Why do stars have different masses ?

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dougettinger
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Why do stars have different masses ?

Post by dougettinger » Mon Apr 25, 2011 5:03 pm

Since the forum is being engaged in star evolution and O and B type stars, I am asking a supposely simple question. Why do stars have different masses. A simple answer would be that there are different sizes of interstellar molecular clouds and hence, protostar disks. But any newly forming star must have a feedback loop to prevent the star from spinning too fast and exploding because of too much centrifugal force. The feedback mechanism is believed to be the polar jets and the fusion process that begins at a certain temperature and pressure and produces solar winds to prevent any further incoming matter.

Assuming that these feedback mechanisms are consistent for all star-making what then causes stars to have different masses ?
Doug Ettinger
Pittsburgh, PA

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Ann
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Re: Why do stars have different masses ?

Post 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
Last edited by Ann on Wed Apr 27, 2011 2:25 pm, edited 2 times in total.
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dougettinger
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Re: Why do stars have different masses ?

Post by dougettinger » Wed Apr 27, 2011 1:48 pm

The answer to the mystery is perhaps there are different mechanisms that create stars from brown dwarfs, the size of 90 Jupiters to R1361a with 265 solar masses. As you had suggested, active open clusters of stars appear to produce the largest stars. The effect of the surrounding environment certainly comes into play.

Currently, I am thinking that the normal protostar, having the size of 1.5 solar masses or smaller are created as astronomers conceive the collapse of a protostar disk. Larger sizes entail the spin of two very close binaries that eventually gather gravitationally surrounding matter to create one massive star. Supermassive stars are created by two massive stars that were already created by binaries that become binaries themselves. The closely rotating stars begin to gather remaining gases in the cluster into a bar similar to bar galaxies. Eventually enough matter initiates the spherical shape due to the internal pressures pushing outward in all directions. In a very young open star cluster, this process may repeat itself many times in the central crowded region. I just cannot visualize a singular point amassing the infall of 265 solar masses without easily increasing its angular momentum to the point that it flies apart. That is why one must look to other ways of performing the star-making business. You gave me this idea after I read about the difficulty of resolving the very large central stars in the center of R136. Thanks.
Doug Ettinger
Pittsburgh, PA

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