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UT AST 301 - The key to the chronology of our Galaxy

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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.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?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 Msun = 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 earlier, but a star has a long-lived source of energy to maintain that pressure. 2. Two nuclear reaction processes can occur in main sequence stars: proton-proton (dominates in stars ~ sun’s mass and lower), and CNO cycle (dominates in stars more massive than sun; see More Precisely 20-1). 3. Stable, lasts most of star’s lifetime. As long as the fuel lasts, any attempt to (say) expand will cool the core, reduce the pressure, and cause the star to contract, returning to its original state. This is what we mean by “stable.”Depletion of H in core [see Fig. 20.2]. Look at the nuclear burning “eating away” at the core H and leaving He behind. Starting in center (hottest), and moving out, the He (the “ashes”) accumulates in core (can’t burn the He as fuel—would require higher temperatures), H-burning only occurs in outer core. This is already the beginning of the end for the star, even though it may have most of its life ahead of it. Contraction of He core. He nuclei can’t fuse, so there is a pressure deficit compared to gravity, so core contracts, heating up (just like for a protostar). But the overlying layers heat as well, causing H-burning just outside the core to become very rapid ⇒ H shell-burning. See Fig. 20-3. So star gets brighter (L rises) even though it has lost its fuel at the center. The layers above the H-burning shell expand (not really understood, although models are undoubtedly correct—explained in class), so surface temperature cools ⇒ subgiant branch, then star ascends the red giant branch. (See Fig. 20-4) Red giants: Don’t let surface appearances fool you: Surface is large, luminous (partly because of large surface area), cool, but the core has shrunk to ~ 1/1000 size of entire star (~ size of earth). The density in core is huge, around 10,000 x main sequence central density (~ 106 gm/cm3 vs. 102 for MS phase). This core will later become a white dwarf when exposed by the loss of the envelope. What will be the next phase for such an object?Helium nuclear fusion. Occurs by triple alpha process whereby three He-4 nuclei combine in a two-step process to form carbon-12 plus some energy. [The lucky placement of a “resonance” without which there would be no carbon or anything heavier is discussed in class.] This reaction requires T~100 million degrees K, which the core has attained by its contraction and heating. But for low mass stars (less than about 2 Mo), when the He gets hot enough to ignite, the core is so dense that the gas is degenerate. (This is an important concept, and will recur when we describe white dwarfs, and later neutron stars.) Only a brief description is given here. Electron degeneracy pressure—due to quantum effect called “Pauli exclusion principle.” Result is that pressure does NOT depend on T, so He ignition increases T, which increases reaction rate, which increases T, … but pressure does not decrease in response ⇒ runaway “helium core flash” (see p. 533): energy gets soaked up by the huge “buffer” of the envelope, so virtually unobservable from the outside—but very well established theoretically—note how almost all of this is stellar evolution theory). Eventually (after only a few hours or less), the huge energy input does cause the pressure to increase enough so that the core expands, making itself non-degenerate; it settles into a new equilibrium between pressure and gravity converting He into C ⇒ called “horizontal branch” phase, or just “core He burning.” In effect, the star is given a new chance to resist gravity but for a shorter time than when it used H (He isn’t as efficient a fuel).Depletion of He in core. He ⇒ C ashes. T too cool to burn C (requires 600 million degrees K!), so core


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UT AST 301 - The key to the chronology of our Galaxy

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