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The H-R diagram of a very young cluster (NGC 2362) will have nearly all its stars still on the Main Sequence. As we view older and older clusters on the H-R Diagram, we note that few of the high mass stars are found. They are either evolved over to be red supergiant, Persei, or giant stars, M11 or cease to exist at all.
The oldest clusters will have the fewest number of stars on the Main Sequence, and the lowest upper mass still on the Main Sequence (see NGC 752 or M67). This point in the H-R Diagram called, the Turn off Point shows where the most massive stars still on the main sequence are found. These stars are just running out of Hydrogen fuel in their centers. The values given in RED show at what age in years would stars at that location leave the Main Sequence. |
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Variable stars as Distance Indicators Certain positions in the H-R Diagram are susceptible to stellar pulsations. This instability strip, shown on the right, includes two important kinds of stars, Cepheids (both Type I-metal-rich and Type II-metal-poor) and RR Lyrae variables. The diagram below shows a star getting bigger and cooler, then smaller and hotter. Cepheids are brightest when they are hottest, close to the minimum size. Since all Cepheids are about the same temperature, the size of a Cepheid determines its luminosity. A large pulsating object naturally has a longer oscillation period than a small pulsating object of the same type. Thus there is a period-luminosity relationship for Cepheids. Since it is easy to measure the period of a variable star, Cepheids are wonderful for determining distances to galaxies. Click here to see a Cepheid light curve. |
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