Stellar Populations

Stars fall into distinct groups or populations. The basic stellar populations are Population I stars and Population II stars. The sun and most stars near the sun are Population I stars. They are young second- to third-generation stars with compositions that include 2% of elements heavier than hydrogen and helium. Population II stars, on the other hand, are older stars whose compositions are just hydrogen and helium. The brightest stars in a group of Population I stars are blue and in a group of Population II stars are red. There are also additional subclassifications within this basic classification.

Population I

Population I stars have properties similar to those of the sun. They are less than ten billion years old and include newly-formed and still-forming stars. Because they are younger second- to third-generation stars, they contain heavy elements that were manufactured in previous generations of stars. (To astronomers, a heavy element is anything heavier than hydrogen or helium. These two lightest elements make up roughly 98% of the matter in the universe.) Population I stars contain roughly 2% heavy elements and 98% hydrogen and helium.

In a group of Population I stars, the brightest stars will be hot blue giants and supergiants. These stars, much more massive than the sun, are in the main part of their life cycles, burning hydrogen in their cores. The dominance of hot blue stars does not mean that cooler, less-massive red and yellow stars such as the sun cannot be Population I stars. Rather, the cooler stars like the sun are not as bright, so in a group of Population I stars viewed from a distance they will be less noticeable.

It turns out that the stellar populations also have dynamic properties in common. Population I stars are concentrated in the disk of the galaxy. They have circular orbits around the center of the galaxy with very little motion in a direction perpendicular to the galactic plane. They tend to have a patchy distribution within the disk and spiral arms of the galaxy. They also tend to be located in regions that have significant amounts of interstellar gas and dust, the raw materials for forming new stars.

Population II

The older Population II stars are usually over ten billion years old. Because they are first-generation stars that formed early in the history of the universe, they are devoid of heavy elements. Their composition is similar to that of the early universe. The brightest stars in a group of Population II stars are red giants. Red giants are stars in the process of dying. They have run out of hydrogen fuel in the core and swollen into cool red giants typically the size of the earth's orbit around the sun. Because they are so large, they are very bright and stand out in a group of Population II stars. Groups of Population I stars do not contain red giants simply because they are younger; they have not had enough time to exhaust the hydrogen fuel in their cores.

The Population II stars also have different dynamical properties. They are not confined to the plane of the galaxy. They have highly eccentric noncircular orbits that often go far above or below the plane of the galaxy to form a smoothly distributed spherical halo around the galaxy. They therefore must have significant components of their motions that are perpendicular to the plane of the galaxy. They also have much higher orbital velocities than Population I stars. Population I stars have orbital velocities that are typically about 5-6 mi (8-10 km) per second. Population II stars zip along at velocities ranging up to 45 mi (75 km) per second in the most extreme cases.