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What kinds of star clusters are there?

Could you please tell me what are the components of an star association, in terms of density, whether it is stable, what stars are in them and how they are formed. This is for a report in astronomy class so I would appreciate this in somewhat non technical terms, something that an advanced astronomy class in high school would comprehend. Thanks.

There are really two classes of the kind of stellar associations you're looking for: globular clusters and open clusters.

Globular clusters are to galaxies a bit like moons are to planets. They orbit the main body of the galaxy and are themselves composed of up to a million stars. They generally have a radius of 10-20 parsecs. See the below image of the center of globular cluster M15. Globular clusters are what we call "metastable." They are technically unstable, and from time to time, stars "evaporate" or escape from them, leaving the rest even more tightly bound together than before. Eventually, they will evaporate a large number of their stars, causing the rest to collapse into a black hole. We call them metastable, however, because this process takes much longer than the age of the universe. No one is really sure how globular clusters form, except that they are all very old, possibly the first things which formed in the galaxy.

M15 taken by HST

The other kind of star cluster is an open, or galactic cluster. These clusters form in the disk of the galaxy itself, and they are generally not gravitationally bound. When stars are born, there are usually many born in one place, and when the gas and dust is blown away, you are left with a collection of young stars. Eventually, the group disperses after a few tens of millions of years. The pleiades are a very beautiful example of an open cluster. take a look at this picture to see them. Open clusters generally contain a few hundred very young stars within a radius of less than 10 parsecs.

January 1999, Dave Kornreich (more by Dave Kornreich) (Like this Answer)

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