The stars, gas, and dust that make up the Milky Way can be grouped into two broad stellar populations that suggest how the galaxy evolved. The spiral arms and central plane of the Milky Way contain the interstellar gas, cosmic dust, and bright young stars categorized as Population I. The halo, spaces between the spiral arms, and central core of the galaxy contain the older, less spectacular stars that are categorized as Population II. This distribution can be explained by an evolutionary model in which an enormous cloud of gas and dust began to condense to form what are now Population II stars. The remaining gas and dust then collapsed, either suddenly or in stages, into the relatively thin disk in which Population I stars were (and still are being) formed.
Like other galaxies, the Milky Way is growing by absorbing small satellite galaxies. It is currently merging with the Large and Small Magellanic Clouds, a process that will be completed in about 100 million years. In 2003 a previously unknown galaxy was found to be colliding with the Milky Way. Its distinctive red stars are slowly being pulled into the Milky Way, and the dwarf will soon lose all its structure. Called the Canis Major dwarf galaxy after the constellation in which it lies, it is about 25,000 light years away from the solar system and 42,000 light years from the center of the Milky Way. This is closer than the Sagittarius dwarf galaxy, discovered in 1994, which is also colliding with the Milky Way. Several other galaxies are also, apparently, on a collision course with the Milky Way. The biggest and most spectacular collision will be with the Andromeda Galaxy. In about 2 billion years, massive tidal gravitational effects will tear spiral arms apart and start to shred the pinwheels from the outside in. The result will be an elliptical rather than a spiral Milky Way.
The Columbia Electronic Encyclopedia, 6th ed. Copyright © 2012, Columbia University Press. All rights reserved.