The Question

How large is the Milky Way?

The Answer

The disk of the Milky Way galaxy is about 100,000 light years in diameter (one light year is about 9.5 x 10¹⁵ meters), but only about 1000 light years thick.

Our Galaxy contains about 200 billion stars. Most of the stars are located in the disk of our galaxy, which is the site of most of the star formation because it contains lots of gas and dust.

The halo, which is a spherical cloud surrounding the disk, contains only about 2% as many stars as the disk. It contains old and cool stars, since it has little gas and dust.

Eric Christian and Samar Safi-Harb
for Ask an Astrophysicist

The Question

Where (is it presumed) within the Milky Way is the Solar System located?

The Answer

The Solar System has been measured to be about 28,000 light years from the center of the galaxy, and about 20 light years above the galaxy's equatorial plane. It is within the "Orion" spiral arm.

There is a picture of it's position on the following page:
http://www.enchantedlearning.com/subjects/astronomy/solarsystem/where.shtml

We hope this helps!

Barbara & Ilana
For the Ask an Astrophysicist team

The Question

I studied a bit of astrophysics at UCLA. I was trying to recall the estimated rate of speed that our Sun is moving in relation to the center of the milky Way galaxy.

And is the whole Milky Way galaxy moving also? I would assume so, but was wondering if there is some calculations.

The Answer

The Sun orbits the center of the Milky Way at about 250 km/second and it takes about 220 million years to complete an orbit.

The Milky Way is part of a group of galaxies known as the Local Group. All of these are moving relative to each other due to their gravitational interaction with speeds of around 100 km/s or less. Calculating the velocities of the galaxies in the Local Group is difficult because there are probably members that have not yet been discovered because they are too dim or are obscured by the plane of the Milky Way. The radial velocities relative to the Milky Way are found by measuring Doppler shifts in the spectra of stars in the galaxies. You will find more information at

http://seds.lpl.arizona.edu/messier/more/mw.html

The Local Group is also moving at about 600 km/second relative to the cosmic microwave background. There's a nice picture of this at

http://antwrp.gsfc.nasa.gov/apod/ap960205.html

Damian Audley and David Palmer
for the Ask an Astrophysicist Team

The Question

What is the best current estimate of the age of our galaxy compared to the time since the big bang?

The Answer

There was a review article in the magazine Science by Lawrence M. Krauss and Brian Chaboyer (Vol 299, page 65, 3 January 2003). If you have access to a library with a subscription to Science, you might want to read the actual article. Here is a brief summary.

There are three independent ways to infer the age of the oldest stars in our galaxy.

Radioactive dating (using Thorium 232 and Uranium 238, both radioactive elements with half-life in billions of years) gives 14.0+/-2.4 billion years for a star called CS 31082-001.

White dwarf cooling method suggests 12.7+/-0.7 billion years (observational errors only, that of the cooling model not included).

The main-sequence turn-off timescale method for oldest globular clusters gives the best estimate of 12.6 billion years and the 95% confidence range of 10.4-16 billion years.

In contrast, the time since Big Bang is estimated to be about 13.7 billion years:
http://map.gsfc.nasa.gov/m_mm/mr_age.html

Best wishes,

Koji & Scott
for "Ask an Astrophysicist"

The Question

How many stars (or what percentage) in our galaxy are 4.5 billion years old (age of the Sun) or older?

The Answer

The Sun is about 4.5 billion years old now, but its total life is estimated to be about 10 billion years. Most stars in the Milky Way live at least this long. And experts estimate that stars have been forming at a similar rate for most of the history of Milky Way (~10 billion years). This means about half the stars are older than 4.5 billion years old.

Koji Mukai
for Ask an Astrophysicist

The Question

What proof do astronomers have when saying our galaxy looks a certain way? All evidence would be indirect I would think. What is this evidence?

The Answer

You have a very good point here: How can we have any idea what our galaxy looks like from the standpoint of an outside observer, when we ourselves are embedded in the middle of it? As you surmised, some detective work is required. If we were to rely only on optical light in trying to formulate an educated guess as to the answer, it would be next to impossible because of all the obscuration due to dust in the plane of our galaxy. When one goes to other wavelengths, however, the project becomes do-able.

The first work which mapped out the spiral structure of our galaxy was done at radio wavelengths by studying the "21 cm" line which is due to the "spin-flip" transition of the hydrogen atom. Basically, this means that the hydrogen atom can have slightly different energy states depending on whether the spin of the nucleus is parallel or anti-parallel with the spin of the whole atom. This change from one state to the other produces this emission, which passes through the dust. By investigating the strength of this emission as a function of point on the sky, early workers (i) determined that we live in a spiral galaxy, and (ii) mapped out the spiral arms. In addition to measuring the strength of the signal at different points of the sky, one also makes use of the Doppler shift of the signal to infer the velocity structure.

