Card Activity, Sep. 27 Determining the Ages of Star Clusters HR Diagrams
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Card Activity, Sep. 27 Determining the Ages of Star Clusters Make a qualitative sketch of the HR Diagram you would expect to find for the following cases. Label the axes and indicate the direction of increasing values. Indicate with Star clusters of dots or shading where the stars should lie. different ages have different turn-off (a) Cluster A: A relatively young cluster, that formed points; younger ones still have high-mass more recently than, for example, the Hyades. Main Sequence stars, (b) Cluster B: A relatively old cluster, that formed about for older clusters, 10 billion years ago. more of the M.S. has (c) Now assume that Clusters A and B were to physically “peeled off.” merge, combining into a single cluster. What would the HR diagram look like then? Explain your reasoning. 9/27/12 Ast 309N (47760) Star Cluster HR Diagrams HR Diagrams of Old Star Clusters The young cluster, (a), will The HR diagram of an show stars extending up to older cluster may show a high luminosity point on concentrations of non- the HR diagram, like for Main Sequence stars, such example, NGC 2362. An as red giants, a horizontal older cluster, (b), will look branch, the second red like M3, with a turn-off giant (“asymptotic”) branch point lower on the Main and some white dwarfs. Sequence. The combined The young cluster will have cluster will be the sum of just a few luminous red (a) + (b), looking like the giants, at most. bold lines in this sketch. 9/27/12 9/27/12 Ast 309N (47760) “Mixed-Age” HR Diagrams Errors and Misconceptions • For (c), some groups showed a turnoff point in The diagram at right shows between those for clusters (a) and (b), reasoning a sample of stars that are that they would “average out.” NO. There are two not in a cluster (we call them “field stars”). They groups of stars, with two different but specific ages, have a range of ages, so so you should see two separate turn-off there are still some short- points. lived, upper Main Sequence • Some groups failed to draw a full HR diagram for stars, but at the same time we see a turn-off just a each cluster; they showed only a Main Sequence, little above the Sun. with (a) and (b) marked as separate points on it. • “The Main Sequence would shift to the left.” Nope. Combining the two clusters doesn’t change the properties of the stars in them. Ast 309N (47760) 9/27/12 Correct Points & Clarifications Errors and Misconceptions • “Even if you merge [the clusters], you still have all • Despite my warnings about misinterpretations of the the same types of stars on the diagram.” Main Sequence, some of you think an individual star can move along it: “… the younger stars are still • “You would see an overlay of the two diagrams, moving down and off, while the old ones are with higher density of stars in the areas of overlap.” gone…”; “Older stars become cooler and lower • “Younger stars have higher temperature because of luminosity…” Nope. mass, older stars the opposite.” This assertion is not • One drawing for (c) showed stars “peeling off” the justified. High-mass stars on the Main Sequence must Main Sequence from both the top and the bottom! have formed recently because of their short “This would be double-ended because there are lifetimes; low-mass MS stars can be old or young. stars at both ends of its life.” After the MS, stars • Another group omitted the lower-main sequence for evolve to be red giants, towards the upper right. Cluster (a). Young stars do not have to be massive! Ast 309N (47760) Ast 309N (47760).