Questions and Answers about Galaxies

1. If 5 galaxies collided is there anyway they could combine and just make one huge galaxy?  Or would they just exchange stars and some matter and move on their way?

  In physics, there are two types of collisions - elastic and inelastic. In an elastic collision, the momentum and kinetic energy are conserved and the two "colliders" have different motions after the collision.  An example of elastic collision can be found on the pool table.  If you want to put the 8 ball in the corner pocket, you aim the cue ball at the 8 ball and then shoot.  Then what happens?  The cue ball hits the 8 ball and then stops; the 8 ball rolls into the corner pocket and you win the game.  Of course, this never happens when I play :) Physically, the momentum in the cue ball is transferred into the 8 ball and kinetic energy is conserved because the cue ball goes from moving to rest and the 8 ball starts to move from rest.  The collision is elastic.    In an inelastic collision, momentum is still conserved but kinetic energy is not necessarily conserved.  So the two "colliders" move (or stick) together or have the same motion after the collision.  An example of this is a bullet and a block of wood.  When a moving bullet hits a stationary block of wood, the bullet gets stuck in the wood after the collision and neither object moves afterwards.  Momentum in this case is still conserved because the block of wood is so much bigger than the bullet, but kinetic energy is not conserved because neither object moves after the collision.

  Galaxy collisions are inelastic collisions.  So when your five galaxies collide (most likely not simultaneously, but even if so), the galaxies will stick together and make one giant galaxy.  Of course, when five galaxies collide at the same time, it is a little hard to predict exactly what will happen because having multiple bodies in a collision complicates things.  Astronomers can actually see this happening in the center of galaxy clusters.  In the center of dense clusters like the Coma cluster, there are galaxies called giant ellipticals (or cD galaxies) which are believed to have formed from collisions.  These galaxies are bigger than most observable galaxies and have masses comparable to many times an "average" galaxy.

Stacy Teng

2. Do elliptical galaxies have a deformed black hole in the center or how come they are so off the normal galaxies?  What do you think the Milky Way is?

First of all, have you heard of the Hubble Galaxy Classification system yet?  This is basically where the names 'elliptical' and 'spiral' came from.  Around 1926, Edwin Hubble developed a classification system for galaxies.  What he did was look at galaxies close to the Milky Way (our galaxy) and divides them up into three major types, elliptical, spiral, and irregular.  He also divided each of these categories into smaller categories, but that's just details.  Elliptical galaxies look like circles or ovals on the sky, spiral galaxies look like, well... spirals, and irregular galaxies are just the leftovers (so they don't look like circles, ovals, or spirals).   The Milky Way is a spiral galaxy.

When Hubble first made this classification system, most galaxies fit into either the elliptical or spiral categories because he was only looking at galaxies close to the Milky Way.  (The reason he only looked at galaxies close to the Milky Way is because the telescopes that were around then could only see galaxies close to us!  They weren't powerful enough to see far away, but nowadays we can see galaxies that are very far away.)  When we look at galaxies far away a lot of them look very different from galaxies close by.  So galaxies that are close by (and therefore fit into the Hubble Classification system) are generally considered 'normal'.   This means that ellipticals, spirals, and irregulars that we see close by are _all_ considered 'normal'.

So, what makes a galaxy abnormal?, you might say.  The main reason that galaxies are considered not normal is because of activity associated with a central black hole.  Nowadays it is generally believed that most galaxies, if not all, have a black hole at their centers.  So, how does a black hole make a galaxy look abnormal?  Well, if a black hole is all by itself, with nothing around, you can't see it because it's black!  But if a black hole has a lot of material around it, that material falls into the black hole.  As it falls in, the material releases a lot of energy!  That energy can do a lot of different things that make a galaxy look not normal.   For example, it can just make the galaxy look very bright near the black hole, or it can make the galaxy have jets.  Jets are just material that is given so much energy that it's thrown out of the galaxy in long straight lines (like exhaust from a jet plane).

So, why do so many galaxies that are far away have material around their black holes (and therefore look abnormal), but galaxies close by have no material around their black holes (and therefore look normal)?  Well, this is just because we see galaxies that are far away how they were when they were younger (because it takes a very long time for their light to get to us).  So by the time galaxies get older the black holes have already eaten up the material around them, so there's nothing left.

