Is it possible for some brown dwarves to be more massive than some stars?
By definition Brown Dwarves are too low in mass to sustain hydrogen fusion. They (as stars) might be more massive than other brown dwarves, but not more massive than a low mass "star" that is main sequence and has happily burned off it's lithium and is in full hydrogen fusion.
A brown dwarf is a failed star - massive enough to fuse deuterium [around 13x Jupiter mass] or lithium [about 65x Jupiter mass], but, not massive enough to fuse hydrogen, which takes about 80x Jupiter mass.
What if it contained a high amount of non-fusible material (ex: iron)? Would it cease being a brown dwarf or would the hydrogen begin to fuse anyway?
If it is mostly iron, with little hydrogen, then it would bear more resemblance to a white dwarf, not brown.
To the contrary, it is claimed that the most massive brown dwarfs should be metal-poor. Namely, the metal ions increase the opacity of hot gases. The clear hydrogen-helium gases of a subdwarf would conduct a large amount of heat out of the core, causing it to cool down and shut down fusion where an equally massive but metal-rich star would sustain fusion as a red dwarf.
What is the current temperature and spectral class of the most massive brown dwarfs in Population II, like globular clusters?
What if iron was hypothetically poured into a brown dwarf until it was more massive than a star? Would it ignite into a star or would it now just be considered a white dwarf?
Fusion occurs in the core of a star. If you could somehow infuse a large amount of iron into a star, the denser iron would displace hydrogen from the core - effectively serving as a stellar fusion extinguisher. If the mass infusion was sufficiently large [like a Chandrasekhar mass], it could even trigger a core collapse supernova.
If the fusible nuclei are there.
No, it wouldn´t. The speed of iron falling into a star is enough to evaporate the iron, which would mix with hydrogen. Once hot iron vapour is mixed with hydrogen, there will be no efficient way to unmix them, unless they are cooled below the boiling point of iron, which is not the case inside even a brown dwarf.
Even if iron were somehow infused into the core, it would cause the fusion to flare up. Look at red giants! They have a large helium core incapable of fusion. Since they also have lost some hydrogen to stellar wind from top, their hydrogen content is much less than it was back when the same star was on main sequence. Yet although there is no fusion in the core, the fusion outside core happens at a much greater rate than the fusion in the core did when the star was on main sequence.
Would you agree the concentration of hydrogen in the core is lowered 'once hot iron vapour is mixed with hydrogen'? Could it be that some of the hydrogen is displaced by iron?
A brown dwarf is not a red giant. It's just a whimpering ember barely able to fuse deuterium. While there is no way to know the effect of dumping a vast amount of iron onto one, I believe it is safe to say it is not going to instantly raise the core temperature high enough to trigger hydrogen shell fusion.
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