Open and Globular Clusters

[tyguy]

Im researching these clusters and am curious if I am correct on an assumption. The brightest stars in a globular cluster, to my knowledge, are red giants...is this because the cluster is so old that none of the other stars are on the main sequence?<br /><br />Subsequently, what would be the brightest star in an open cluster and why? Type B stars?<br /><br />One more question...<br />Lets say a black hole with 1 solar mass replaced the sun overnight...there would be no change in an orbital pattern for earth, correct? To my knowledge the black hole would still contain the same mass so nothing would change...<br /><br />Thanks

 

[MeteorWayne]

I don't know the answer to the Open and Globular Cluter question, but the part about replacing the sun with a 1 solar mass BH is correct.<br /><br />The orbits would stay the same, but it would get pretty cold pretty quick <img src="/images/icons/smile.gif" /> <div class="Discussion_UserSignature"> <p><font color="#000080"><em><font color="#000000">But the Krell forgot one thing John. Monsters. Monsters from the Id.</font></em> </font></p><p><font color="#000080">I really, really, really, really miss the "first unread post" function</font><font color="#000080"> </font></p> </div>

 

[doubletruncation]

Hi tyguy,<br /><br />To answer your question regarding clusters, the brightest star in a cluster depends very much on the age of the cluster. <br /><br />In our galaxy, at least, globular clusters are all very old (typically about 10 billion years old) so, as crazyeddie discussed, all the O-A and even often F stars have long since died. The brightest stars in these clusters are the stars that are presently in the last stages of their lives (red giants). The smaller stars are still on the main sequence (M dwarfs, for example, can live more than 100 billion years - so none of them have ever died through ordinary single star evolution).<br /><br />Unlike globular clusters, you can find open clusters over a large range of ages. While they tend to be young (many are only a few million years old, or less than a hundred million years old), there are some that are billions of years old. Because open clusters lose their stars to the general galactic milieu over time (due to tidal forces) they usually disperse in less than a billion years, unless they are particularly rich. The oldest open cluster known is Berkeley 17 (which is more than 10 billion years old), and another really old and interesting one that people study a lot (myself included) is NGC 6791 which is about 9 billion years old. In these old open clusters the brightest star tend to be red giants, just like for the globular clusters. In fact this tends to be true for clusters older than a few hundred million years. In young clusters like the Orion Nebula Cluster, the brightest stars tend to be the really hot, luminous massive O/B/A stars. The most massive stars go through their evolution very rapidly, but they're also very rare. So in a given, rich young cluster the brightest star may be a luminous main sequence star, or the star could be an evolved red supergiant or a wolf rayet (very luminous but bluer than stars on the main sequence) depending on where the cluster's handful of very massive/luminous stars currently <div class="Discussion_UserSignature"> </div>

 

[doubletruncation]

just one more note:<br /><br />a good example of how the brightest star in an open cluster depends on its age is to compare the Pleiades with the Hyades. These are two really lovely clusters that you can easily see with your naked eye throughout the evening this time of year. The Pleiades are about 100 million years old, and their brightest star is Alcyone, which is a hot B-type giant that is just evolving off the main sequence and is about 6 solar masses. The Hyades, on the other-hand, are about 600 million years old, and their brightest star is a red giant that is about 2.5 solar masses.<br /><br />You can compare the color-magnitude-diagrams (the observed version of the H-R diagram that you might have heard about) for yourself by looking at:<br />http://www.univie.ac.at/webda/cgi-bin/plot_cmd.cgi?mel025+Hyades for the Hyades<br />and<br />http://www.univie.ac.at/webda/cgi-bin/plot_cmd.cgi?mel022+Pleiades for the Pleiades <div class="Discussion_UserSignature"> </div>

 

[tyguy]

Thank you very much for that very detailed explanation!