I'm no physicist, but it is my understanding that after a certain size a star has to have fusion happening inside or it will collapse under its own gravity.
Several things going on here. The balance of fusion "pressure" versus gravity is what determines the fate of all main sequence stars, so yes that's true for "normal" stars. White, brown and black dwarfs aren't normal stars though...
You might be thinking of Chandraskhar's Limit where he showed that fusion
plus electron degeneracy pressure in a white dwarf would still not be enough beyond 1.4 solar masses. So yes a black dwarf with 116 solar masses is unphysical and it must be a Stellar Forge issue. But even in a white dwarf it's not fusion that's preventing it from collapsing.
Brown dwarves aren't on the main sequence and the mass involved is WAY smaller. The limit between "actual brown dwarf with fusion" and "sub-stellar object" (as FFE calls them) or "hot Jupiter" as astro press releases call them, which have no fusion, is about 13
Jupiter masses. Not solar masses! 13 J-mass is enough to start deuterium fusion, which requires deuterium as a fuel and will stop when that runs out. At 65 masses you can "burn" lithium. At 80 J-masses you get "normal" hydrogen fusion and that's the line between brown dwarf and white dwarf.
The size of brown dwarfs and white dwarfs doesn't depend on "how much fusion" anyway though - there isn't enough fusion pressure to keep them at that size in the first place. They both depend on "electron degeneracy" which is what prevents them from shrinking down to a density where they would become a neutron star.
The limit of
electron degeneracy pressure is about 1.4
solar masses so brown dwarfs are way, way off that limit and in no danger of further collapse.
All of which means these here "black" stars in ED are unphysical yes - not because they can't form in theory but because the theory would take
trillions of years to play out, and we haven't had trillions of years yet...
I don't remember what the maximum size is, but I would guess that somewhere between Jupiter and maybe about twice as massive, give or take.
13 J-mass (or 65...) as above. After 80 it's a white dwarf.
In this case the "black" star in the screenshot is listed to have 116 solar masses, so I doubt it's physically possible for it to be that cold (its surface temperature is listed as 31 kelvin).
Yep this is a Stellar Forge issue for sure then. There is a pathway for a white dwarf to cool into a black dwarf in theory, but the theory requires the star to cool for longer than the present age of the universe. Order of 10 trillion years, so three orders of magnitude older than the universe. And as above, a white dwarf over 1.4 solar masses - so again, this black star is two orders of magnitude too massive - should have collapsed into a neutron star long before it got down to this temperature.
The screenshot is from the Phimbeau AA-A h76 system.
There's another famous black star in the Splojeia AA-A h19 system (in this case with a surface temperature of 113 kelvin), if you want to make a tour.
Thanks!
Splojeia AA-A h19 - through the core and out the other side... blimey. That would be a "tour" alright!
Here's more stars for the tour, just looked this up:
All in the Bubble.