Stellar Populations in the Galaxy
Walter Baade's Discovery of Population I and II Stars
Discovery
During World War II American scientists served on the Manhattan Project. Walter Baade was however an exception. He was born in Germany and his citizenship documents were lost. He was therefore not allowed to work on war related projects.
Baade was a staff astronomer at Mount Wilson Observatory, just north of Los Angeles. Without the usual competition for telescope time from other astronomers, such as Edwin Hubble, Baade had more access to the Hooker telescope, which at 100 inches was the world's largest at the time. Wartime blackouts reduced the light pollution from nearby Los Angeles and gave Baade better sky conditions.
With ideal conditions, Baade obtained excellent photographs that for the first time resolved individual stars in the Andromeda Galaxy. Baade discovered two distinct populations of stars: Population I and Population II. These stellar populations also occur in the Milky Way and other galaxies.
Population I and Population II Stars
Population I stars are less than 10 billion years old, including newly formed stars. Population II stars are older. At about 5 billion years age, the Sun is a Population I star and shares their other characteristics.
Astronomers call any element that is heavier than hydrogen or helium a metal. Population I stars contain roughly 2% metals and 98% hydrogen and helium, by mass. Population II stars on the other hand are virtually devoid of metals.
In a group of young stars, the brightest stars will be massive O and B spectral class stars. These hot luminous stars are bluish in color, so galaxies or regions of galaxies consisting of Population I stars will appear blue. There are cooler red and yellow Population I stars, such as the Sun, but they are fainter than the hot blue Population I stars. The red light is overwhelmed by the blue light originating from a group of Population I stars.
Because they are beginning to die out the brightest stars in a group of older stars will be red giants and red supergiants. A galaxy or region of Population II stars will therefore appear red in color. After 10 billion years the hot luminous blue stars will have evolved to the red giant stage and died out in supernova explosions.
Population I stars have approximately circular orbits around the center of the galaxy that are confined to the plane of the galaxy. They are located in the disk and spiral arms of the Milky Way and other spiral galaxies. By contrast Population II stars will have highly elliptical orbits that are not confined to the plane of the Milky Way galaxy. They are found throughout the spherical halo of the galaxy. These elliptical orbits require Population II stars to zip around their orbits much faster than Population I stars.
Other Populations
Modern astronomers have expanded Baade's initial classification of Population I and II stars based on how strongly stars display the Population I or II properties. The five populations are: Extreme Population I, Older Population I, Disk Population II, Intermediate Population II, and Halo Population II. Some astronomers also think that there was a short lived initial generation of massive Population III stars that no longer exists.
Further complicating the classifications, there are Population I stars in the Magellanic that contain few metals. There are also Population II stars near the core of the Milky Way galaxy that contain metals.
Why the Populations?
The initial big bang made no metals other than trace amounts of lithium and beryllium. Elements other than hydrogen and helium were made in stars and blasted back into space by supernova explosions of massive stars. The first generation Population II stars therefore contain only hydrogen and helium. Later generation Population I stars contain 2% metals that were manufactured by the massive Population II stars that have short lifetimes and have died out in supernovas.
Stellar populations help us understand the evolution of stars, galaxies, and the universe.
Further Reading
Morrison, D., Wolff, S., and Fraknoi, A., Exploration of the Universe Saunders 1995.
Zeilik, M., Astronomy: The Evolving Universe,. Cambridge, 2002.