More recently it has become feasible to perform this exercise at other wavelengths. For instance, the COMPTEL instrument on the Compton Gamma Ray observatory has mapped out the spatial distribution of radioactive aluminum 26 which produces an emission line at 1.8 million electron volts. To see what kind of galactic structure they have inferred, take a look at the figure in the following paper. (The paper itself is rather technical, but the figure just shows a picture of the spiral structure of the galaxy in our vicinity.) The paper is just 2 pages, with one figure.

(i) Go to this site http://adsabs.harvard.edu/abstract_service.html

(ii) In the author field enter: Chen, Gehrels, Diehl, Hartmann (put these in with just one name on each line of the search box), then select the Boolean "and" search option, and click "send query". This will get you�the following paper:

"On the spiral arm interpretation of COMPTEL 26 Al map features" Astronomy & Astrophysics Supplement Series vol. 120, 315. (1996)

It has just been pointed out to me that it may be difficult for you to access this reference in the manner I suggest if you are on other than a UNIX machine. In that event, it may be necessary to find a university library to get this paper.

Of possible interest, here is the Multiwavelength Milky Way poster site

http://adc.gsfc.nasa.gov/mw/milkyway.html

J.K. Cannizzo, K. Smale, D. Palmer
for Ask an Astrophysicist

The Question

I am in 5th grade. Do you have any pictures of the Milky Way from above it?

The Answer

There are no pictures of the Milky Way galaxy taken from above. That would require us or a space probe to be able to get far enough away. That hasn't yet been done. The furthest a space probe has gotten from the earth is Pioneer 10, which is now more than 10 billion km from the earth. (For comparison, Pluto is about 6 billion km away). However, Pioneer 10 would have to go 3 billion times farther to get far enough away to take a picture of the entire galaxy.

However, astronomers think they know what the Milky Way galaxy would look like from outside it. They have long thought that it is a spiral galaxy, like the Andromeda Galaxy or the Whirlpool Galaxy. However, recent research indicates it may be a barred spiral, like M91.

You can find a picture and a description of each of these galaxies on these web sites:

Andromeda: http://www.maa.mhn.de/Messier/E/m031.html
Whirlpool: http://www.maa.mhn.de/Messier/E/m051.html
M91: http://www.maa.mhn.de/Messier/E/m091.html

Jim Lochner (with help from Gail Rohrbach and Koji Mukai)
for Ask an Astrophysicist

The Question

I am curious to know why with the HST we can see the galaxies in the deep space image that are billions of light years away, but we can't study or even see the center of our own galaxy?

The Answer

It's not true that we can't study or even see the center of our galaxy. This is a very active research topic for astronomers, particularly using radio, infrared, X-ray and gamma-ray telescopes.

As for seeing the Galactic center: a search in the "Astronomy Picture of the Day" site has turned up 3 nice pictures:

http://antwrp.gsfc.nasa.gov/apod/ap960605.html
http://antwrp.gsfc.nasa.gov/apod/ap970121.html
http://antwrp.gsfc.nasa.gov/apod/ap971111.html

However, it is true that it's hard to see the Galactic center with visible light. This is because of the dust in our own Galaxy, which can be seen in the first of the 3 "APOD" pictures above as dark patches in the Milky Way. Such dust clouds make stars behind them appear much fainter.

Hope this helps.

Best Wishes,
Koji Mukai
for Ask an Astrophysicist

The Question

I am studying about 21cm wavelength or 1420 MHz radio waves coming from the sky. Do you know where I can find information about sky mapping with a radio telescope at 21cm?

The Answer

There is a map of the Milky Way in the 21 cm H line, and other wavelength bands at -

http://adc.gsfc.nasa.gov/mw/milkyway.html

There are also links to key papers describing 21 cm studies. These links lead into the abstract service of the Astronomical Data System, from which you can search for similar papers.

Another interesting link is -

http://www.haystack.mit.edu/edu/undergrad/srt/

- which describes the Small Radio Telescope Project at the Haystack Observatory. They have been able to make a 21 cm map of the Milky Way using a 9-foot satellite TV dish antenna. This work was discussed in the August 96 Sky and Telescope.

I hope that this information is helpful.

Paul Butterworth and Leonard Garcia
for the Ask an Astrophysicist Team

The Question

I teach astronomy, and a student once asked me which there are more of in our galaxy -- Pop. I stars or Pop. II stars. I wasn't sure! Do you know? More specifically, what is the ratio of the number of Pop. I stars to the number of Pop. II stars in the galaxy?