Also, black holes can't be deformed.  Scientists theorize that all black holes are a single point, which has no shape.

Melanie E. Freed

3. The next question that I have has to do with a central engine.  We went through this part of the chapter really fast and their was some things that I wanted to know about this.  We talked about how it might be a black hole and I was just wondering if it could be anything else.  Also, if there was a giant black hole, then wouldn't it suck in the rest of the galaxy?

One of the reasons we believe there are black holes at the centers of some galaxies is because we see so much light and energy shining out of such a small space. Even though the black hole itself cannot be seen there is a lot of gas orbiting around it, and as the gas slowly spirals into the black hole it gets very, very hot, and it is the light from this hot gas that we see. It is impossible to get so much light out of such a small volume in the center of a galaxy from a collection of normal stars, or anything else we can think of, other than a black hole. It is actually very difficult for a black hole to suck in the rest of the galaxy, which may sound a bit strange at first. The reason is that the gas and stars in the galaxy have angular momentum -- i.e. they are orbiting around the nucleus, rather than diving directly towards it. One way to think about this is get (or imagine) a funnel and a marble. If you place the marble at the top of the inside wall of the funnel and let it go it will quickly roll to the bottom. If, on the other hand, you roll the marble sideways it will roll around in circles (i.e. orbit) many times before friction slows it down and it falls to the bottom. This is a bit like a galaxy, with the black hole at the center, except that friction is much lower in a galaxy so it is much harder to get sucked into the black hole. It is also the reason that the sun does not suck the earth into it and we are happily orbiting!

Andy Young

4. Are there certain places in different galaxies where a maximum number of stars are produced at a greater rate than in other galaxies?

Yes, the rate of star formation varies within a single galaxy and from galaxy to galaxy as well.  Within a single galaxy, star formation is concentrated in the spiral arms where the gas to form the stars has collected.  In between these arms, very few stars can form. Some galaxies are known as "starburst galaxies" meaning that they are currently undergoing a burst of star formation at a rate much higher than normal.  This can happen when two galaxies collide.

Marc Pound

5. Can any new interstellar matter be developed from quasars after they die?

Quasars are very distant galaxies with extremely bright centers. Because quasars are so bright for their size, astronomers think that they are powered by material being sucked into a VERY massive black hole at their centers.  This black hole may be as massive as 100 million Suns.  As the black hole gets more and more massive, from sucking up all that matter, it becomes less efficient at converting that energy into light and the quasar dims. Eventually the quasar probably resembles a normal galaxy again. But the stuff that vanishes into the black hole is never seen again, so no new interstellar matter is created from it.

Marc Pound

6. What is the gravitational pull holding out galaxy together?

You have identified exactly what makes a galaxy what it is.  For galaxies to be unique bodies, they need some glue to hold them together which makes them separate from other stuff in the universe.

Gravity is one of the four fundamental forces of nature (the other three are called the 'strong,' 'weak,' and 'electromagnetic' forces).  In general, it is the only one that is significant in considering the motions of large systems like our solar system or galaxies.

The way in which gravity holds together the solar system is slightly different from the way in which gravity holds our Galaxy together.  The solar system possesses one massive body, the Sun, which is much more massive than anything else in the solar system.  The motions of each of the planets is governed mostly by the Sun's mass, though the planets can certainly affect one another.

In the case of the Galaxy, instead of a single massive body, we have billions of stars that are all of similar size, plus gas in between the stars.  These things collectively, gas + stars, govern the motions of each star or atom.

Dave Rupke

7. I've always wondered was how many galaxies there are and what ones are the most important ones.

The exact number of galaxies in the universe is unknown, partly because we can't see them all to count them.  However, we can make indirect counts of how many there are in various ways.

For instance, we can count the number we see in a certain part of the sky at various times in the universe's evolution.  By assuming that galaxies are distributed more or less evenly in the sky, we can guess at how many there are in total.

