Stellar populations

With the general outline of the Galaxy developed, astronomers turned to working out its structure in more detail. One new insight came by accident, helped by World War II. In 1944 Walter Baade was working at the 100-inch telescope at Mount Wilson Observatory. As it was wartime, the German-born Baade was restricted to staying in the Mount Wilson-Pasadena area. Because his colleagues were off working for the government, Baade had access to plenty of time on the 100-inch telescope. In addition, Los Angeles and neighboring towns were blacked out, resulting in an unusually dark sky. Using film sensitized to red light, Baade photographed the Andromeda Galaxy, M31, and resolved its central bulge into stars. When he made an H-R diagram of the stars, he found that most of the luminous stars in the central bulge were red giants, whereas the most luminous in the disk were hot blue stars. [The same is true for other galaxies: note the difference in color between disk and bulge for M51.]

He was impressed by the similarity of the stars in the bulge to those in the globular clusters studied by Shapley. He also noted the differences between these stars and those found in the solar neighborhood or in galactic clusters. From those observations

Baade proposed that the stars in a galaxy-like ours or M31-fall into two groups, which he called Population I and Population II. Baade noticed also that the two populations were found in different regions of the Andromeda Galaxy and inferred the same situation in our Galaxy. Astronomers had found the Galaxy to be shaped like a thin disk. Viewing some other galaxies, such as Andromeda, they noted a bright nuclear bulge in the central region; its core is the Galactic Center. Globular clusters mark out the third general region of the Galaxy: a halo that is spherically distributed around the nucleus. The halo consists of globular clusters, some stars, and a little hot gas.

What kinds of star populate these three regions of the Galaxy? Baade concluded that the stars in the disk outside the nuclear bulge are mostly Population I. The nuclear bulge he labeled as primarily Population II. And the halo stars, which are mostly in globular clusters, he also assigned to Population II.

Although it marked a first step toward working out the details of galactic structure, astronomers now recognize that Baade's division of stars into two and only two populations was too simple. We have since learned that the stars in globular clusters are not the same as those in the nuclear bulge. Although both groups contain numerous red giants, the globular cluster stars contain a much smaller abundance of heavy elements, whereas the nuclear bulge stars have about the same abundance found in the sun. Astronomers today, therefore, refer to stars in the nuclear bulge as old Population I, those in the disk as young Population I, and only those in the halo as Population II. Some astronomers make even more divisions, for there is no strict boundary between groups, but for simplicity, we will stick with these three.

We've encountered the idea of stellar populations before, particularly in Chapter 20, in the context of stellar evolution, where we discussed the concept in terms of age as well as chemical abundance. Interstellar matter, as well as the young stars formed from it, such as OB associations, belongs to young Population I.

More evolved groups of stars, the galactic clusters with many red giants, belong to old Population I. The planetary nebulae, ejected from evolved red giants, belong to this population also. The very oldest stars, those in globular clusters, also have low metal abundances. They belong to Population II. The supernovae that are produced by core collapse in a massive star (Type II) belong to the young Population I, whereas those that evolve from low mass stars (Type I) belong to Population II. Similarly, there are cepheids in both populations: massive stars that cross the instability strip as they evolve are Population I (classical] cepheids, low-mass stars that evolve into the instability strip after core helium burning are W Virginis stars, and those that settle on the horizontal branch at the position of the instability strip during core helium burning are RR Lyrae stars.