The Answer

According to 'Galactic Dynamics' by Tremaine and Binney (1987: Princeton) the ratio of luminosities of the spheroid population to the disk population is 1/30. In order to translate to numbers of stars, we can use the fact that the mass/light ratio of the spheroid is greater than that for the disk (they suggest values of 5 and 12, respectively). Also, the mass distribution of disk stars has a large contribution from the more massive stars, which are not present in the halo. Taking all these things into account is tricky, and the best I can do is to suggest that the disk population probably wins, but not by a large factor (less than 10).

I hope this helps!

Tim Kallman
for the Ask an Astrophysicist Team

The Question

I heard on the news tonight that a dark matter galaxy recently collided with the milky way. And that astronomers aren't worried because it has previously happened ten times. I can't find any more information about this anywhere on the Web. My astronomy knowledge is somewhat limited. I am not a student, I am a screenwriter.

The Answer

The announcement of a small galaxy colliding with ours was made by astronomer Rosemary Wyse at the annual meeting of the American Association for the Advancement of Science in Philadelphia in February.

The dwarf galaxy is not a "dark matter" galaxy, but rather the dark matter mixed in with the rest of the material in the dwarf galaxy appears to be holding it together under the gravitational tidal forces exerted by our galaxy. The proximity of this dwarf galaxy, however, will allow astronomers to better study the nature of dark matter.

Try http://www.seds.org/messier/more/SagdEg.html

for a description of the Saggitarius Dwarf in more details.

Jim Lochner
for Ask an Astrophysicist

The Question

Was the Milky Way a quasar in the distant past?

The Answer

The answer is that we aren't really sure. Our Galaxy does have a supermassive black hole at the center, which is what astronomers believe powers the enormous emission in quasars. However, whether or not it was ever a quasar is still up for debate. It's entirely possible that it was, but we don't have any proof one way or the other.

We hope this helps!

Barbara and Stefan
For the "Ask an Astrophysicist" team

The Question

The Hubble Space Telescope (HST) site estimates there are hundreds of billions of galaxies in the universe. A recent German super-computer simulation estimates that the number may be as high as 500 billion! Can someone please clarify the accepted educated ballpark figure? Thanks a billion!

The Answer

Your inquiry is definitely in the minds of many scientists who are trying to obtain a good estimate for the number of galaxies in the universe. The methods used to achieve such number varies, and therefore, the results would vary, too. Also, as new and improved technology becomes available, astronomers can detect fainter objects that were not seen before. These objects that have come into view will in turn change the estimated number of galaxies.

For example, in 1999 the Hubble Space Telescope estimated that there were 125 billion galaxies in the universe, and recently with the new camera HST has observed 3,000 visible galaxies, which is twice as much as they observed before with the old camera. We're emphasizing "visible" because observations with radio telescopes, infrared cameras, x-ray cameras, etc. would detect other galaxies that are not detected by Hubble. As observations keep on going and astronomers explore more of our universe, the number of galaxies detected will increase. For more about the Hubble Space Telescope, check out this web site:

http://www.stsci.edu/hst/

Hope this helps,
Georgia & Veronica
For "Ask an Astrophysicist"

The Question

What galaxy is closest to ours and how far is it from the earth?

The Answer

For a long time, the Large Magellanic Cloud, an irregular type satellite galaxy of our own, was held to be the closest galaxy to the Milky Way. It is 179,000 light-years away.

But in 1994 the Sagittarius Dwarf Elliptical Galaxy was discovered at 80,000 light-years. It now holds the honor.

Amy C. Fredericks and Michael Loewenstein
for Ask an Astrophysicist

PS. For a more recent update, see the "Nearest Galaxies information" page:
http://heasarc.gsfc.nasa.gov/docs/cosmic/nearest_galaxy_info.html
which is part of our Cosmic Distance Scale site:
http://heasarc.gsfc.nasa.gov/docs/cosmic/

The Question

Is it true that the 2 Magellanic Clouds are moving away from us at a speed greater than we thought, making our galaxy either more massive or the 2 Magellanic Clouds not our satellites? Are these measurements reliable?

The Answer

Thank you for your question. Indeed, their speeds were recently measured to be roughly twice as high as previously thought. This implies that the clouds are unbound to the Milky Way unless the dark matter halo is twice as massive as assumed (or perhaps significantly non-spherical).

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

The Question

What exactly happens when two galaxies collide?