We can also look at something called the 'extragalactic background light.'  This is light that is the sum of the contributions from stars and gas and cosmic dust at many different times and places.  This light is hard to see at some wavelengths but fairly easy at others.  By using models to describe this light, we can make estimates of how many galaxies there are.

In all, however, there are a lot--loose estimates I've heard start at least in the billions.

As for what the most important ones are, they're all important!  But there are so many different kinds, and they are each important for helping us understand different things.

For instance, I study a type of galaxy that is very bright in infrared light (which we can't see with our eyes, but which is the kind of light people see in night-vision goggles and traces things that have relatively high temperatures).  Many of these galaxies are the result of collisions between two other galaxies.  They form many stars at once, and we thus call them 'star-bursts.'  Many may also possess very massive black holes at their centers.  Understanding these galaxies helps us understand the evolution of stars and massive black holes, as well as what types of galaxies are formed from these big collisions.

Dave Rupke

8. What are the signals we receive from quasars? What kind of energy to they release? How far out into space are we receiving these signals from?

Quasars are one of the most distant objects that astronomers study.  The most distant quasars are thought to be at a distance of about 13 billion light years (!) -- The known Universe is about 15 billion light years, so that's pretty far out there.

Quasars are observed in many different kinds of light.  As you may have learned in class, they were first observed as radio sources.  However, most quasars now known are "radio quiet' -- meaning they aren't very bright when you look at them in radio waves.  Quasars are also seen in optical light, X-rays, and gamma rays.

If you get a chance, you can see a bunch of images of quasars in different wavelengths in the "Astronomy Picture of the Day" web archives -- click on the "search" link at the bottom of the page and search for "quasar".  The descriptions of the images also have links to more information on quasars.

Barbara Mattson

9. How do we know where the earth is located at in our galaxy?

This was one of the major points of debate in astronomy in the first couple decades of the 1900s.  The tricky part at that time was that because only optical astronomy was possible, observations were very limited due to absorption and extinction by dust and gas.  It was therefore like trying to survey the Black Forest from somewhere inside: if you can't see the edge because trees are blocking your vision, you don't know how large the forest is.

However, it was possible to see large clusters of stars called globular clusters, because these are out of the plane of our galaxy and therefore observers didn't have to look through the gas and dust rich middle of the Milky Way.  These clusters seemed centered around a region about 25,000 light years away, in the direction of the constellation Sagittarius, so people correctly determined that this was the center of our galaxy, and our distance was about 25,000 light years.  To continue the forest analogy, it would be like observing birds and noticing that there were always more in one direction than the other.  You could deduce that the trees continued farther in the direction with more birds.

The other key development was the ability to see at other wavelengths, in particular infrared, radio, and X-ray, where the absorption and extinction are minimized.  That means that the galaxy is transparent, so we can see it all easily.  The forest analogy gets a little strained here, but perhaps it would be like looking in winter, when the leaves are off the trees.  It is then easier to see long distances, so you can get a better idea of the true size of the forest.

Cole Miller

10. A) The Milky Way and other galaxies is what we are currently learning about.  There are all sorts of cool stuff that makes up are galaxies--clusters being one of them.  Why do they surround us in the Galactic Halo and just what is the Galactic Halo?

B) In regards to your article, I was wondering how a super cluster of that size can actually exist?  Wouldn't the matter that formed the stars be pulled together into fewer, more gigantic stars?  Or if the stars formed, and then later traveled through space and met each other in this super cluster, why wouldn't their gravitational forces tear one another apart?

C) Earlier this week we read Science News about a star cluster, Westerlund 1, that was found in the Milky Way and was wondering if any of the stars had exploded in that cluster and if we'd be able to see it from Earth?

D) Hello Maryland astronomers!! We read Dr. Miller's article in Science News yesterday about the Super Cluster in the Milky Way.  How can so many stars survive there and not destroy each other?

E) What characterizes a group of stars as a Super Star Cluster like Westerlund 1, and how does Westerlund 1 compare to other star clusters?

Good questions all!  Let me give you some general background about clusters, then I'll address your individual points.

