![]() Nearly all stars appear to be composed mostly of hydrogen and helium, but their abundances of the heavier elements differ. These are based on analyses of the stars’ detailed spectra. We also now have good determinations of the compositions of stars. Population II, on the other hand, consists entirely of old stars that formed very early in the history of the Galaxy typical ages are 11 to 13 billion years. But so are the massive young stars in the Orion Nebula that have formed in the last few million years. For example, the Sun, which is about 5 billion years old, is a population I star. While some are as old as 10 billion years, others are still forming today. Population I includes stars with a wide range of ages. Today, we know much more about stellar evolution than astronomers did in the 1940s, and we can determine the ages of stars. The stars in globular clusters, found almost entirely in the Galaxy’s halo, are also classified as population II. Examples include stars surrounded by planetary nebulae and RR Lyrae variable stars. Some are in the disk, but many others follow eccentric elliptical orbits that carry them high above the galactic disk into the halo. These objects are found throughout the Galaxy. Population II stars show no correlation with the location of the spiral arms. Interstellar matter and molecular clouds are found in the same places as population I stars. Examples are bright supergiant stars, main-sequence stars of high luminosity (spectral classes O and B), which are concentrated in the spiral arms, and members of young open star clusters. Population I stars are found only in the disk and follow nearly circular orbits around the galactic center. We now know that the populations differ not only in their locations in the Galaxy, but also in their chemical composition, age, and orbital motions around the center of the Galaxy. Note the bulge of older, yellowish stars in the center, the bluer and younger stars in the outer regions, and the dust in the disk that blocks some of the light from the bulge. This neighboring spiral looks similar to our own Galaxy in that it is a disk galaxy with a central bulge. On this basis, he called the bright blue stars in the spiral arms population I and all the stars in the halo and globular clusters population II.įigure 25.20 Andromeda Galaxy (M31). He also noted the difference in color between all these and the bluer stars found in the spiral arms near the Sun ( Figure 25.20). We will discuss other galaxies in the next chapter ( Galaxies), but for now we will just mention that the nearest Galaxy that resembles our own (with a similar disk and spiral structure) is often called the Andromeda galaxy, after the constellation in which we find it.īaade was impressed by the similarity of the mainly reddish stars in the Andromeda galaxy’s nuclear bulge to those in our Galaxy’s globular clusters and the halo. His observations were aided by the darker skies that resulted from the wartime blackout of Los Angeles.Īmong the things a large telescope and dark skies enabled Baade to examine carefully were other galaxies-neighbors of our Milky Way Galaxy. As a German national, Baade was not allowed to do war research as many other U.S.-based scientists were doing, so he was able to make regular use of the Mount Wilson telescopes in southern California. The discovery that there are two different kinds of stars was first made by Walter Baade during World War II. (b) In this image, you see the motion of stars in the Galaxy’s halo in randomly oriented and elliptical orbits. (a) In this image, you see stars in the thin disk of our Galaxy in nearly circular orbits. Let’s first see why age and heavier-element abundance are correlated and then see what these correlations tell us about the origin of our Galaxy.įigure 25.19 How Objects Orbit the Galaxy. The stars in the thick disk are intermediate between these two extremes. Halo stars can plunge through the disk and central bulge, but they spend most of their time far above or below the plane of the Galaxy. The stars in the halo are old, have low abundances of elements heavier than hydrogen and helium, and have highly elliptical orbits randomly oriented in direction (see Figure 25.19). Young stars lie in the thin disk, are rich in metals, and orbit the Galaxy’s center at high speed. Look back at Table 25.1 and note some of the patterns. In the first section of his chapter, we described the thin disk, thick disk, and stellar halo. Explain why the oldest stars in the Galaxy are poor in elements heavier than hydrogen and helium, while stars like the Sun and even younger stars are typically richer in these heavy elements.Distinguish between population I and population II stars according to their locations, motions, heavy-element abundances, and ages.By the end of this section, you will be able to:
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