The Answer

Collisions of galaxies are tremendous things (a galaxy is a LOT bigger than anything on Earth that you can imagine colliding!) and generate a lot of energy, heating and mixing up the gases in the two galaxies, making a good place for star formation. Unlike car collisions, galaxies collisions take a very long time - as many as a billion years or more for large galaxies!

There is lots of interesting information to be found on the web about what happens when galaxies collide, and even some recent images of galaxies that are in the process of colliding. A good brief explanation and images can be found at:

http://astrowww.phys.uvic.ca/~patton/openhouse/collisions.html

Galaxy collisions are complex interactions and there are many people trying to figure out how galaxies interact when they get close enough together, and how they affect each other. One of the ways scientists do this is by studying numerical simulations of colliding galaxies. The simulations capture much of the important physics but can be run on a much faster timescale.

There are also movies of computer simulations at the University of Victoria site (above).

Allie Cliffe and Jim Lochner
for Ask an Astrophysicist

The Question

After reading a book about galaxies and learning that galaxies can sometimes change shape when one galaxy brushes past another galaxy, one of the students in our Third Grade asked if part of a galaxy can break off and join another galaxy when they brush past each other?

The Answer

This is a very good question. Unfortunately I can't give a definitive answer. Here is what I can tell you: galaxies consist of 3 kinds of material: gas, stars, and 'dark matter' (material that we know must exist because its gravity is needed to hold the galaxy together, but we can't observe it directly). When 2 galaxies interact at a distance, they affect each other through their gravitational forces. These are of 2 types: first is the ordinary gravitational attraction which holds us onto the earth, and which holds solar system together; second is the tidal force, which is due to the fact that gravity decreases with distance. The tidal forces due to the moon and the Sun are responsible for the earth's tides. If they were very much stronger, they could actually rip the oceans off of the earth, or rip the earth apart. There is no danger of this, but it can happen in galaxy interactions -- the tidal forces can disrupt one of the galaxies, or remove the gas from one of them. This has been suggested as a way of explaining why some galaxies (ellipticals) have little gas while others (spirals) have a lot more. In this case some of the gas is probably transferred to the bigger galaxy. Other possible interactions include total disruption of one of the galaxies, or merging of the two galaxies. Which of these occurs depends on how closely the galaxies approach each other and the masses of the two galaxies.

I hope this helps,

Tim Kallman
for the Ask an Astrophysicist Team

The Question

I would like to know about the history in brief about the discovery of galaxies in the universe. If this seems to be a lengthly question please give me references.

The Answer

Immanuel Kant, who was a famous philosopher, was the first to suggest around 1755 that the spiral nebulae were island universes. Prior to this century there was no distinction made between the things that today we call nebulae, such as the star forming region in Orion or the crab, which are luminous gas clouds within our galaxy, and other galaxies. This is exemplified in the Curtis Shapley debate of 1920. You can read about how this was resolved and other related topics at:

http://earthguide.ucsd.edu/virtualmuseum/ita/04_1.shtml

I hope this helps,

Tim Kallman
for "Ask an Astrophysicist"

The Question

How can one calculate the distance to other galaxies? Do you have an example to give me bettter understanding in this matter?

The Answer

To determine the distance a galaxy is from Earth, astronomers look for celestial objects within that galaxy they can determine the luminosity (or absolute magnitude) of. These objects are known as standard candles. Once the luminosity is known from observation they can make a measurement of how bright the object appears here on earth and determine the distance. If one has two objects of the same luminosity, the one further away will appear less bright. A good discussion of how the distance can be calculated knowing the absolute magnitude of the star is available here:

http://ceres.hsc.edu/homepages/classes/astronomy/spring99/Mathematics/sec19.html

The key to making the distance measurement is to find and use a standard candle. One such class of objects are cepheid variable stars. These are stars whose luminosity varies periodically due to ionization of helium in the star's atmosphere followed by expansion and deionization. Observation has shown there is a relation between the stars absolute magnitude and the pulsation period. Another class of stellar objects that are used as standard candles are Type Ia supernovae which are explosions of white dwarf stars in a binary star system. These explosions always release roughly the same amount of energy and have a known peak magnitude which allows them to be used as a standard candle. There's a good discussion of these and other standard candles here:

http://universe-review.ca/R02-07-candle.htm

Hope this helps
Jason and Koji
for "Ask an Astrophysicist"

The Question

What is the error factor in the distances to galaxies? For instance, the Andromeda galaxy (M31) is 2.8 million light years. How accurate is that? Is the error plus or minus 1 million light years or what? And does that error factor get much worse the farther out we go?

The Answer

Nearby galaxies, where you can see individual stars, probably have about a 10% distance uncertainty.

