Galaxy Snapshots; Zsolt Frei, Institute of Physics, Eötvös University, Budapest
As the Universe cooled and expanded after the Big Bang, we know that gravity pulled the matter in small regions of the Universe together to form the first galaxies of stars. Those stars shone, using up their fuel and evolving into white dwarfs, neutron stars and black holes, and producing the metals that would be incorporated into later generations of stars. At the same time, more matter was collected by these first galaxies, causing them to grow, and providing material for yet more stars.
Although this basic idea seems simple, the results that we observe in nature are rather complex. Astronomers often classify galaxies into three broad types:
Elliptical galaxies have oval shapes, and their stars move in all directions within the galaxy. Elliptical galaxies contain very old stars, and rarely are seen forming new stars. This is because the raw material for stars --- gas and dust --- has been eliminated from the galaxy. It is not certain where the gas has gone. It seems unlikely to astronomers that the gas was used up completely by forming stars, so it is generally thought that some violent process ejected it from the galaxy. What that process is is still a matter of debate.
Sprial galaxies might be better called disk galaxies, because they are flattened like plates. The stars all move in the same direction, orbiting the center of the galaxy. Large amounts of gas and dust reside in these galaxies, which provides material for stars to form continuously. Moreover, in the rotating disk, gravity amplifies small disturbances in the density of gas, causing stars to form in great waves that travel around the galaxys' disks. These form the spiral arms that give the galaxies their names. Our own Milky Way Galaxy is a spiral galaxy. We reside about half-way out from the center of the Galaxy. When you see the Milky Way in a dark night sky, you are looking toward the center of our Galaxy.
Dwarf galaxies get the name because they are small, rarely more than 10% of the mass of our Milky Way Galaxy. The young ones tend to be irregularly-shaped, presumably because their gravity is to weak to have to enforce more regular spiral or elliptical shapes during the short span of time they have been forming stars. The older ones are often shaped similarly to elliptical galaxies.
So how did these very different types of galaxies emerge? It is generally thought that when a galaxy first forms, it is likely to have a disk-like shape. The disk is a common shape throughout astronomy, because random motions get amplified into rotation when matter collapses to form galaxies on a large scale, and stars on a small scale. The disk is the most stable shape for a rotating object, whenever the centrifigal force is comparable to the gravitational force. This is consistend with the idea that young galaxies will tend to be spiral galaxies.
Elliptical galaxies are thought to form over time when galaxies collide and merge. Since elliptical galaxies tend to be old, plent of time should have passed for collisions to occur. Moreover, elliptical galaxies tend to be found in regions where there are lots of other galaxies, such as at the centers of clusters of galaxies, where collisions between galaxies might occur more often.
This description is a good starting point, but it is over-simpliefied. When we look at our own Galaxy, we see two distinct populations of stars. There is a young population, and that tends to be found in the disk of the Milky Way, where there is plenty of gas and dust to form stars. However, there is also an old population of stars, whose members all trace random orbits around the center of our Galaxy. Viewed on its own, the old population looks a bit like an elliptical Galaxy. Many spiral galaxies contain spherical halos of older stars, which may be produced when smaller galaxies collide with them, but fail to completely disrupt the disk.