning"), and Abrams ("The Birth of the Universe: The Big Bang and After") are worth a look.

For discussions of the origin and evolution of life, books by Steven Jay Gould might give you a place to start.

For more on the scientific method, Bronowski's "The Ascent of Man", Morrison's "Nothing is too Wonderful to be True" and Sagan's "Cosmos" and "The Demon Haunted World" contain interesting discussions of how science works. You might also want to check out the bi-monthly magazine "Skeptical Inquirer".

I had hoped to be able to recommend a much longer list of sites for you to visit on the World Wide Web, but I was very disappointed when I explored what is currently available. The average quality of information for the areas you are interested in is extremely low -- because anyone can make material available on the Web, so the few good sites are lost in the noise. I suggest that you will make much more progress by visiting good libraries and bookstores and reading widely.

I hope that both you and your daughter will continue to enjoy reading about, thinking about, and discussing these important questions.

Paul Butterworth
for Imagine the Universe!

Question ID: 970217e

The Question

A prophetic astronomical picture has recently intrigued me. Revelation 12:1,2,3 says a great sign will appear in the heavens and then projects the following alignment of the stars, Sun and moon in the heavens followed by what sounds like a massive comet. Is there any way of determining what the possible dates associated with the following description might occur? It could have taken place around the time of the birth of Christ or could be in the near future (next several decades).

A woman (aren't there several star configurations or galaxies which would fit this description?) clothed with the Sun (I assume that means that the Sun will be in the midst of this "woman" star configuration) and the moon under her feet (I assume the moon will be at the bottom of the star configuration and on her head a crown of 12 stars (Are there any 12 star groupings?).

Is there anyone in your organization who might know about this or be willing to research it? Any assistance which you can give would be greatly appreciated.

Thank you

The Answer

No, this is not an area of expertise or professional interest of people working in our lab. Rather, the heasarc is an organization within nasa of professional high-energy astronomers who utilize satellite data to examine a wide range of phenomena exhibited by objects in the Universe such as pulsars, supernovae, and active galaxies. While it is not NASA's role to help interpret Biblical passages, we can help you with the astronomy and a historical perspective. For assistance, we asked Dr. Brad Schaefer who used to work in our lab (but is now at Yale), and has done work in historical astronomy. He contributed what follows.

The primary trouble with trying to identify any particular event (past, present, or future) described in a text is to understand what the text is describing. This is the primary problem with scholarly research into medieval and ancient annals in search of old astronomical records of utility for modern science. Historically, this has also proven the biggest uncertainty for identifications of the Star of Bethlehem. Modern scholars have tried to identify it as a triple conjunction of Jupiter and Saturn, a nova, a comet, a massing of three planets in Pisces, an occultation of Jupiter by Venus, a supernova, and the stationary point of Jupiter. With such a wide array of attested astronomical phenomena from the time 10BC to 1AD, we see that the texts cannot be of reasonable utility to uniquely identify the event. Many events written about by ancient and medieval chroniclers cannot now be identified for the same reason. The trouble is that many events can be fitted into the sketchy information provided, so that no certainty or even likelihood is possible.

With regard to great sign in the heavens you referred to, here the problem is that the sky is always showing some magnificent and beautiful display. Statistical studies show that the skies give a spectacular spectacle of order once a year. In recent years, we have had beautiful solar eclipse across America, Comet Hyakutake's awesome tail, a fantastically colored lunar eclipse, and some brilliant sky-filling aurorae. In the next few years, astronomers have predicted (for decades in most cases) a Christmas Day solar eclipse across North America (2000), fantastic Leonid meteor storms in 1998 and 1999, a rare transit of Venus across the face of our Sun, a bright comet this year, and a massing of five planets in early 2000. But these series of spectacles is the ordinary condition of the sky. So with a description only promising a great sign in the sky, there is no way to identify any event.

You specifically asked about a woman in the sky and 12 star groupings associated with the Sun. A major problem here is that figures in the sky and groupings of stars change rapidly in time and are widely different from culture-to-culture. Studies of even medieval sky maps shows no uniformity in either constellation identification or star counts - despite being a somewhat uniform cultural environment. This by itself would render any interpretation dubious.

Historically, the questions you have raised have been raised many times over the centuries. Millenialistic fervor has been widespread in the years ~320, 1000, 1254, 1543, and 1843. The motivation in each case has been the same text, but the application to the sky has always been greatly different. Thus, history teaches us that the same text yields many disparate interpretations that gain adherents, and all of which have not resulted in consummation of the predictions.

We hope you find this information useful.

James Lochner
for Imagine the Universe!
(and with thanks to Brad Schaefer)

Question ID: 961217b

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