Galaxies

Stars "hang out" in galaxies. Our sun (along with 200 billion other stars) make up our galaxy called the Milky Way. Perhaps you've seen the Milky Way (best seen in the summer months) and wondered what it was. It is difficult to picture our Milky Way because we are "inside". It is similar to asking a person to describe their house if they have only been in one room the whole time. Fortunately, there are many other galaxies we can see ... and this helps us understand our own galaxy much better.

Edwin Hubble studied galaxies in the 1920's and noted that they come in many shapes and sizes. He classified them into three groups:

Spiral Galaxies
- Spirals are flat with most stars concentrated in the center.
- Our Milky Way is this type.
- The closest spiral to us is the Andromeda Galaxy.
- Spirals contain much gas and dust - nebulae.
- Here are some other examples: The Whirlpool Galaxy , M83, The Southern Pinwheel Galaxy, a barred spiral galaxy (which may be the shape of our Milky Way ??), and yet another barred spiral, and a spiral seen edge on.
Elliptical Galaxies
- By far, the most common type of galaxy
- Very little gas and dust
- Have may old red stars
- Some examples are here, and here.
Irregular Galaxies
- The Small Cloud of Magellan (SMC) and The Large Cloud Of Magellan (LMC) very close to our Milky Way
- Other examples are here, and here
- The irregular shapes of these galaxies are most likely caused by gravitational interactions with larger nearby galaxies.

Our Milky Way

The Milky Way is a spiral galaxy, and the solar system is in one of the spiral arms (Orion Arm). We are in the plane of the Milky Way but far from its center (red arrow). The visible part of our Milky Way is about 100,000 light years in diameter (a light year is the distance light travels in one year) and contains about 200 billion stars. There is much gas and dust (nebulae) in the plane of the Milky Way, so it becomes difficult to see very far in that direction. An analogy would be a very dense low lying fog. If you look horizontally, you see nothing but if you look up, you have no problem viewing the sky. There are several small groups of stars ... known as globular clusters which orbit around the Milky Way (and concentrated towards the center.

How are galaxies distributed in space?

When we look out into distant space we see a lot of galaxies, but they are not evenly distributed. Galaxies tend to lump together into groups. Groups of galaxies tend to lump together to form clusters. Clusters of galaxies tend to lump together to form super clusters. Click here to see a picture.

The Milky Way is part of a gravitationally bound group called the local group. It consists of a few dozen galaxies including 3 spirals, a few irregulars and mostly small elliptical galaxies.

How many galaxies are there?

Certainly there are many, many galaxies ... several billion at least! (Wow! ... and each one has billions of stars.) The problem is .... every time someone builds a better telescope, astronomers find more galaxies!!!!

What are galaxies made of?

Now that's an interesting question. One might guess stars, gas and dust (nebulae), but that only makes up an estimated 10% of the mass of a galaxy. Astronomers are convinced that galaxies have much more mass than they can presently see ... and they don't have a clue what form it takes. This is based on the known rotation rates of galaxies and their motion within a group and also the strange way light is "bent" around distant galaxies. This "hidden matter" is known as dark matter and it may make up 90% of the mass of the universe! In fact, astronomers believe this dark matter resides in a "halo" surrounding the outside of the galaxy (just the opposite from the way the stars seem to be distributed ... that is, most stars are found in the galactic nucleus). What form could this matter take? Possible answers are:

White Dwarfs
Brown Dwarfs (objects whose mass lie between a star and a planet)
Black Holes (all three collectively known as MACHOS)
Neutrinos (or other very small atomic particle known as WIMPS)
Some form of matter no one even knows about

How are galaxies moving?

The galaxies within a group are gravitationally bound to the members of the group, so they are just moving randomly within the group itself (much like a swarm of bees). That means that galaxies within our local group may be approaching the Milky Way or moving away. However, when we look at all the other galaxies in the universe ... they all appear to be moving away from us! In fact, the farther a galaxy is from us ... the faster it appears to be moving. Edwin Hubble noticed this in the 1920's and is known as Hubble's Law. Do not think that this implies that the Milky Way is the center of the universe. An observer viewing the universe from any galaxy would make the same observation. The fact is ... the universe is expanding! Imagine you place dots on a balloon and then add air to the balloon. If each dot represents a galaxy, then no matter which dot you observe from ... you would make the same conclusion ... and that is ... everything appears to be moving away from you! Will the universe continue to expand? Say tuned.

Do Galaxies change?

Hubble felt that galaxies "evolve" from one type to another. Although his scheme was wrong ... his idea was correct. Galaxies within a group can interact (gravitationally) in ways that alter their shape and appearance. In fact, galaxies can have head-on collisions! When you look further in space, you are looking back in time as well. Astronomers look in deep space (distant past) and see a higher percentage of spiral galaxies than we see closer to us (more recent times). This implies that spiral galaxies are disappearing. Models show that collisions between spiral galaxies can "convert" them to elliptical galaxies!

What are Quasars?

When astronomers look to extreme distances ... they are looking back to the very early universe. There they see strange "star like" objects called quasars. Some are at distances of 12 billion light years away and more!!! That means that light from these objects have been traveling for 12 billion years (at 186,000 miles per second) to reach us. It also means that these objects represent a glimpse of the very early universe. So what are these things? If these objects are really as far away as we think, they must be producing enormous amounts of energy (to be seen at all).

The key to understanding quasars became clearer when it was discovered that super massive black holes lurk in the center of several nearby galaxies. These objects have masses many billion times that of the sun!!! There is evidence that our Milky Way has a super massive black in its center. How could these be made ... or maybe a better question is when? If in the early stages of galactic formation a black hole is formed in the center of a galaxy ... it could produce huge amounts of energy by "sucking up" debris in its vicinity. All this time it would be gaining mass. Later, after most of the debris was "cleaned up" ... it would become less energetic. This would explain why we don't see any quasars relatively close to us. When we look at a quasar we may be watching an extremely energetic event which occurred very early in the life of the universe - the formation of super massive black holes!