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. A Population III classification is sometimes also used; however, no stars within the Milky Way galaxy exist as Population III stars. There are also additional sub–classifications within the basic classification for Population I and Population II stars.
German–born American astronomer Wilhelm Heinrich Walter Baade (1893–1960) discovered the stellar populations during World War II (1939–1945), which contributed to his discovery. During the war, most scientists worked on war–related projects, such as the Manhattan Project. Baade, who was on the staff of Mount Wilson Observatory near Los Angeles, California, was not allowed to work on war projects because he was German–born. He did, however, conduct research at Mount Wilson Observatory, which at the time had the world’s largest telescope. The wartime blackouts in Los Angeles contributed to darker skies on Mount Wilson. There were also few astronomers wanting to use the telescope because they were working on war–related projects. So Baade had plenty of time to use the world’s largest telescope under better than normal sky conditions.
During this time Baade was able to resolve, for the first time, the stars in the Andromeda galaxy. He noticed that there were two distinct populations composed of predominantly red and blue stars. He labeled them Population I and Population II. Further study since that time has produced the additional sub–classifications.
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.
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 non–circular 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 to 6 mi (8 to 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.
Like most initial classifications, the division of stars into Population I and Population II stars is a bit of an oversimplification. Astronomers now classify stars into five distinct populations based on how strongly they exhibit the Population I or II characteristics. These populations are: Extreme Population I, Older Population I, Disk Population II, Intermediate Population II, and Halo Population II. Astronomers are currently arguing over whether these groups represent distinct populations, or a gradual blending from Population I properties to Population II properties.
There are also some exceptions to the rule that young Population I stars have heavy elements while older Population II stars do not. For example, the Magellanic Clouds, irregular companion galaxies to the Milky Way galaxy, are Population I stars with few heavy elements. The core of the Milky Way also contains Population II stars that do contain heavy elements. Why? Most likely, in the core of the galaxy there was a very rapid generation of very massive stars. They are gone now, but they enriched the core with heavy elements very quickly. The high concentration of stars near the core contributed to this process. The opposite occurred in the Magellanic Clouds. Star formation proceeded so slowly that there was not an early generation of massive stars to produce heavy elements.
These different populations can be understood in the context of stellar evolution. When the universe formed in the Big Bang, only hydrogen and helium were made. The heavy elements were made later in the cores of stars. Therefore, the older Population II stars are deficient in heavy elements, while the younger Population I stars contain heavy elements that were made by the massive first generation stars. Massive stars manufacture and recycle heavy elements. These stars are blue giants and supergiants most of their lives, which are very short by stellar standards. These stars are the brightest blue stars in a group of Population I stars. In a group of older Population II stars, these stars have finished their life cycles, so the brightest stars are the red giants formed near the end of stellar life cycles. The distribution of these different populations is then related to the evolution of the galaxy.
Blue giant, supergiant —The most massive stars in the hydrogen burning stage.
Heavy elements —To astronomers, anything that is not hydrogen or helium.
Population I —The younger second– to third–generation stars.
Population II —The older first–generation stars.
Stellar population studies help astronomers understand stellar evolution, evolution of the galaxy, and the history of the universe.
Arny, Thomas. Explorations: An Introduction to Astronomy. Boston, MA: McGraw–Hill, 2006.
Bacon, Dennis Henry, and Percy Seymour. A Mechanical History of the Universe. London: Philip Wilson Publishing, Ltd., 2003.
Chaisson, Eric. Astronomy: A Beginner’s Guide to the Universe. Upper Saddle River, NJ: Pearson/Prentice Hall, 2004.
Paul A. Heckert