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Assuming Ton 618, the largest ultra massive black hole wasn’t the result of feasting on near by matter but was hypothetically once a single star body. How large in terms of size would TON 618 have been if it was a single star? TON 618 is about 11 times larger then the solar system so how much bigger would it have been before collapsing? Again, hypothetically and how long would it have lived for before collapsing into the ultra massive black hole?

I’m making a game, but I would like to get that figure somewhat right for the shock value to the player to read the ungodly size of such a star!

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    $\begingroup$ The most massive stars we know of are about 200–250 times more massive than the Sun. The black hole in Tonantzitla 618 has a mass of 66 billion times that of the Sun; it’s thus impossible for it to have originated from a single star. $\endgroup$ Nov 20 '21 at 0:08
  • $\begingroup$ @pierrePaquette that is the answer here imho $\endgroup$ Nov 20 '21 at 7:04
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    $\begingroup$ @PierrePaquette those stars are late stage stars, typically Wolf-Rayet stars, and it is unclear how massive their progenitors were, especially since a Wolf-Rayet star only lives for about 0.5 Myr and thus they are unlikely to be primordial. It is possible for stars to form in the early Universe to be much much much more massive, and could be even larger via accretion. See my answer for references. $\endgroup$ Nov 20 '21 at 15:28
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It's not possible to obtain a star that large without accretion, however you can still consider it to be a "single star" because it's not bound to other stars. Many people think that there is an upper limit on the mass of a star, but it is based on outdated stellar evolution models and physics, or is only relevant for present galaxies and not for the early Universe environment. This has been known for at least a decade.

IT is true that the most massive stars that we have observed to date are only up to $\sim 250$ M$_{\odot}$, but those are late-stage stars, usually Wolf-Rayet stars, that have long evolved off the main sequence and lost their envelopes, so they are irrelevant for the question posed by the OP which concerns the progenitors of supermassive black holes.

Stars that are likely seeds for supermassive black holes have been dubbed "supermassive stars," unsurprisingly. Although it is still uncertain exactly how such supermassive stars accrete mass, recent studies suggest that these stars would be slowly rotating due to the large amount of angular momentum needed to be extracted in order for the gas cloud to collapse gravitationally. These supermassive stars at high redshift might be detectable by the James Webb Space telescope.

Here is a review about the properties of these supermassive stars. The role of metallicity is important, as high metallicity stars will very likely not be massive enough to form a supermassive black hole, but rather a stellar mass black hole. Nevertheless, the conditions of the early universe are likely to not be rich in metals, so it is not impossible to expect that a Population III supermassive star could form into a seed of a supermassive black hole, i.e. a black hole of mass $\sim 10^4 - 10^6$ M$_{\odot}$, and then accretion of nearby gas over cosmic time brings the black hole's mass to much larger values, such as observed for Ton 618. However, keep in mind that the existence of supermassive stars is uncertain so this should be taken with a grain of salt, but it is within the realm of current possibility, theoretically speaking, which ought to be sufficient for your video game.

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There is a theoretical upper limit to star size which is A LOT smaller than the mass of that black hole: simply speaking, a star is in a pressure equilibrium where the outward working radiation and gas pressure are balanced by the gravitational pressure. The limit here seems to be around about a few hundred solar masses at most, going by the largest stars we know (e.g. Eta Carinae) as well as theoretical calculations which also include the formation processes. As such a star of several billion solar masses cannot form. And if existed, it could never create enough radiation and gas pressure to balance the gravitational force, it would become a black hole instantly, before fusion could even ignite.

As such: An object of this mass can never have originated from a single star.

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  • $\begingroup$ And if existed, it could never create enough radiation and gas pressure to balance the gravitational force, it would become a black hole instantly, before fusion could even ignite. This is not accounting for accretion rates that can sustain the gravitational collapse, and very massive clouds of pristine, atomically cold gas. $\endgroup$ Nov 20 '21 at 15:29

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