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enter image description here (Our sun compared to some of the known stars)

I know that the star is born in a nebula.Do only a extremely gigantic nebula give rise to large-radius star or is there any other factors related?

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  • $\begingroup$ Just to confirm, are you asking about physical size or mass? $\endgroup$
    – user10106
    Jun 11, 2018 at 9:56
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    $\begingroup$ @Kozaky i have edited the question $\endgroup$
    – Paran
    Jun 11, 2018 at 10:30
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    $\begingroup$ Regarding "give rise to super-massive star" in your question note that "massive" would be taken to mean "mass" whereas I think you mean "radius". Could you clarify that in both the question body and title ? Thank you. $\endgroup$
    – StephenG - Help Ukraine
    Jun 11, 2018 at 13:19

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The size of the nebula doesn't matter at all, what matters to the size of a star is the mass and age of that star. Even the mass of the nebula doesn't really matter, because giant molecular clouds fragment dramatically as they make many many stars. It's a very complicated and detailed process that controls how they fragment, so usually one merely assumes a statistical distribution of stellar masses, called the "initial mass function," which applies more or less universally to any sized nebula.

Once the mass of the star is determined (statistically), the star contracts with age, so very young stars have all kinds of different sizes while they contract. Eventually fusion initiates in the core (which is sometimes considered the "birth" of the star, though it was pretty much already a star before that), and then the size of the star is determined by its mass because a higher mass star doesn't need to contract as small to reach fusion temperatures. Later, when core fuel runs out, stellar envelopes expand dramatically into giants or supergiants, which to a point reverses the process for the envelope as what happened in the original contraction of the star. The size varies a lot after that.

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  • $\begingroup$ It's important to note that there is a feedback loop which prevents star larger than around 100 Solar masses from forming. As bits of the GMC collapse, the new star grows and gets hotter and after a point the radiation of the star begins to push the remaining nebula away and stop accretion. (Oversimplified as the Eddington Limit.) So larger stars can't form no matter how big the chunk of GMC which begins to collapse. $\endgroup$
    – Mark Olson
    Jun 11, 2018 at 14:52

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