Posted by Monica Wen on November 26, 1997 at 11:24:51:
Professor, I have a question on #37 on midtermII. It asks how could we tell the age of a young cluster from its HR diagram. The correct answer was d)by the turnoff point. But from pp258-259 in the book under "Plotting a star cluster on an H-R reveals its age" and the two diagrams, I was given the impression that we tell the age of a young cluster by looking at when the low-mass stars start to fall to the main sequence (which was answwer c). Anyway I thought we can tell the age of an OLD cluster by looking at the turnoff point. I would appreciate it if you could clarify this.
Also, on p339 in the book, it said that we know about the dark matter in the universe because the rotation curves of many galaxies are flat contrary to what Kepler's 3rd law predicts (little changes in orbital speed as distance changes); but later on in the book and also in lecture we covered that spiral galaxies form because of DIFFERENTIAL rotation, it seems like a contradiction to me, I would appreciate any pointers on this as well.
Thank you.
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