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Fraser
2005-Feb-04, 05:17 PM
SUMMARY: New research from the University of Michigan has determined that there's definitely an upper limit to the mass that stars can reach - between 120 and 200 times the mass of our own Sun. The team examined a wide range of stellar clusters, and determined the distribution of the mass of stars in those clusters. They couldn't find any stars above this 120-200 stellar mass limit. But this brings up a new mystery. Is this as big as stars get because they run out of material, or is there a fundamental limit in physics that stops them from getting any bigger?

View full article (http://www.universetoday.com/am/publish/upper_limit_star_size.html)

What do you think about this story? Post your comments below.

StarLab
2005-Feb-04, 08:10 PM
this as big as stars get because they run out of material, or is there a fundamental limit in physics that stops them from getting any bigger?
Good question. ;)

wstevenbrown
2005-Feb-04, 08:39 PM
This is confirmation (not discovery) of an order-of-magnitude estimate known before Chandrasekhar.

I think the answer to the question"How big?" is sensitively dependent on environment. I would paraphrase it. We know that there is a delay between the production of energy by fusion and its appearance at the surface of the star, on the order of a million years or so, and probably a slightly longer time until the surface reaches thermal equilibrium. The new question becomes: For a molecular cloud of a particular density and metallicity, for how long can the protostar continue to accrete material after nuclear ignition? Because, at some point, as the star's radiation exceeds the Eddington limit, its outgoing wind will be so strong as to interdict the accretion process. Depending on environment, you might end up with an 'ordinary' O star, a Wolf-Rayet star, a Luminous Blue Variable. Current estimates for really singular stars like the Pistol cause me to think the 'absolute' limit is very close to 300 Sols. Best regards-- Steve

alfchemist
2005-Feb-05, 04:58 AM
I'm not an expert on this but I think the main factor to consider is "stability" in stellar sense. Systems which would otherwise be unstable in a certain condition could achieve stability at other conditions , which means the upperlimit given in the article could be breached. I believe though that if there are no "effective" external influence, there would definitely be an upperlimit to the stellar mass

greenone
2005-Feb-05, 07:58 AM
i don't know the numbers on this, but it seems that at some point the mass would become too great and the gravitgational pull would prevent light from escaping...ie a singularity. so maybe "stars" do form above the 200 solar mass limit, and we just never see them.

antoniseb
2005-Feb-05, 11:12 AM
Originally posted by greenone@Feb 5 2005, 07:58 AM
i don't know the numbers on this, but it seems that at some point the mass would become too great and the gravitgational pull would prevent light from escaping...ie a singularity. so maybe "stars" do form above the 200 solar mass limit, and we just never see them.
That's an interesting idea. I suspect that there is a forbidden range. Below but near 120-200 SM [depending on metalicity of the cloud] and a star can form that will blow most of itself away during its life.

In the forbidden range, something like a star can begin to form, but it is so hot so fast that it blows away incoming material before it can collapse. This initially ejected gas goes on to form other stars later.

Above the upper boundary for the forbidden range [say about 5000 solar masses], an intermediate sized black hole forms, and a globular cluster forms around it.

alfchemist
2005-Feb-06, 03:14 PM
Looks like we've got a consensus that the said limit can be breached but consider this, we can always speculate based on logic and prevailing theories but maybe, they may not be applicable or existing in our known universe. We can always arrive at infinite possibilities but I guess an empirical approach just like in the study is more in order.

Littlemews
2005-Feb-07, 01:35 AM
I even heard that stars could form in a Black Hole, why is not possible for a star that exist above the C.limit - 100 times, 200 times.

The Near-Sighted Astronomer
2005-Feb-07, 02:52 AM
Hi All,

It's probably not so much how stable a star may be after it forms. More importantly is the process through which a star passes to condense out of an accretion disk. If matter rushes too quickly toward the core, tremendous shock waves may be set up. These shock waves could result in a form of spontaneous fusion that releases energy at far too high a rate. The result is to blow matter well away from the star - much of which ultimately ends up in interstellar space in the same way that supernoval displays create remnants like the Cygnus loop.

Data indicates that many such breeder stars condensed within a couple hundred million years after the Big Bang. Its entirely possible that such super-massive breeders went through multiple cycles of such implosions as they formed and later at least once as they collapsed into black holes. (In fact they may have esperienced multipl noval displays as different types of matter were exhausted and new types of fusion supervened.)

Hope this helps,

jeff

alfchemist
2005-Feb-07, 02:55 AM
Hi, Littlemews!

