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Is there a particular reason(s) why stars cannot grow as massive as they want to?

And why doesn't this limit apply to supermassive black holes?

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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 for compositions similar to the Sun. 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.

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 without ever forming a stable star.

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