by Ann » Sun Jun 08, 2014 11:30 pm
Indeed, clusters are not closed, but they can be gravitationally bound. That is why globular clusters have not dispersed after billions of years. On the other hand, some globular clusters probably
have dispersed, so that we can't detect them any more. Those clusters were not rich enough for their mutual gravitational attraction to prevent dispersion.
Here is a picture of rich globular cluster M53 and sparse globular cluster NGC 5053. These clusters are made of stars with the same "ancient" chemical composition, and they were probably born at the same time, 10-12 billion years ago. But NGC 5053 likely contained less mass than M53 right from the beginning, and it is likely that NGC 5053 has been "shedding stars" at a higher rate than M53 ever since it was born.
The crucial difference between globular and open clusters is their chemical composition. Open clusters were born from "metal-enriched" gas, which gives them different characteristics than the stars in the globular clusters. They were also born at a time when there was less free gas available, making it very hard (or impossible) for open clusters to be as rich in stars as the rich globulars, such as M53. However, some open clusters are indeed rich and massive and can stay intact as a cluster for billions of years.
Here you can see two open clusters, M35 and NGC 2158. On the left is bluish cluster M35. It is relatively rich for an open cluster, but nowhere near as rich as more distant, yellowish cluster NGC 2158. M35 is going to disperse long before NGC 2158 "loses its integrity as a cluster".
Ann
Indeed, clusters are not closed, but they can be gravitationally bound. That is why globular clusters have not dispersed after billions of years. On the other hand, some globular clusters probably [i]have[/i] dispersed, so that we can't detect them any more. Those clusters were not rich enough for their mutual gravitational attraction to prevent dispersion. [url=http://www.grinderphoto.se/pics/Med_6.jpg]Here is a picture of rich globular cluster M53 and sparse globular cluster NGC 5053[/url]. These clusters are made of stars with the same "ancient" chemical composition, and they were probably born at the same time, 10-12 billion years ago. But NGC 5053 likely contained less mass than M53 right from the beginning, and it is likely that NGC 5053 has been "shedding stars" at a higher rate than M53 ever since it was born.
The crucial difference between globular and open clusters is their chemical composition. Open clusters were born from "metal-enriched" gas, which gives them different characteristics than the stars in the globular clusters. They were also born at a time when there was less free gas available, making it very hard (or impossible) for open clusters to be as rich in stars as the rich globulars, such as M53. However, some open clusters are indeed rich and massive and can stay intact as a cluster for billions of years. [url=http://24.media.tumblr.com/7e034659ad0c077ac568579b8c4e08cb/tumblr_mg2dg5Mw541r1lgmgo1_1280.jpg]Here you can see two open clusters, M35 and NGC 2158[/url]. On the left is bluish cluster M35. It is relatively rich for an open cluster, but nowhere near as rich as more distant, yellowish cluster NGC 2158. M35 is going to disperse long before NGC 2158 "loses its integrity as a cluster".
Ann