For galaxies at distances where you can't see individual stars, the distance is found by multiplying the redshift by a number called the Hubble constant H0. The value of H0 is a contentious issue, but the two extreme camps are arguing for ~55 or ~70 km/s/Mpc, which means that there is about a 30% range in how far away people think any given galaxy is. The accuracy with which the redshift is measured doesn't depend much on the distance to the galaxy, so this is a constant factor: you know that one galaxy is 3.0 times as far away as another, and not 3.1 or 2.9 times.

At distances which are a good fraction of the age of the universe away, the question is how constant the Hubble constant is. Depending on how much mass there is in the universe (and thus how much the universe has been slowed down by gravity--so how much faster it was expanding in the early days) this can add an additional uncertainty range of 50%.

David Palmer
for Ask an Astrophysicist

The Question

How far away is the furthest known galaxy?

The Answer

The most distant galaxy known today is called IOK-1, with a redshift of 6.964 which puts it about 12.88 billion light years away from earth. Here is more information:

http://www.universetoday.com/2006/09/15/subaru-finds-the-most-distant-galaxy/

http://www.nature.com/nature/journal/v443/n7108/full/nature05104.html

A galaxy called Abell 1835 IR1916 was found in 2004 and was originally thought to be at redshift 10, or about 13.18 billion light years away, but subsequent attempts to confirm the observation did not see the same results.

Here is the original report:

http://www.universetoday.com/am/publish/record_furthest_galaxy_broken.html

And here is a recent paper that discusses the attempts to confirm the original observation:

http://arxiv.org/abs/astro-ph/0601181

Jay and Jeff
for Ask an Astrophysicist

The Question

I recently saw a TV program exploring new theories about the relation between black holes and galaxies. People thought that there was a black hole in the center of every galaxy, and that it was possible that galaxies are created by black holes. Is this true and if so, what do you think?

The Answer

It's great that you're interested in this subject, and that you got your information from such a quality program.

It is true that observations show a tight relationship between the masses of the central black holes and the masses of their host galaxies.

It is also true that many scientists believe there is a black hole at the center of each large galaxy. (This doesn't apply to smaller galaxies, by the way, like the Large and Small Magellanic Clouds, which are satellites of the Milky Way Galaxy.)

Nobody knows for sure exactly how galaxies formed --- the details are very hard to reconstruct. Whatever they turn out to be, one thing you need is a region of enhanced density (once you start the process, it increases the gravity, which in turn increases the density, so it builds up). Can the central black hole be the seed that started the creation of the galaxy? Possibly, although the galaxy is usually something like a hundred thousand times more massive than the central black hole, so the numbers may not work out. Other possibilities are that the black hole grows to "fit" the size of the galaxy in some ways (i.e., the black hole came later) or that the two are both the results of a single underlying cause (i.e., the two were created at the same time). It's an exciting area of research --- check back with us in a few years, and we may have a more definite answer by then.

Hope this helps.

Koji & Bish
for "Ask an Astrophysicist"

The Question

I've heard that Andromeda and the Milky Way galaxy will collide in 3 to 4 billion years, and that Andromeda and the Milky Way have black holes in their centers. What would happen to the black holes when the two galaxies merge?

The Answer

Thank you for your question. For some period of time the merged Andromeda/Milky Way galaxy will have two massive black holes in the central region, as is sometimes observed in merger remnant galaxies. Dynamical friction caused by interactions with stars cause them to sink to the center of the new galaxy, where they will form a binary system that will get tighter due to further encounters with stars. Eventually as the separation gets smaller than a light-year or so, gravitational radiation takes over and leads to an inevitable merger into a single supermassive black hole. If there is enough gas around to be accreted, the black hole may exhibit quasar-like behavior.

Michael Loewenstein & Amy Fredericks
for "Ask an Astrophysicist"

The Question

What different types of galaxies are there? What are their similarities and differences?

The Answer

There are indeed different types of galaxies. The main types are spiral galaxies (like our own MilkyWay), elliptical galaxies and irregular galaxies. An irregular galaxy has an undefined shape and has lots of young stars, dust and gas. A spiral galaxy is shaped like a disk, usually with a bulge in the center and with arms that spiral outwards as the galaxy rotates. Spiral galaxies tend to contain more middle-aged stars along with clouds of gas and dust. Elliptical galaxies contain older stars and very little gas and dust. They can be different shapes ranging from round, to flattened, elongated spheres.

J. Allie Cliffe
for Ask an Astrophysicist

The Question

Do you think you could send me any links to good web-pages that have information on Lenticular galaxies or do you know where I can get any good information on them at?

The Answer

I am not an expert on galaxy morphology, but here is my understanding.

