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Star Clusters

Distribution of Globular Clusters in the Milky Way

Nearly all stars are formed in groups or clusters of some kind. There is no real dichotomy between globular clusters and open clusters but rather a continuity of properties and perhaps of formation processes. Figure 06-02a shows the blurred dividing line between globular clusters (triangles) and open clusters (circles) in a plot with age against metal content. The halo of the Milky Way was built up over a period of at least a few billion years, perhaps by an inside-out accretion process. This process continued at least until the youngest outer halo clusters were formed about 10 billion years ago. The fact that the oldest stars and clusters in the galactic disk are about as old as the youngest halo clusters suggests that the galactic disk may have formed only after cluster
Distribution of Globular Clusters formation in the halo was completed. The globular clusters are more massive because they formed earlier in an environment of high density and low angular momentum. Apparently, the first systems to condense in the early universe had no angular momentum initially, since all the matter had emerged from the same infinitesimal volume of space; angular momentum was generated only later by tidal torques. Therefore, it seems to point to a close connection between globular clusters and cosmology, and suggests that the globular clusters are fossil remnants of the earliest condensed structures to form in the densest parts of the universe. Observations outside the Milky Way also

Figure 06-02a Star Clusters [view large image]

show the high frequency of globular clusters in some giant galaxies such as M87 and near the center of the Sagittarius dwarf galaxy where matter density should be much higher.

Distribution of Globular Clusters The left diagram in Figure 06-02b shows the distribution of globular clusters in the halo with low metal content (older); while the right one shows the distribution in the galactic disk with higher metal content (younger). Most of the younger globular clusters are concentrated toward the galactic nucleus where the matter density should be much higher. [Fe/H] is the ratio between abundances of iron and hydrogen in logarithmic scale, it is a measure of the metal content of the object.

Figure 06-02b Globular Clusters in the Milky Way [view large image]

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