A Split Disk
The hazy band of the Milky Way puts in its best showing on summer nights. It arcs high across the east at nightfall, and directly overhead by around 11 o'clock. It stretches from W-shaped Cassiopeia in the north, to the Summer Triangle overhead, to the teapot-shaped Sagittarius constellation in the south.
The band of light we see as the Milky Way is the combined glow of millions of stars in the disk of the Milky Way galaxy.
As astronomers discovered a couple of decades ago, the disk consists of two main components. The main part of the disk, which contains most of the galaxy's stars -- including our own solar system -- is the thin disk. Like a layer of meat inside a pita bread, it's surrounded by a thick disk.
Several studies are shedding new light on this portion of the galaxy, including one headed by Texas astronomer Carlos Allende Prieto. The Texas team examined readings of almost 23,000 stars made over the last few years by a telescope in New Mexico. All the stars are of a similar class to the Sun. The data included information on each star's composition, temperature, and motion through space. From this, the astronomers determined each star's age and location.
Like other studies, this one indicates that stars in the thick disk are older than those in the thin disk. They also orbit the center of the Milky Way more slowly. That suggests that these two parts of the galaxy's disk had different beginnings. More about that tomorrow.
Script by Damond Benningfield, Copyright 2006
For more skywatching tips, astronomy news, and much more, read StarDate magazine.
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