Various observations suggest that most stars aren't formed alone. Instead, they form near other stars.  In some cases this can mean that large clusters of stars form at basically the same time. In a super star cluster (or a globular cluster, which is a much older version of the same thing), you might have a million stars within a few parsecs of each other.  That's at least a hundred thousand times the number of stars per volume in the vicinity of the Sun.  To answer Jessica's question, a "super star cluster" is such a cluster that is young enough (just a few million years) to contain bright, massive stars.  We're not sure, but Westerlund 1 might be the largest such cluster in the Milky Way.  However, it is dwarfed by some of the super star clusters in other galaxies such as M82. To answer A), it seems that some of the first stars formed in the universe did so in such clusters.  These are now called globular clusters, and they apparently formed before some galaxies were fully assembled.  That means that, rather than forming in the disks of such galaxies, they formed all over, and that's what people call the halo of a galaxy.  In other words, globular clusters orbit around their host galaxy, but in all directions and inclinations, and at some pretty large distances (some around our galaxy are ten times farther from the center than we are).

B) and D) asked about how the stars in a super star cluster would interact with each other, and in particular whether there would be collisions, destruction, and other mayhem :).  These are excellent questions, and Jacqueline was particularly close to the mark when she asked about stars coming together to form bigger stars.

Let me approach this by giving you a sense of scale.  Suppose that we have a really massive cluster with a million stars that is extremely compact, with a radius of only one parsec.  The average distance between stars is still huge: about four hundred thousand times the radius of the Sun!  That is big enough that collisions aren't as common as one might have thought, but they do occur sometimes. In particular, there is a group of scientists who have suggested that when the cluster is very young (but still a few million years old!), the heaviest stars might sink to the center where they have a better chance to hit other stars.  If they do hit other stars, they can make yet bigger stars, and when the cores of these stars collapse they could form extra-large black holes, maybe a hundred times or more the mass of our Sun.

However, as you may have learned, the most massive stars don't last long; instead, they explode as supernovae.  Therefore, if you wait longer, the cluster will contain only lower-mass, smaller stars. These can't collide as easily.  Globular clusters, which are really old (maybe 12 billion years!) are like that: the stars undergo a complicated dance, but collisions are really rare, so the cluster can continue to exist in a kind of peace.

To answer C, Westerlund 1 is old enough (about five million years) that it is likely that its most massive stars have indeed exploded as supernovae.  If such a star exploded now we would be able to see it, but not necessarily in ordinary (optical) light because there is a lot of gas and dust in that direction.

Cole Miller

11. How do galaxies form?  

A current frontier of astronomy is study of the formation of galaxies, so people can't give a detailed answer to why they form.  The general picture people have is of giant clouds of gas in the early universe, which slowly draw themselves together by their own gravity.  If such a cloud was rotating (even slowly) to begin with, then as it contracted its spin would speed up (like an ice skater when she spins and then draws her arms in).  The galaxy would tend to flatten as a result, and this might be how we get spiral galaxies.

In addition, however, galaxies tend to form near each other, in groups or clusters.  They can therefore, sometimes, collide.  A collision does not involve stars hitting each other (they are way too small compared to the galaxies), but the gas in the galaxies does collide.  This leads to a variety of things, including enhanced star formation and extra feeding of the central super massive black holes (!).  Most people think that this leads to most of the gas being driven out of the merged galaxy.  That means that new stars can't form, so the stars in the galaxy just get old and die, leading to a redder population.  In addition, when galaxies collide they are likely to do so at some random angle, so the merged remnant is puffed up and doesn't look like a disk any more.  That's how people think you get an elliptical galaxy.

But, like I said, there is active study going on, so parts of this picture could be changed in the future.

Cole Miller

12. How are scientists able to determine the shape of our own galaxy while being inside of it.  And since we have discovered a blackhole inside the center of our galaxy, what is the estimated amount of time it will take until our solar system has been consumed within it? Or will it?

