by Ann » Wed Apr 27, 2011 3:12 am
I don't know much about that at all, Doug. I know, however, that the most massive stars tend to be found inside the most massive clusters, and that the most massive clusters tend to form when galaxies interact or collide, so that gas clouds collide violently. Then the rules that govern star formation under ordinary circumstances don't seem to apply any more.
But in any case, the biggest stars are rare and the smaller stars seem to be increasingly common. That is probably due to the fact that starforming clouds fragment easily, perhaps due to rotation. If you have a big starforming cloud, it will fragment into many "rotating pieces", but some of those pieces may still be quite big and lead to the formation of an O- or B-type star. If you have a small starforming cloud and it fragments, you are only going to get small stars out of it.
Here is an image of the Pleiades:
http://apod.nasa.gov/apod/image/0601/pl ... er_big.jpg
The brightest stars in the Pleiades are not supermassive by any means, but the biggest of them are massive enough to be B-type stars. There aren't that many bright stars in the Pleiades. Depending on which stars you count as bright, there may be, perhaps, nine stars that are bright and perhaps twenty or thirty more that are moderately bright. But the number of stars that belong to the Pleiades cluster is estimated to be around 500, or so I think. That means that most of the stars in the Pleiades must be little red dwarfs.
The cluster R136, which ionizes the Tarantula Nebula in the Large Magellanic Cloud, is a super star cluster which contains very many very massive stars, including very many of the hottest class O stars, those of spectral type O3. R136 also contains the most massive star that astronomers know of, R136a1, which is estimated to contain 265 solar masses and have a luminosity ten million times that of the Sun. (Actually, if that refers to the bolometric - total - luminosity, I think it sounds a bit low for a star which is so unbelievably massive.)
Here is an image of R136:
http://arweenn.files.wordpress.com/2010 ... escope.jpg
Read more about R136 at
http://en.wikipedia.org/wiki/R136.
R136 contains an estimated total mass of 450,000 solar masses, so according to Wikipedia, it may become a globular cluster in the future. Clearly it will be a smaller cluster than, say, the Hercules cluster, M13, which still contains a greater mass than 450,000 solar masses, even though this cluster is about 12 billion years old or so. But most of the stars in M13 are low mass stars of spectral class M. Undoubtedly the majority of the stars in R136 are also little red runts of spectral class M.
Here is an image of M13:
http://thebigfoto.com/wp-content/upload ... ier-13.jpg
Note the bright red giants in M13, the not quite so bright but still obvious blue horizontal branch stars, and a profusion of small, mostly neutral-colored stars. Most of those we can see easily are probably of spectral class G.
Ann
I don't know much about that at all, Doug. I know, however, that the most massive stars tend to be found inside the most massive clusters, and that the most massive clusters tend to form when galaxies interact or collide, so that gas clouds collide violently. Then the rules that govern star formation under ordinary circumstances don't seem to apply any more.
But in any case, the biggest stars are rare and the smaller stars seem to be increasingly common. That is probably due to the fact that starforming clouds fragment easily, perhaps due to rotation. If you have a big starforming cloud, it will fragment into many "rotating pieces", but some of those pieces may still be quite big and lead to the formation of an O- or B-type star. If you have a small starforming cloud and it fragments, you are only going to get small stars out of it.
Here is an image of the Pleiades: http://apod.nasa.gov/apod/image/0601/pleiades_gendler_big.jpg
The brightest stars in the Pleiades are not supermassive by any means, but the biggest of them are massive enough to be B-type stars. There aren't that many bright stars in the Pleiades. Depending on which stars you count as bright, there may be, perhaps, nine stars that are bright and perhaps twenty or thirty more that are moderately bright. But the number of stars that belong to the Pleiades cluster is estimated to be around 500, or so I think. That means that most of the stars in the Pleiades must be little red dwarfs.
The cluster R136, which ionizes the Tarantula Nebula in the Large Magellanic Cloud, is a super star cluster which contains very many very massive stars, including very many of the hottest class O stars, those of spectral type O3. R136 also contains the most massive star that astronomers know of, R136a1, which is estimated to contain 265 solar masses and have a luminosity ten million times that of the Sun. (Actually, if that refers to the bolometric - total - luminosity, I think it sounds a bit low for a star which is so unbelievably massive.)
Here is an image of R136: http://arweenn.files.wordpress.com/2010/11/grand_star-forming_region_r136_in_ngc_2070_visible_and_ultraviolet_captured_by_the_hubble_space_telescope.jpg
Read more about R136 at http://en.wikipedia.org/wiki/R136.
R136 contains an estimated total mass of 450,000 solar masses, so according to Wikipedia, it may become a globular cluster in the future. Clearly it will be a smaller cluster than, say, the Hercules cluster, M13, which still contains a greater mass than 450,000 solar masses, even though this cluster is about 12 billion years old or so. But most of the stars in M13 are low mass stars of spectral class M. Undoubtedly the majority of the stars in R136 are also little red runts of spectral class M.
Here is an image of M13: http://thebigfoto.com/wp-content/uploads/2009/02/messier-13.jpg
Note the bright red giants in M13, the not quite so bright but still obvious blue horizontal branch stars, and a profusion of small, mostly neutral-colored stars. Most of those we can see easily are probably of spectral class G.
Ann