Draft:Supermassive star

A supermassive star (SMS) is a hypothetical type of extremely massive and luminous star, with its mass being on the order of more than a thousand times the mass of the Sun. Such stars may have existed early in the universe (i.e., at high redshift) and may have produced high-mass black hole seeds like direct collapse black holes (DCBHs).

Description
Although the term "ultramassive star" ("UMS") is rarely used, it has been once used to refer stars between $1,000$ and $10,000$.

Formation and properties
The environmental physical conditions to form a supermassive star are the following:


 * 1) Metal-free gas cloud (gas containing only hydrogen and helium).
 * 2) Atomic-cooling gas.
 * 3) Sufficiently large flux of Lyman–Werner photons, in order to destroy hydrogen molecules, which are very efficient gas coolants.

Depending on models, several studies had predicted that supermassive stars could have evolved "red supergiant protostars" as high accretion rates would prevent stars to contract, resulting lower temperatures and radii reaching up to many tens of thousands of, comparable to some of the largest known black holes. Researchs predicted that metal-rich supermassive stars may have been able to form from merging of metal-rich protogalaxies.

A computer simulation reported in July 2022 showed that a halo at the rare convergence of strong, cold accretion flows can create massive black holes seeds without the need for ultraviolet backgrounds, supersonic streaming motions or even atomic cooling. Cold flows produced turbulence in the halo, which suppressed star formation. In the simulation, no stars formed in the halo until it had grown to 40 million solar masses at a redshift of 25.7 when the halo's gravity was finally able to overcome the turbulence; the halo then collapsed and formed two supermassive stars that died as DCBHs of $31,000$ and $40,000$.

Direct collapse
A 2018 study proposed a new model for the direct collapse route.

A scenario whereas a DCBH is formed without stellar phase is called "dark collapse".

Alternative scenarios for the fate for a supermassive star have been proposed by various other researches, although this depends directly on the star's properties.

Bar-mode instability
Although thermal emission from a rotating supermassive star will cause the configuration to contract slowly and spin up, the contracting and cooling star may rotate differentially if internal viscosity and magnetic fields are enough weak and will likely encounter the dynamical bar mode instability, which may trigger the growth of nonaxisymmetric bars.