Lenticular galaxies are best described as 'spiral galaxies without the spiral'. The spirals highlight the places where bright new stars are forming, but lenticular galaxies (like ellipticals) have lost the interstellar gas which forms new stars. Lenticular galaxies are most often found in dense clusters of galaxies, so the most likely conclusion is that collisions with other galaxies or intergalactic gas clouds have stripped the gas out of these galaxies.

Hubble classified the shapes of galaxies in a 'tuning fork' diagram, with spirals and barred spirals as the tines of the fork, and elliptical galaxies as the handle. Lenticular galaxies on this classification system are where the tines meet the handle.

Go to

http://www.seds.org/messier/lenticul.html

to see some images of lenticular galaxies.

A search engine may turn up over references to lenticular galaxies. Since lenticular galaxies are not a topic popular on the 'fringe', most of these pages should be reasonably accurate. As always, consider the source of the information before deciding how much to trust its validity.

David Palmer
for Ask an Astrophysicist

The Question

Why do the galexies posses different shapes such as elliptical, spherical, spiral, etc?

The Answer

This is a very interesting question, and still an active area of research. We do think we have a pretty good understanding, at least at a basic level, but there's much more to learn in detail.

The main types of galaxies are disk (or spiral) galaxies, elliptical galaxies, and irregular galaxies. "Irregular" covers pretty much everything that isn't a spiral or nice ellipse. The short answer is that galaxies form originally as a spiral disk. As nearby galaxies interact and collide, they eventually end up as an elliptical shape. So the elliptical galaxies are all much older structures.

Irregular ones are probably the result of galaxies being disrupted by gravitational interactions with other galaxies.

There is a nice article about galaxies on wikipedia, which has a lot more detail:
http://en.wikipedia.org/wiki/Galaxy_formation_and_evolution

We hope that helps.

-Kevin and Hans,
for "Ask an Astrophysicist"

The Question

I was woundering what the most common type of galaxy is. I have searched different scorces and they all say different things. Some say spiral, some say eliptical.

The Answer

The references I was able to find all agree that elliptical galaxies are the most common, accounting for about 60 percent of all galaxies.

Perhaps what some of the references were referring to the numbers of known galaxies. Since sprial galaxies tend to be brighter than ellipticals, more of them have been named and cataloged. But in a given region of the sky astronomers see more ellipticals, so they infer that ellipticals are more common everywhere.

Cheers,

Hans Krimm for "Ask an Astrophysicist"

The Question

What is the average size of a galaxy?

The Answer

Thank you for your question. Our Milky Way galaxy is a pretty typical large galaxy. Most of the stars are in a disk that is about 100,000 light years across in diameter and 3000 light years thick. Most of the galaxies in the universe are actually smaller than the Milky Way. For example, most of the dozens of galaxies in our Local Group are at least ten times smaller in diameter.

-- Michael Loewenstein and David Marsden
for "Ask an Astrophysicist"

The Question

I know there are different types of galaxies, so does the size of the galaxy vary according to the type?

The Answer

Well, yes, sort of, but it's a little bit more complicated.

The biggest galaxies we know of are elliptical - but on the other hand, there are small ones, too (called "dwarf elliptical galaxies").

The spiral galaxies tend to be in the medium large range.

The smallest galaxies are often irregular.

See, for example,

http://cas.sdss.org/dr6/en/astro/galaxies/galaxies.asp

for more.

Hope this helps,

Koji & Kevin
for "Ask an Astrophysicist"

The Question

First, why do stars clump together into arms in spiral galaxies and are there commonly a specific number of prominent arms in a typical spiral galaxy? Is there a correlation between star masses, rotational speeds, galactic size, etcetera and the spiral arms?

Second, and this will settle an argument with a coworker :). What is the typical evolution of a cluster of stars forming into a galaxy? My friend seems to think that we start with spirals then move to disks. He uses as an analogy, chocolate syrup mixing in a glass of milk (where the chocolate particles are stars and the milk is space). I think this is wrong. Is he correct? And if he is, why does this happen? Or if he is wrong then what is the right answer?

The Answer

The answers to your two questions are, not surprisingly, related. What causes spiral arms are density waves propagating through the stars and gas of the galaxy. This means that stars in an arm may not be in the arm after the density wave has moved on. What exactly causes these density waves is not known. Because of the increased density of gas in the arms, star formation in a spiral galaxy is concentrated in them, and so newer, bigger, and brighter stars tend to be in the arms.

