Astrophysicists know quite a bit about the life history of stars. In particular, they can be simulated by computer programs, since stars are basically gigantic hydrogen bombs and we know quite a bit about nuclear physics. 

But astrophysicists, by contrast, know very little about the life history of galaxies since it is so difficult, even with our supercomputers, to calculate the motions and evolution of hundreds of billions of stars all at once. There is considerable uncertainty about how galaxies first formed and how they evolve. Ironically, the Milky Way galaxy comes out every night, clearly visible as a gigantic swath of light cutting across the entire night sky, but physicists are clueless about the precise way in which it formed and evolved. 

One interesting piece of data, however, has come from analyzing our nearest galactic neighbor, Andromeda. The results of a five-year study of a very thick disc of older stars within the Andromeda Galaxy has yielded results that will essentially give us a greater understanding of how these galaxies form. The Andromeda Galaxy has often been referred to as Messier 31, M31 and even NGC 224 and is in fact the nearest spiral galaxy to our own at a distance of 2.5 million light years away or 1.46962495 x 10^19 miles. Last year, a team of astronomers reported that Andromeda was formed out of the collision of two smaller galaxies between 5 and 9 billion years ago. (We now know that galaxies often collide and in fact cannibalize or devour smaller galaxies. It is believed, for example, that our own Milky Way galaxy will eventually collide and merge with the larger Andromeda galaxy billions of years from now, eventually creating a gigantic elliptical galaxy from the collision.) 

The Andromeda galaxy however is not alone and in fact is a member of the Local Group which is a group of more than 30 galaxies including the Milky Way and is about 10 million light years in diameter. It's estimated that over half (approximately 70%) of the total stars within the Andromeda Galaxy currently reside within the stellar disc. The stellar disc of Andromeda is relatively flat and surrounding the central bulge of the galaxy which is comprised of much older stars that formed billions of years ago. The generalized formation of these discs have in a sense been a mystery and otherwise not well understood even though we understand the composition and evolution of the galaxy itself.