If you were living a hundred years ago, it would be easy to say what a galaxy was. There was one galaxy, the Milky Way, and people lived in it. It was the Galaxy, all that existed. Today we call the whole of everything the Universe, because there are billions of other galaxies.
Galaxies have large numbers of stars kept together by gravity - but they also have star systems like the Solar System.
The stars make the galaxies visible to us, but a galaxy is more than a collection of stars. For one thing, stars have orbiting planets and other bodies. NASA's Kepler Mission spacecraft has found hundreds of planets. Astronomers think that there are at least 50 billion planets in the Milky Way Galaxy. There must also be moons, asteroids and comets, but these are still too small to find.
Between the stars there is gas and dust.
The gas and dust is very spread out – we say it is diffuse. But there's a lot of space between the stars, so some galaxies have large amounts of gas and dust. It's important, because it's the material for making new stars.
Galaxies also have two surprising things: black holes and dark matter.
We know that the Milky Way and many other galaxies have a black hole in the center. Its mass is millions of times the mass of the Sun. The gravity of a black hole is so strong that light can't escape, so we can't see it. But its gravity is also so strong that it has a big effect on the orbits of nearby objects. We can see that.
Dark matter is dark because we can't see it with any kind of radiation. It's also dark because we don't know what it is. That's embarrassing, because most of the Milky Way seems to be made of it. The only reason astronomers think it exists is because of the effects of its gravity.
We classify galaxies using Edwin Hubble's system.
Most galaxies that astronomers see are either elliptical or spiral. Elliptical galaxies are shaped like watermelons, eggs, cigars and similar things. Their stars are old and reddish in color, so it makes the galaxy look rather yellow. There's very little dust, so we don't find new stars in elliptical galaxies.
Spiral galaxies are the really pretty ones you see in pictures. Click here to see a side view of the Milky Way, which is a typical spiral galaxy. It shows a disk with a bulge in the middle. The disk just looks like a line when you see it from the side.
The stars in the disk are in spiral arms that come out from the center. There are many young stars there because this is where new stars are made. Often young stars are very hot, so they give a blue color to the disk. The disk also has lanes of dust and gas which are the material for new stars.
The bulge is full of stars too, but they're older stars like those in elliptical galaxies. The galactic center is in the middle of the bulge and this is where the black hole is.
A spherical halo surrounds the whole galaxy. It isn't labeled on the diagram, but the globular clusters are part of it. Globular clusters are large groups of stars pulled together by gravity into a globe-shape. The halo stars are the oldest ones in the galaxy.
There is an even bigger dark matter halo surrounding the one we can see.
Some galaxies don't seem to have much shape. They are called irregular galaxies. Some of them are an odd shape because two galaxies are coming together to make a new one. We call this merging.
There are giant galaxies and dwarf galaxies.
Dwarf galaxies come in different shapes, but there aren't any spirals. The very smallest dwarfs may have only ten million stars, but a large dwarf can have ten billion.
Most of the dwarfs we know about are companion galaxies to a larger ones. A companion galaxy is separate from the large galaxy but kept close to it by gravity. Sometimes the dwarf is pulled close enough for the large galaxy to swallow it up.
The Milky Way is a giant spiral galaxy. It's over 100,000 light years across and has between 200 billion and 400 billion stars. Our neighboring large spiral Andromeda has three times as many stars as that. But Andromeda's not the biggest galaxy we know. There are giant elliptical galaxies that are much bigger. The largest known galaxy is about a hundred times bigger than Andromeda.
The large elliptical galaxies probably got so big through mergers. A long time into the future, Andromeda and the Milky Way will collide to produce such a giant.
My Pinterest board Galaxies has images of different kinds of galaxies.