Origin of the Milky Way

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How old is the Milky Way? How did it form? Did the galaxy form "all at once" or did it form in stages? These are the central questions for this section.

Age of the Milky Way

In Unit 4 you learned how the HR-diagram (colour-magnitude) for a star cluster can be used to determine the age of the cluster. The turn-off point is the position along the main-sequence where stars are just beginning to move above and to the right of the main sequence. By carefully noting the spectral type for which main-sequence turn-off occurs and comparing this to stellar models, an age can be deduced. When this technique is applied to open clusters one finds ages that range from very young (Pleiades for example at 60 million years) to old (M 67 at approximately 4.5 billion years). Globular clusters are much older yet. Figure 12.13 shows the very old globular M15 in the constellation Pegasus. Also shown is the colour-magnitude diagram for M15.

Figure 12.13 Globular Cluster M15 and its colour-magnitude diagram (Image courtesy The King's University College Observatory)

The colour-magnitude diagram for M15 has a turn-off consistent with an age of about 12 billion years and makes it one of the oldest known globular clusters in the galaxy. The tremendous range in ages between open clusters and globular clusters suggests that the galaxy originally formed about 12 billion years ago but that star formation and the creation of open clusters is on-going on the galaxy.

Stellar Populations

By the end of the 1940's a peculiar anomaly had arisen. The best estimates for the distance of the nearby galaxy M31 (The "Andromeda Nebula") put the galaxy at roughly 1 million light years from the Sun. This was determined by observing Cepheid variables in M31. If that were true, however, it would imply that the globular clusters surrounding M31 were much fainter than the globular clusters surrounding the Milky Way. If one made the assumption that the globulars around M31 should be not different than those around our own galaxy then the distance to M31 would be more than two million light years. The mystery was solved by the American astronomer Walter Baade in 1952. Baade discovered that there are two distinct kinds of Cepheid variables. When a correction was made for this then the Cepheid-implied distance for M31 grew to 2.2 million light years. Furthermore the distinction between variables was also shared by other stars and this led to the idea of stellar populations. Figure 12.14 helps illustrate how subtle differences in the composition of stars leads to the idea of stellar populations. Both spectra are from stars that have essentially the same spectral class (G-type). Yet there are subtle but significant differences in the composition of the stars. The top star shows two very prominent Hydrogen lines and very faint traces of lines due to heavier metals. The bottom spectrum shows the same Hydrogen lines but now the metal lines are much more prominent.

Figure 12.14 Comparison of Population II and Population I spectra.

Stellar populations are related to the epoch or time in the history of the galaxy in which the stars formed. Population I stars are younger stars and are much richer in heavy elements when compared to Population II stars.

Example 12.7 Explain why it is reasonable to suspect that Population I stars formed after Population II stars.

Solution: Recall from Chapter 10 that heavy elements (metals) are created during supernova events. This would imply that the abundance of heavier elements is increasing over the history of the galaxy and that stars that form later will consist of material that has been enriched with heavy elements.

Baade discovered that population I stars that were Cepheid variables were intrinsically much brighter than population II Cepheids. Population II Cepheids are sometimes called W Virginis stars after the first population II cepheid that was discovered. Population I Cepheids are also called "Classical Cephieds".

Example 12.8 A Cephied variable is discovered to have a period of 20 days. Use the applet VariableStar to compare the absolute magnitude a population I and population II Cepheids that have this period. If you assumed that the variable you were observing was a population I star but it was in fact population II how would this effect any distance estimate that you would make?

Solution: As the applet indicates, a population I Cepheid with a period of 20 days has an absolute magnitude of about -5.1 while a population II Cepheid of period 20 days has an absolute magnitude of about -2.6. If you had mistaken the star for a population I Cepheid you would conclude that it was about 2.5 magnitudes fainter than it really was and hence you would over estimate its distance away from you.

Table 12.4 summarizes stellar populations.

How Stellar Populations Tell the Milky Way's "Story"

If you inspect Table 12.4 you will quickly notice that there is a pattern to how the stellar populations are distributed in the galaxy. The halo region of the galaxy is made up of old, population II stars. These stars are traveling around the galaxy on highly elliptical orbits that take them far above or below the galactic plane and, as they pass through the galaxy they move with high velocity at right angles to the plane of the galaxy. For this reason these stars are also referred to as high velocity stars. This is also the region in which, as you saw in the previous section, we suspect large amounts of dark matter exist. The nuclear bulge also contains population II stars that are not quite as old as the halo stars.

Population I stars are found exclusively in the galactic plane with the very youngest (extreme population I) stars found in the spiral arms of the galaxy. What do all of these "clues" tell us?

Stellar populations are the galaxies chronometers that tell us when specific events occurred in the history of the galaxy. The commonly accepted model is that the galaxy "condensed" from a vast cloud of primarily hydrogen and helium. The outer halo stars (including the globular clusters) formed first. Any initial rotation of this large cloud would have to be preserved (to conserve angular momentum) and the collapsing cloud would spin and flatten into a disk. Figure 12.15 and Table 12.5 summarize the steps astronomers think the galaxy went through as it collapsed.

	Phase 1: A spherical cloud of Hydrogen and Helium gas begins to collapse. Smaller regions collapse more quickly and these become globular clusters and halo stars
	Phase 2: The collapsing cloud is spinning and beginning to form a disk. The globular clusters and halo stars continue in their original orbits high above the galactic disk that is beginning to form. Stars are forming in the nuclear bulge region.
	Phase 3: Collapse has ended, the galactic disk has formed and a new generation of Population I stars is beginning to form
Figure 12.15 Phases in the formation of the galaxy.	Table 12.5 Steps in the formation of the galaxy.

Astronomers now understand that globular clusters are among the oldest objects in the universe - forming before the galaxy itself and with ages approaching 13 billion years which also marks the time at which the galaxy began to form. As you shall see in Unit 6, this indicates that star and galaxy formation began very early in the history of the universe.

Practice

1. Why do we think halo stars and globular clusters formed much earlier than stars found nearby - the Pleiades for example?
2. Population II stars still have some metal content. Why is this problematic and suggests the possibility of an even older population of stars?
3. An astronomer measures the period of a Cepheid variable to be 23.6 days. What additional information would she need before using the Cepheid as a "distance indicator"?
4. A Cepheid variable with a period of 12.8 days is discovered in a star cluster. Spectroscopy shows that the star has a very low metal abundance. If the apparent magnitude of the star is 12.6 estimate the distance to this star.