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A composite color-magnitude diagram for a "metal-poor" globular cluster, constructed from real photometric data from several Milky Way globular clusters including M3, M55, M68, NGC 6397 and NGC 2419. [William Harris, McMaster University, STScl/NASA 10th Anniversary Symposium Proceedings (STScI, Baltimore), May 2000]

Fig 13.11.

A composite color-magnitude diagram for a "metal-poor" globular cluster, constructed from real photometric data from several Milky Way globular clusters including M3, M55, M68, NGC 6397 and NGC 2419. [William Harris, McMaster University, STScl/NASA 10th Anniversary Symposium Proceedings (STScI, Baltimore), May 2000]

clusters. For globular clusters, only the lower (cooler) part of the main sequence is present. All earlier spectral types have turned off the main sequence. This tells us that globular clusters must be very old. Globular clusters contain a large number of red giants. In Chapter 10 we saw that the red giant state is symptomatic of old age in a star.

13.5 I The concept of populations

There is another important difference between the stars in galactic and globular clusters. It concerns the abundances of "metals", elements heavier than hydrogen and helium. Many globular clusters have stars with very low metal abundances, while galactic cluster star are higher in metal abundance. We refer to high metal stars as population I stars and low metal stars as population II stars. We have a general sense that population I stars represent younger, more recently formed stars. We interpret the metal abundance differences as reflecting the conditions in our galaxy at the time each type of star was formed. When the older stars were formed, our galaxy had only hydrogen and helium. When the newer stars were formed, the galaxy had been enriched in the metals. This enriched material comes from nuclear processing in stars, followed by spreading into the interstellar medium, especially through supernova explosions.

The differences between galactic and globular clusters start us thinking about old and new material in our galaxy. The globular clusters are older, and form a spherical distribution, while the galactic clusters are newer and are confined to the galactic disk. This suggests that, a long time ago, star formation took place in a large spherical volume, but now it only takes place in the disk. This is supported by the fact that globular clusters are free of interstellar gas and dust, the material out of which new stars can form, while galactic clusters are sometimes associated with gas and dust.

The concept of stellar populations is important in our understanding of the evolution of our galaxy. This will be discussed farther in Chapters 14-16.

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