Determining the shape of our Galaxy has occupied astronomers since Galileo first pointed his telescope toward the heavens.  The most basic method, first employed by William Herschel in the late 1700s, is to look in different directions with a telescope and count the number of stars you see in each.  In some directions you will see many more stars than in others.  Herschel reasoned correctly that he must be looking through more of the Galaxy in those directions where he counted more stars. So he was able to deduce that the Galaxy must be roughly shaped like a pancake. There are complications, for instance there is material called "dust" in some directions that blocks out starlight, so you undercount stars in those directions. (This dust had not yet been discovered in Herschel's time).  But now we know how to correct for these complications and know quite well the Galaxy's shape. It is a barred, spiral galaxy that probably looks a lot like this one:

(That's another hint at the Galaxy's shape--galaxies only come in a handful of shapes, so ours must look like some other galaxies).

To answer your second question, the black hole in the center of the Galaxy will not consume our solar system.  We are too far away from it.

Marc Pound

13. I have a question concerning galaxies. I understand that sometimes galaxies pull on each other, and almost come in contact with each other. My question is will all galaxies eventually collide? And if so, what will happen?

Galaxies exist in groups, or clusters, of many galaxies. Each galaxy attracts every other galaxy due to its gravitational force. Since the gravitational force depends on mass, more massive galaxies have a more of an influence on other galaxies than less massive ones. For example, our Milky Way galaxy (MW), which is a large, relatively massive spiral galaxy, is attracting two less massive irregular galaxies, which are satellites to the MW, known as the Large and Small Magellanic clouds (LMC and SMC). Since the LMC and SMC are much smaller and less massive than the MW, the collision will not be dramatic. Basically, the LMC and SMC will just "merge" into the MW, and the MW will still remain a spiral galaxy.

However, two or more large galaxies can also collide, and such collisions are much more dramatic. For example, our Milky Way and Andromeda, another large spiral galaxy, are attracting each other, and will eventually collide in about 2 billion years. This collision will involve the stripping of gas from the disks of both galaxies, and the violent event will also cause gas to compress and form stars. Eventually, after this encounter settles, what will remain is 1 extremely massive elliptical galaxy.

Numerous colliding galaxies have been observed, and 1 well known encounter is called the "Antennae," which has been observed with the Hubble Space Telescope and the Spitzer Space Telescope:

Also, galaxy mergers have been studied extensively using simulations. Using our understanding of gravity and the flow of fluids, theorists perform numerical simulations to describe an encounter. One simulation (movie) can be found at:

You can do a google search for "galaxy collision movie" (or similar keywords) and you will find many interesting movies from theorists that show what we think will happen when galaxies collide.

I hope this answers our question about what happens when (and if) galaxies collide.


Rahul Shetty

14. My question has to do with what happens when galaxies collide. The text book we are using has a cd in it, and on the cd there are video clips. One of those clips is of two spiral galaxies colliding. My question is if our galaxy collided with another would we be able to tell before hand, and what are the possible things that could happen to us? would we simply be incinerated by a passing star, or would it be possible for us to assimilate our two galaxies together without harm?

Our galaxy is actually currently "interacting" with small galaxies surrounding it, and has done so for a long time now. I say "interact" rather than "collide," because a collision implies a galaxy running into another head-on, as in an automobile accident. However, nearby galaxies can still affect each other with their gravitational forces, even if they don't run into each other. This is true of many of the small galaxies surrounding ours: the Milky Way is pulling apart these smaller galaxies with its gravity. Eventually the remaining bits of these galaxies may get swallowed up by the Milky Way.

However, suppose that a galaxy of a size similar to ours approached the Milky Way. We would know well in advance, because things in the universe move *very slowly*. It wouldn't happen in a single person's lifetime, or even over the span of all human history. It would take over a billion years for a major galactic collision to occur, and we would probably know several billion years before it even started that it was going to happen.

Once it did happen, though, the two galaxies would get seriously changed by the event (as you probably saw in your video clips). The Sun's orbit in the Galaxy would change, and the Sun might even get flung out of the galaxy. In this case we won't encounter many other stars. Alternatively, the Sun might end up in the Galactic center, where there's lots more stars than there are around us now, and the chances of interactions between the Sun and another star go up. There would also be plenty of radiation from various stellar events that would destroy life on Earth (if it still existed then!).

For more information, here's a nice website that talks in more detail about what would happen:

Best wishes,

Dave Rupke

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