General Astronomy/Galactic Evolution
(This section is incomplete.)
The question of which came first, stars or galaxies, was once the "chicken and egg" question with no real answer. However, more recent evidence points to vestigial structures in space that ultimately led to the formation of galaxies, long before the formation of any stars. Thus, galaxies came first and stars formed within them.
Astronomers have pieced together the evolution of galaxies not from any one particular galaxy, because of the enormous time scales involved, but by reference to many galaxies in different stages. It is possible, for example, to note common characteristics in all spiral galaxies. Astronomers also can view various spirals at various ages to develop a relative time sequence. If we look at a galaxy that is 5 billion light years away, we know that it is at least 5 billion years old. The distance, and hence minimum age, can be determined by the shift in spectral lines from the light of the galaxy via the concept of Red Shift and the Hubble Law.
Galaxies form from immense clouds of intergalactic gas in three basic types: spiral, elliptical and irregular. Ultimately, the type of galaxy is determined primarily by the dimensions and mass of the gas cloud, and the amount of angular momentum it contains. The angular momentum determines how much the cloud begins to spin as it contracts under the force of gravity. Clouds with large angular momenta spin faster and flatten into a disk-like structure that evolves into a spiral galaxy. Clouds with little angular momenta spin little if at all, and the central force of gravity pulls them into a near-spherical or elliptical shape, appropriately forming an elliptical galaxy. Most large galaxies are of these two types.
There is good evidence to believe that when two similarly-sized spiral galaxies collide, the ultimate result can be a large ellipical galaxy. Although no such single process has ever been observed, there are numerous examples of colliding galaxies.
Smaller clouds of gas may be so loosely bound by gravity that they have not specific or definable shape, and are known as irregular galaxies.
In addition, there are also "strange" or "peculiar" galaxies, likely the result of not of the original formation of the galaxy, but rather some subsequent event, such as the collision of two galaxies.
Once a galaxy forms, its subsequent evolution is determined primarily by the conditions in its nucleus, and the amount of gas left over for further star formation. Astronomers have determined that the core of most newly formed galaxies is dominated by a supermassive black hole (section 13.5). Young galaxies typically are quite energetic due radiation formed from material that spirals down into this supermassive black hole (SBH). This is likely the source of energy of active galactic nuclei and quasars (Section 13.6) Over time, all of the material that is close enough to the SBH to be pulled into it diminishes and with it the strong radiation from the core. Thus, young galaxies are very energetic, but they calm down with age.