[kerryfalk]

Wait. What? A star more massive than a black hole?

I must be missing something here. Once something reaches the mass of a black hole, it becomes a black hole. No? How is it possible for a mass to be greater than a black hole but not a black hole? Mass over distance?

[hxegon]

no, there isn't some set mass that will make a black hole. A black hole is gravity overcoming the other fundamental forces. When a star dies and forms a black hole the force pushing outward from the fusion is overcome by the gravity of the star, but there isn't some threshold of size where this happens.

[saagarjha]

While true, this answer is a bit misleading. There is a minimum threshold for stellar black holes–the ones created through stellar evolution–it's called the Tolman–Oppenheimer–Volkoff limit.

[pdonis]

> There is a minimum threshold for stellar black holes–the ones created through stellar evolution–it's called the Tolman–Oppenheimer–Volkoff limit.

That limit gives the maximum size of a neutron star, but it does not say an object smaller than that cannot form a black hole. In fact, it is considered likely that supernova explosions could cause an implosion that could form a black hole smaller than the limit, simply because it happens too fast for neutron degeneracy pressure to stop it before the implosion reaches the Schwarzschild radius.

[725686]

I don't know absolutely nothing about black holes, but I think that amount of mass has nothing to do with it. It is the density what matters. Wasn't the LHC trolled about the possibility of creating tiny black holes?

[gnaritas]

No, stellar black holes have less mass than the stars they came from. When a star goes supernova it ejects much of its mass, the core can collapses into a black hole. Black holes don't form due to mass alone, but density of mass. Collapse the earth to the size of a golf ball and it'll become a black hole, but of course the earth doesn't have the necessary gravity to do that. Neither do stars, until their cores collapse and get dense enough to cross that line and become black holes.

[dredmorbius]

No.

What you're missing is that there's energy potential in nuclear structures, which can be released through nuclear fusion (and in some cases: fission, though that's not materially significant for stars).

What a star is, basically, is gravity and the strong nuclear force duking it out. Eventually, in most cases, the strong nuclear force wins. How long that takes, and where that ends up, depends almost entirely on the initial mass of the star itself.

For low-mass stars ("brown dwarfs"), there's only just barely enough gravitational pressure to stimulate nuclear fusion. The stars may radiate only in the infra-red, and have lifespans of trillions of years, after which they gradually cool to the background temperature of space, having exhausted their hydrogen. These are from a few times the mass of Jupiter to significantly less than the mass of our Sun.

A star such as our Sun has a lifespan of around 10 billion years. Gravity compresses its hydrogen and helium sufficiently that the atoms are no longer distinct, but form a plasma. In that, hydrogen atoms fuse (through several different chains, it can be complicated), producing first helium, then, as lighter elments (or at least their nuclei) are consumed, temperatures rise, and those in turn fuse, through carbon, nitrogen, oxygen, silicon, and finally resulting in iron, with core temperatures increasing all the while.

Iron is at the bottom of the nuclear potential curve -- you cannot release energy by fusing it (as you can lighter nuclei) or fissioning it (as you can heavier nuclei). Which means that gravity at this point wins, and the collapse of the star progresses to its next stage(s).

Incidentally, the rate of energy release by unit mass of the Sun is about 1/5 that of your own body. It's not that the Sun is highly energetic, but that there's so much of it.

Larger stars are fighting a stronger pull of gravity, so must release more energy with time to overcome its pull. They burn faster, and hotter. Basically: the bigger the star, the shorter its life, with some of the largest having a lifespan of only a few tens of millions of years.

There's a maximum star size (through normal formation) given that once a star begins hydrogen fusion the generated pressure drives off any additional potentially in-falling gas. This is roughly 40x a solar mass as I recall.

There may be models in which more massive stars can form through collisions, though I've no specific information on this.

In all of this: the star that forms may have enough mass to form a black hole, but so long as it can undergo fusion it will be able to resist that.

The ability to resist stops when silicon fuses to iron in the star's core. Once the silicon is exhuasted, a life-stage which takes about one Earth day, the core collapses. Depending on total mass, this may result in a white dwarf, a neutron star, or a black hole. It also generates absolutely immense heat (through pressure and kinetic energy) which rips through the rest of the star -- a nova or supernova. Much of the star's mass may be lost, and additional heavy elements (beyond iron) are formed. Which is to say: all of you that isn't hydrogen or helium, was once part of stellar fusion or a supernova (some heavy elements -- gold and platinum-series elements, may come from neutron-star collisions).

Which is how you can have stars more massive than black holes. For a while.

[colordrops]

What would the be the properties of the surface of these brown dwarfs that cooled to low temperature after trillion of years?

[dredmorbius]

Good question. Probably close to a gas giant.

Keep in mind that surface temps could be anywhere from, say, a few low-thousands of K (dull red) to hundreds, possibly even tens. Constitution would likely be roughly 75% hydrogen / 25% helium, and up, with some lithium and possibly higher metalicity constituents.

What you wouldn't have though is a high-silicate crust with a bunch of CNO -- carbon, nitrogen, and oxygen.

Temperature and chemistry can both be ascertained through spectral emissions (wavelength corresponds to temp via blackbody, emission or absorption spectra to chemistry).

I'm not an astronomer though I've got a pretty good lay grasp of the subject. There may well be some literature on dwarf star chemistry and temperature, and Wikpedia is likely a good starting point.