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I'm given the HR diagram seen below and I need to identify the different regions. I know them all except for the one circled which I have no idea what it is... Can anybody help me out with this? Thank you. enter image description here

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closed as off-topic by Kyle Oman, John Rennie, stafusa, glS, Anders Sandberg Aug 23 '18 at 9:40

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The stars within the circle are "blue stragglers." They inhabit the blue end of the original main sequence, and should have left the main sequence long ago, but for some reason remain. Various theories exist as to their origin/longevity, but none are yet conclusive.

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  • $\begingroup$ Another question, if I may, regarding the diagram. How can I determine the age of the cluster based on the given diagram? $\endgroup$ – FlyGuy Jun 29 '17 at 17:27
  • $\begingroup$ The position of the main sequence turnoff point should determine the age. More accurate ways exist, but if you can find a chart of main sequence lifetime vs B-V, you can get a rough estimate. $\endgroup$ – probably_someone Jun 29 '17 at 17:49
  • $\begingroup$ Can I say that the main sequence mass at the turnoff point gives the current age of the cluster? This is why I believe there's an arrow right about the turnoff point. $\endgroup$ – FlyGuy Jun 29 '17 at 19:47
  • $\begingroup$ Yes, that's precisely it, since there's a color-mass relation on the main sequence, the color of the turnoff point gives you the mass of a star, which gives you a lifetime on the main sequence for that star. $\endgroup$ – probably_someone Jun 29 '17 at 20:01
  • $\begingroup$ I would like to ask you a couple more questions, if I may, but I don't want to go off-topic here. Can we open up a chat? $\endgroup$ – FlyGuy Jun 29 '17 at 20:06

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