As for the history of galaxies, the current thinking is that galaxies that have a relatively low total angular momentum form elliptical galaxies, and high angular momentum galaxies form spirals. Spirals probably start more like ellipticals, then collapse down to a disk (and a more spherical center bulge), and then the spiral arms form. Spirals all tend to be similar in mass, as opposed to the elliptical galaxies which vary in size from very small to the largest of galaxies. How these galaxies continue to develop is a topic for speculation, or you can ask me again in 5 or 10 billion years.

Thanks for your questions

Eric Christian
for Ask an Astrophysicist

The Question

Mature elliptical galaxies have been spotted 1 billion years after the big bang. Considering that the stars within these type of galaxies should be at least 10 billion years old, and assuming that the same laws of physics apply to the early universe as apply to the present universe, why do these galaxies exist at all in the early universe.

The Answer

Indeed apparently fully assembled elliptical galaxies have been observed as early as 1 billion years after the Big Bang. The stars within these galaxies, however, are not as old as the stars in nearby elliptical galaxies. The best-estimate stellar ages are somewhat uncertain at the moment, but they are consistent with being formed after the Big Bang, so there are no problems here. This is quite interesting, since it has told us that galaxy assembly proceeds very rapidly after the big bang.

Bret & Antara
for Ask an Astrophysicist

The Question

What exactly determines the size of a galaxy?

The Answer

Galaxy formation is a very active area of scientific research today and the exact means by which they form and hence their size is determined is not exactly known. The generally accepted view of the process by which a galaxy forms is that dust and gas come together within a region, are gravitationally attracted and coalesce into stars and other celestial bodies inside the galaxy. The amount of material and the angular momentum that material has (that is to say how much spinning movement the particles of dust and gas have) are believed to be the main contributers to the ultimate size of the galaxy.

The story doesn't quite end here however. In recent years there has been observations made about the mergers of galaxies whereby an even larger galaxy can be formed as material from one galaxy is added to another. Whether or not there may be other factors or events in the formation or evolution of a galaxy that can contribute to its size is still being studied.

Some links you might find useful that discuss galaxy formation and evolution:

http://en.wikipedia.org/wiki/Galaxy_formation_and_evolution

http://www.astro.washington.edu/larson/Astro101/LecturesBennett/Galaxies/galaxies.html

Hope this helps
Jason and Koji
for "Ask an Astrophysicist"

The Question

Okay well, I have really been wondering about something I've heard of before and that I know isn't too widely known. How much do you know about Dark galaxies? I've heard that possibly dark galaxies are comprised of most of the matter in the universe from dark matter, or perhaps it contains tons of black holes and dead stars. I'm not sure and I would like to know what you know about the topic since I have been looking around and didn't see a topic on Dark galaxies just yet.

The Answer

First, good luck with your studies. Actually most of the mass of most galaxies is already composed of dark matter. It is this additional mass that we can't see that keeps rotating galaxies from flying apart. However, there are some candidates for truly dark galaxies, which have very little star formation. These are still detectable via radio waves from the hydrogen gas mixed in with the dark matter.

Details can be found at:
http://www.newscientist.com/article.ns?id=dn7056
http://arxiv.org/abs/astro-ph/0310192
http://en.wikipedia.org/wiki/Dark_galaxy_(astronomy)

Hope this helps,
Mike and Georgia
for "Ask an Astrophysicist"

The Question

How are the "Local Group" of galaxies selected? Are they just galaxies that are close to each other or are they connected by other means (By gravity, similarities, etc)? What other galaxies, apart from Andromeda and The Milky Way, are included in the "Local Group?"

The Answer

Galaxies are not uniformly distributed in space, but tend to be concentrated in groups and clusters. In some clusters, the density of galaxies is so much higher and the combined gravitational pull so deep that membership is easy to determine. Groups are looser associations, and so are harder to define cleanly.

To quote http://www-hpcc.astro.washington.edu/papers/localgroup/lg.html
(a page which appears to have disappeared):

"What is the criterion for inclusion in the Local Group? Proximity is the cleanest and often used to the exclusion of any other. If we grant membership to all galaxies within 4 million light-years, we have a club with 30 members, three of which barely got in. We can also use velocities to find out if the last three are on their way in or out. That is, we can accept all the fellow travelers. If we do this, the three squeakers become full members and we are pressed to include a few more distant objects such as Leo A and Pegasus, both small irregular galaxies."

The motions of all the Local Group galaxies are strongly influenced by the combined gravity of M31 and our Milky Way Galaxy, the two dominant members.

For a full list of members, see
http://www.seds.org/messier/more/local.html

Hope this helps,

Koji & Bish
for "Ask an Astrophysicist"

The Question

I'm curious as to the recent need for teraflop and petaflop computers for the sole purpose of calculating the evolution of clusters (gravitational pulls, twin star formations, and other collisions).