Antoniseb, Greenone, and I are in agreement that the stellar mass limit cited in the paper can be breached. What i'm pointing out is that an empirical approach like in the study is in order since we don't fully understand star formation. True, we know so much of the birth and death of stars and their other transformations but so much is unknown about the stars. Knowing the limit of stellar mass, if there's really such a thing, is fundamental in our understanding of these systems. As it is, we hardly know yet. There were studies, yes, but methodology is not presented in the article given. Are the star samples representative of the whole population? We can come up with only an approximate number of stars in the MilkyWay unless somebody have counted them already! :D How much more if you consider all the stars in the known universe? If you're familiar with counting cells, well, there are millions or even billions of them just like the stars. The current method is to put a grid, count the cells in one grid and multiply the result by a factor and that gives you the total cell count. Methodology such as this must be presented for the study to be credible. I'm sure they have one in their study.

Clear skies to you!

Richard

alfchemist
2005-Feb-07, 03:03 AM
Hi, Jeff!

Stability is a relative term. Would you not consider the supermassive breeders as stars when they start fusion even if they are shortlived? seconds, minutes, hours, years, etc. I guess that would matter to those who would encounter such things

saski
2005-Feb-07, 03:21 AM
Hi folks

I wrote an essay on this for my MSc in astronomy (the Swinburne Astronomy Online program, which I can recommend). It was rather technical and condensed, so I shan't try to post it here. If anyone is interested, I could email it.

Things are basically as wstevenbrown says. A supermassive star at first nuclear ignition is luminous enough to repel surrounding matter - millions of times more luminous than the Sun - and so ceases to accrete new mass.

What I found in writing the essay was that supermassive stars don't exist in isolation. They are borne in dense nebular "clumps", surrounded by other stars, and protostars, and volumes of gas as massive as they are themselves. They live out their short, intense lives - mere millions of years - inside the nebulae where they are born.

Stellar formation begins with vast clouds of gas (galactic nebular clouds; in the early universe, possibly galaxy-scale dark matter haloes). Part of a cloud begins to fall inward under its own gravity, expels some matter and enegry, and can't climb out of its own gravitational well. Then denser clumps form within the cloud, and denser ones within those, until finally there are spherical protostars.

Looking at the statistical distribution of clump masses, it seems that there should be some protostars massing as high as 750 Suns. In fact, there is a fall away from this "power law" between about 115 and 155. Presumably this is where the Eddington luminosity limit comes into play.

There are some twists on this scenario. For one, it is likely that neighbouring stars can capture one another as close binaries, spiral in and coalesce; there is nothing that even super-star luminosities can do to prevent that. It could also happen that a new star possesses a large disk of surrounding matter, which shields external infalling matter from the ignition luminosity. So on top of the "regular" population, cutting off at 155 MSun, there should be a small secondary population of coalesced superstars with higher masses - maybe up to 200 or so.

Another twist is that carbon dust absorbs light, which increases the effectiveness of luminosity in expelling matter; the dust is a natural lightsail, so to speak. The first stars formed from hydrogen and helium only, so we are expecting a mass distribution with a higher maximum to apply in the early universe.

To the best of my knowledge, all this happens at densities too low for black holes to form just from gravitational "clumping". It takes a supernova to compress matter to the Schwartzschild threshold. But once a black hole has formed, in the environment of a dense star forming nebula, there is no limit to how much matter it can then absorb.

antoniseb
2005-Feb-07, 04:20 AM
Originally posted by saski@Feb 7 2005, 03:21 AM
I wrote an essay on this for my MSc in astronomy (the Swinburne Astronomy Online program, which I can recommend).
Thanks Saski, and welcome to the UT forum.

This was a nice first post, and had some good stuff in it. I especially like the part of the spiralling companion.

Thanks again.

Planetwatcher
2005-Feb-07, 06:08 AM
I don't believe stars could form inside a black hole, but that doesn't mean it's impossible.

I also find it hard to fathom an upper limit on how big a star can get.
I seriously doubt that 120 to 200 solar masses is the largest a star can get.
It's only just the largest we have seen.

While it does seem fessible that the igniting of larger fussion reactions may blow away accredition matter, it is equally plausible that the amount of mass required for such reactions would also have suffencient gravity to keep it from blowing apart.

Which would mean upper size limits can be determined by either the amount of material available to the developing protostar, or the gravity of such a body approaching the speed of light.
There is a very long way to both, and I couldn't say which limit would come first.

I mean logicly one might think that gravity approaching the speed of light would occur first, but that is without considering the astronomical amount of mass for one body (which isn't a black hole) to have in order to reach such levels of gravity.

I would go with the others that such massive stars would be very short lived, because the amount of energy required to maintain the most massive stars is much greater then the stars will have available.

Meaning the eventual fate of such super bodies is collapse and life as black holes.

alfchemist
2005-Feb-07, 06:09 AM
Hi, Saski! Welcome to UT. your post is very informative and authoritative. If I get you right, there exist massive stars, though relatively few, above the 200 limit cited in the paper. Why then these people can't find one? Are they looking at the wrong places? How could you detect one? Second, from your post, since massive stars upon ignition would be luminous enough to repel matter stopping accretion of more materials, setting a limit to their size or does it? if it does, I guess an empirical approach to the problem is no longer very useful and mere theoretical determination would suffice