Cosmic Clocks 
Cosmic clocks are ticking all across the Milky Way galaxy. They tell us the ages of the galaxy’s first stars and its broad, flat disk. And they tell us that the galactic retirement homes are outside the disk, in a huge bubble of space known as the halo.
The clocks are white-dwarf stars -- the final stage of life for the Sun and about 98 percent of all the other stars in the galaxy.
A white dwarf is a star’s dead core. Because it no longer produces nuclear energy, it shines only because it’s very hot. Over time, though, it cools and fades. So by measuring a white dwarf’s temperature and brightness, astronomers can determine its age -- how long it’s been a white dwarf.
The oldest white dwarfs in the Milky Way reside in globular clusters -- densely packed families of hundreds of thousands of stars in the halo, which extends far beyond the galaxy’s disk. The oldest white dwarfs in the clusters are about 12 billion years old, and perhaps a wee bit older. That means they formed when the universe itself was quite young.
But the Milky Way’s disk is another matter. The disk contains hundreds of billions of stars, including the Sun. It’s outlined in our sky by the glowing band of light known as the Milky Way. The oldest white dwarfs in the disk are only about nine billion years old. That means the disk took shape long after the globular clusters -- two auspicious events marked by the ticking of cosmic clocks.
More about white dwarfs tomorrow.
Script by Damond Benningfield, Copyright 2012