When we finish calculating many of these virtual clusters of galaxies, will we be able to understand if our cluster is really the center of the universe and if it is truly an average cluster? Has this already been explored?

The Answer

Thank you for your question about evolution of galaxy clusters, and the need for fast (or special purpose) computers for this work.

The basic reason why the investigation of the dynamical evolution of galaxy clusters (as well as the evolution of single galaxies, or even globular clusters) is so computer intensive actually is due to a fundamental mathematical property of the equations that determine this evolution. The gravitational attraction between all objects is described by Newton's Laws, which you are probably familiar with. One law states that the gravitational force between two objects is a constant multiplied by the product of the two masses, divided by the distance separating the objects squared. In most of the solar system examples we are presented with on a 'day to day' level, the system can be described as two bodies. For each of the planets, we can treat their orbital evolution largely as if they were a single object in orbit about the Sun (the other planets produce only minor perturbations to this simple two-body orbit). Likewise, the Moon's orbit about the Earth can be treated largely as a two-body problem, since the distance between the Earth and Moon is much smaller than that between the Earth-Moon system and the Sun. Mathematically, the two-body problem is one that we refer to as 'integrable'. What this means is that it is possible to write down the solution to the equations of motion in closed form. Then for any set of initial conditions, we can use this closed form solution to determine the positions and velocities of the two bodies for all time.

When even one more body is added to the mix, the problem becomes 'non-integrable'. This has two important consequences. The first is the equations that determine the evolution are no longer in closed form. The second is that the system can now have parameter ranges for which the evolution is extremely sensitive to the initial conditions of the system. Very small changes in the initial conditions (positions and velocities) can lead to drastically different evolutions. Putting these two consequences together, you can probably see now why one needs a lot of computer power: galaxy clusters are comprised of numerous objects (galaxies) which are themselves made up of individual stars, interacting with each other. There are clever ways to make the calculation of the cluster evolution less computationally intensive, such as concentrating only on the interactions of nearest neighbor stars, and treating the contribution from the numerous more distant stars as a smooth gravitational potential. You still need to have a lot of computer power to do this. The sensitivity to initial conditions means that researchers often try a very large number of initial conditions so they can get an idea of the statistical behavior of the interactions.

To answer your specific questions: the need for teraflop or faster computers to do these calculations is not recent. However, the development of special purpose computers (that are hard wired to do nothing but the cluster evolution calculation) and novel ways of networking computers to achieve greater speeds, are currently very active areas of computational astrophysics research. The sophistication of the cluster evolution models is constantly growing. None of these calculations are aimed at trying to determine if our cluster is the center of the Universe. One of the fundamental assumptions of modern cosmology is that no single location in the Universe is special, and that there is no meaning to the concept 'the center of the Universe.' However, the average observer in any location in the Universe would observe galaxies to be receding from her position, and so might erroneously suppose herself to be at the center of the Universe. In any case, the dynamical evolution of the cluster is a local phenomenon, not connected to the overall expansion of the Universe.

Sorry if this is long-winded, but your questions touch on topics which are not easy to answer without going into the details.

Cheers,

Padi Boyd
for the Ask an Astrophysicists Team

The Question

The great attractor has been described as an agglomeration of matter. What would it look like, a galaxy? What are its proportions and what are its effects on the Milky Way and other local galaxies.

The Answer

The Great Attractor is far bigger than a galaxy. In the terminology of astronomers, there are clusters of galaxies containing maybe hundreds of galaxies, and superclusters containing many clusters. The Great Attractor is a supercluster, or something even bigger (the terminology becomes a bit fuzzy when it comes to the largest scale structures in the universe!).

The gravity of the Great Attractor has been pulling the Milky Way in its direction --- the motion of local galaxies indicated there was something massive out there that are pulling the Milky Way, the Andromeda Galaxy, and other nearby galaxies towards it. For a while, nobody could see what it was, because it lies behind the plane of our Galaxy --- that means the gas and dust in our Galaxy obscures the light from the Great Attractor, and it is outshone by the stars and other objects in our Galaxy.

X-ray observations with the rosat satellite then revealed that Abell 3627, a previously known cluster of galaxies, was much more massive than originally suspected, containing many more galaxies. Optical astronomers had missed a great number of galaxies, because of the obscuration, but with hindsight (and with better observations), could spot many more galaxies. It is now thought that the Great Attractor is probably a supercluster, with Abell 3627 near its center.

There is an optical image of Abell 3627 at:

http://antwrp.gsfc.nasa.gov/apod/ap960218.html

Hope this helps.

Koji Mukai, Rich Mushotzky & Maggie Masetti
for Ask an Astrophysicist