Is there a particular reason why stars cannot grow as massive as they want to?
And why doesn't this limit apply to supermassive black holes?
Why don't (or can't) stars be more than 325 or so times the mass of the sun? What limits their size?
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1$\begingroup$ Related: astronomy.stackexchange.com/q/27553/16685 $\endgroup$– PM 2RingCommented Jul 24, 2020 at 21:51
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2$\begingroup$ A black hole doesn’t really have “mass” in the same sense as moons and planets have. A black hole (formed via collapse) is a gravitational field emanating from collapsed (or, causally correct, collapsing) matter. Its dynamics is entirely different from dynamics of a material object. Models of a black hole consider it as a General Relativity’s empty space (except for charged holes where EM field is not exactly “vacuum”). These models are not realistic about formation of holes, but describe their dynamics well enough. In short, black holes are not about matter and its limits. $\endgroup$– Incnis MrsiCommented Dec 22, 2020 at 18:34
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$\begingroup$ @Kurt Hikes you might want to see my answer here astronomy.stackexchange.com/questions/47550/… $\endgroup$– Daddy KropotkinCommented Nov 26, 2021 at 18:35
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$\begingroup$ stars don't "want" to do anything... $\endgroup$– MichaelCommented Jul 17 at 12:57
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There is basically an upper limit to the mass of a star because their luminosity is so great that the radiation pressure prevents the accretion of further mass.
However, the upper limit depends on the composition of the accreting material. This is because the effect of the radiation depends on the opacity of the material - stuff that is more metal-rich is more opaque. The upper limit you refer to is a bit too high for compositions similar to the Sun (I think more like $200 M_\odot$ would be the limit for that). For stars born in the distant past that were metal-poor, or even born from primordial material with no metals, the upper limit could be much higher.
For primordial material, with no metals at all, the physics that determines the upper limit is likely to be fragmentation. Primordial gas is quite transparent to radiation, but this also means it is poor at radiating away the heat from gravitational contraction. In principle, much larger bodies can only begin to contract towards being very massive stars if some of the gas is ionised or if there is hydrogen in molecular form. However, this cooling also reduces the Jeans mass, allowing the collapsing cloud to fragment into a cluster of smaller objects before a supermassive $(>10^6 M_\odot)$ star/black hole could form.
Nobody knows for sure how supermassive black holes formed. It seems likely that they grew from seed intermediate black holes that formed from very massive primordial stars - perhaps thousands of solar masses - or even by direct collapse to black holes of mass $10^4-10^6 M_\odot$ without ever forming a stable star.
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2$\begingroup$ I've just asked What was the absolute limit to the possible sizes of the first stars formed from "primordial material with no metals"? $\endgroup$– uhohCommented Jul 25, 2020 at 6:32
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1$\begingroup$ But is it really 325 solar masses? Isn't it closer to 100 solar masses with the current composition and 200 solar masses for the early starts? $\endgroup$ Commented Dec 19, 2020 at 16:35
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3$\begingroup$ @PeterMortensen Yes, 325 is probably too high for a solar metallicity, but 100 would be too low. $\endgroup$– ProfRobCommented Dec 19, 2020 at 16:51