7. EVOLUTION OF THE MILKY WAY:

In 1944, Walter Baade, the same scientist who predicted neutron stars in supernova remnants, had a very important insight. He noticed that the Milky Way is a composite of two distinct stellar systems, which he called Population I and Population II. The Population I stars were located in the disk and spiral arms of the Milky Way, while the Population II stars were located in the central bulge and halo of the Milky Way, as indicated in the diagram below.

Baade noticed that the Population I stars were associated with gas and dust in the disk and spiral arms of the Milky Way. Population I includes young stars that must have formed fairly recently, while Population II has no stars younger than 10 billion years. But Population II stars are not concentrated in the disk of the Milky Way; they are distributed in the bulge and halo of the Milky Way, which have a more spherical distribution. Finally, Baade recognized one more remarkable fact: there was a correlation between the locations of the stars and the concentration of heavy elements (such as C, N, O, Fe, etc.) seen in their spectra. The Population I (young disk) stars had relatively high (about 2%) fractional abundance of heavy elements (compared to hydrogen and helium); while the Population II (old bulge and halo) stars had much lower (<1%) concentration of heavy elements. These observations are summarized in the table below.

Actually, most of the facts in the above table were already known in the 1940s, but just as isolated facts. Baade's great contribution was to recognize that they all formed a consistent pattern. Taken together, these facts lead to an understanding of how the Milky Way was formed in the first place. Some 10 - 14 billion years ago, there were no stars. The Milky Way was just a giant amorphous cloud of gas that was collapsing because of its own gravity. This primordial gas cloud contained virtually no heavy elements, only hydrogen and helium. Then, as the cloud collapsed and became more dense, it fragmented into many smaller gas clouds, from which the first generation stars (Population II) began to form. (We will discuss star formation in Lesson 9). They probably formed in clusters. Indeed, the globular clusters seen in the halo of the galaxy are the residues of these primordial star clusters. Once formed, these stars and clusters will move forever in orbits similar to the chaotic motions of the collapsing gas cloud as illustrated below.

The young Population II stars included massive stars, which, as we have learned, make supernova explosions. The supernovae produce heavy elements and return them to the interstellar gas. Thus, as the primordial gas cloud of the Milky Way continued to collapse, it gained an increasing concentration of heavy elements produced by this first generation of stars. Also, as the gas collapsed it began to rotate, just as water begins to swirl as it goes down the drain. Because of this rotation, the gas of the Milky Way began to flatten. It continued to flatten as it shrank, ultimately forming the thin disk that we see today. Because this first generation of Population II stars formed before the gas of the Milky Way had settled into a disk, they continue to move about the Milky Way in random orbits, as shown in the diagram above. The gas cloud collapsed to a disk in less than a billion years, so no new stars have formed in the halo of the Milky Way for more than 10 billion years. Thus, the only stars remaining in the halo of the Milky Way are low-mass (< 0.8 solar) main sequence stars, which have lifetimes greater than 10 billion years, and stellar remnants such as black holes, neutron stars, and white dwarfs, all of which are very difficult to detect. In contrast, the Population I stars are formed in the rotating disk of gas, which has already been salted with heavy elements from the first generation of supernovae. Since star formation is ongoing in the disk, the concentration of heavy elements continues to increase in the gas there. The Sun (which was formed about 5 billion years ago) has a fractional abundance of heavy elements of about 2%, which must have been the abundance of heavy elements in the interstellar gas at that time. Moreover, since the Sun was formed from gas that was already rotating in a thin disk, the Sun has continued to circle the center of the Milky Way along with the disk gas from which it was formed. Actually, the dichotomy between Population I and Population II is not that sharp. There is a continuous gradation of heavy element abundance in the stars, with the stars in the outer halo having the lowest abundances, the stars in the bulge having higher abundances, and the stars in the inner disk having the highest abundances. This gradation is entirely consistent with the picture above.

For a detailed summary, see Origin of the Milky Way, by Prof. Barbara Ryden of Ohio State University.

This scenario for the origin of the Milky Way makes sense, but we have a long way to go before we can say that we understand it fully. It would be great if we could reconstruct the history of the collapse of the primodial gas cloud that produced the Milky Way and give a detailed accounting of the correlation of the abundances of heavy elements in stars with their locations. But to do that, we need to understand in detail just how collapsing gas clouds fragment into stars. As we shall describe Lesson 9, that is one of the great challenges of modern astronomy.

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Last modified March 3, 2002
Copyright by Richard McCray