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stellar_evolution - Before proceeding to Chapter 20 More on...

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Before proceeding to Chapter 20… More on Cluster H-R diagrams: The key to the chronology of our Galaxy Below are two important HR diagrams: 1. The evolution of a number of stars all formed at the same time in a cluster. Note that both during the approach to the main sequence and evolution away from the main sequence the most massive stars evolve most rapidly . This is what gives us a way to obtain ages of clusters from the extent of the main sequence.
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2. This illustration shows some HR diagrams of real open clusters, illustrating directly how ages of clusters can be estimated. The oldest cluster known, M67, is only about 4-5 billion years because almost all clusters dissolve or “evaporate” long before this. M67 is just a “lucky survivor.” At this point you should be able to look at Figures 19.17 (open cluster) and 19.18 (globular cluster) on p. 519 of your book, and understand clearly how we know that one was “just born” recently (about 100 million years ago), while the other must be extremely old, about 10 billion years. Can you now guess why so many globular clusters have about the same (old) age, and are still “in one piece” (not dissolved), while not a single open cluster is older than about 5 billion years, and nearly all of them are younger than about 100 million years? Does this suggest that there were few open clusters forming until recently, with our Galaxy in a “lull” or “off” state for nearly 10 billion years? Why would all the globular clusters form 10 billion years ago?
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Stellar Evolution (ch.20) So far all we know about stars’ lives is that they are formed within interstellar clouds by contraction under self-gravity, contract until they are hot enough in their cores to burn nuclear fuel, and that the lowest- mass stars live longest. Next we summarize the stages of a star’s life after it begins it’s main sequence stage of evolution, going into some detail. Note that the statements made about stellar evolution are almost entirely theoretical, but form one of the best-established and tested parts of astronomy—stars are not so complex that we can’t make models for them (compare with biological organisms). And there are some observational tests: The cluster H-R diagram above is one of them. Main Evolutionary Phases for Stars ~1Mo to ~8Mo (Notation from here on: Mo means M sun = mass in units of the Sun’s mass) Main sequence equilibrium 1. Pressure balances gravity at every depth, so hottest in center nuclear reactions H He, “core H-burning”, which is what the main sequence in the H-R diagram represents. This is the same “pressure equilibrium” that we have discussed
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