Lewis R. Dove, age 11, of Tucson, Arizona, for his question:
What are star populations I and II?
New stars are born as old ones die, so there are various age groups in the dazzling assortment. It seems that most medium aged stars tend to associate with the youth generation in certain celestial neighborhoods. These types are rated in the population I group. Aging stars in the population II group tend to be apart in remote neighborhoods. Astronomers suspect that these old timers may hold secrets about the ancient history of the Galaxy.
No two stars are exactly alike, but they can be classified in certain similar groups. When thousands of them are charted according to their brilliance and their particular colors, they arrange themselves in very definite classes. When plotted on a graph, the visible stars in our part of the Galaxy form a curve called the main sequence. This majority group is made up of population I type stars.
Most of its members are average stars in classes similar to our sun. Others are double and triple star systems. A small percentage are variable stars and a few are exploding stars. A group of colorful giants arrange themselves above the main sequence curve and a group of dwarfs are below it. All these are population I type stars. They belong in the flattened disc of the Galaxy, where the neighborhood is strewn with dusty gases for creating new stars. As a group, they favor the blue end of the spectrum.
Our enormous Galaxy also wears a stupendous halo, like a sphere of dazzling diadems around its crowded hub. It is set with billions of remote single stars and brilliant globular clusters. And every gem in the halo rushes around as though it were winding a celestial ball of yarn. Each globular cluster is a crowded globe of stars, like a swarm of golden bees. Its brilliance may outshine a 100,000 million suns.
These halo stars also are charted according to their colors and brilliance. But they do not arrange themselves in the neat classes of population I. Their colors favor the red end of the spectrum and neglect the blues. Their remote neighborhood beyond the dusty disc has little or no gaseous raw material to create new stars. So the gems in the halo are old timers. These and other differences set them apart from the disc stars and astronomers classify them in the population II group.
Our night sky is studded with population I type stars. Most of the population II group is too remote to be visible. A few globular clusters can be seen dimly. Telescopes have revealed about 100 others and no doubt more are hidden behind cosmic clouds. Apparently these aging clusters formed about five billion years ago, when our Galaxy was young. Other galaxies, young and old, have various systems of globular clusters. These patterns of old timers may provide clues to the life stories of the galaxies themselves.