Star Populations 
The Milky Way arches high overhead on December evenings. This faint band passes from the Northern Cross, which is in the west, to W-shaped Cassiopeia high overhead, to near the face of Taurus, the bull, in the east.
The Milky Way glows a little more softly at this time of year than it does in summer. That’s because we’re looking away from the galaxy’s crowded center and toward the thinly settled edge of its disk. And beyond that edge is the halo — an extended volume of space that contains the galaxy’s oldest stars.
Astronomers classify the stars in the halo as Population II, while most of those in the disk are Population I. The difference isn’t just where the stars reside — it’s what they’re made of.
All stars consist mainly of hydrogen and helium, which were created in the Big Bang. But they also have a smattering of heavier elements. These elements were created in the hearts of other stars, then hurled into space as the stars died, where they could be incorporated into new stars.
Population II stars formed when there were almost no heavier elements around, so they have only tiny amounts of them. But Population I stars — stars like the Sun — are younger, so they have higher proportions of heavy elements. It’s still not much, but it’s enough to set these stars apart from their older kin.
One class of Population II star is in a stage of life that helps astronomers measure the size of the universe. More about that tomorrow.
Script by Damond Benningfield, Copyright 2012
This program was made possible in part by a grant from NASA.