Thick and Thin
Our home galaxy, the Milky Way, is a giant. Its bright disk spans a hundred thousand light-years, and contains several hundred billion stars. And it's getting bigger all the time as it pulls in small "dwarf" galaxies and incorporates their stars and gas into its own body.
Like other galaxies, the Milky Way probably grew from the mergers of a lot of dwarf galaxies -- a process that gave birth to early generations of stars. The process also split the galaxy's disk into two regions -- a "thick" disk, and a younger "thin" disk that's sandwiched inside it.
Right now, astronomers are trying to work out the details. Astronomers at McDonald Observatory, for example, have studied the composition of many stars to determine which part of the disk they belong to. This information will help narrow down the possible ways in which the galaxy came together.
In one scenario, the thick disk formed by taking stars from the smaller galaxies it was gobbling up. This process shut down starbirth in the thin disk. Eventually, though, starbirth began again, creating later generations of stars.
And in another scenario, as the smaller galaxies came together, their gas gave birth to many stars -- the thick disk. Later, more gas fell inward, providing the raw materials for the stars in the thin disk.
Detailed studies of stars throughout the galaxy should help astronomers decide just how the Milky Way's two-part disk was born.
Script by Damond Benningfield, Copyright 2006
For more skywatching tips, astronomy news, and much more, read StarDate magazine.
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