But it's possible to have a star that is heavier than a black hole.
Say you had a binary star system of two supermassive stars. The smaller one goes nova because it runs out of fuel first. The core starts to collapse, meanwhile the twin is scooping up the ejecta, and the star and the black hole start getting closer together...
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.
No, it is of a mass that according to our still standing understanding of astrophysics, is impossible to create by the collapse of "a single star".
But you can very well have this mass by the colapse of two black holes, or by one black hole that ends up consuming one or more stars nearby in due time.
My poor internal physicist thinks this is unlikely. When you read about density of black holes, it is often mentioned that the supermassive ones aren't very dense (in the sense of mass/Schwarzschild volume). I take it the smaller a black hole is, the higher its density must be. Therefore it should be unlikely for much smaller parts of a star that does not form a black hole as a whole to become black holes as they must achieve much higher density than that of the star.
It doesn't, it's just a lot denser. A star has a lot of inner pressure from the ongoing fusions inside of it. When these stop or reduce, the star collapses from its own gravity, as soon as its radius is smaller than the event horizon (i.e. the radius at which the escape velocity exceeds the speed of light) you have a black hole.
Well, not really. The mass of the black hole created equals the amount of mass that falls into it, of course, and to first order the mass of the BH will be equal the mass of the iron core of the star. That mass is however quite a bit smaller than the mass of the star, and very much smaller than the initial mass of the star. (A star has to start out with a mass > 8* the mass of the Sun to go supernova, and that will only guarantee that it forms a neutron star whose mass will be of order 1 Solar mass. The rest is lost first during the life of the star, as massive stars are so luminous they drive their own matter off from the surface, and then ejected in the explosion itself.
Edit: The article has a number off, as it states that the black hole has a mass of 15 Solar masses and that the companion is 19 times larger. The original article says the companion is 19 Solar masses, which makes more sense.
Let's assume the Star A and the Black Hole B hole orbited each other when the black hole was Star B. If Star B collapsed into a black hole of roughly the same mass, I don't think Star A & its planets would be affected much at all in terms or orbits. Maybe the Star A system would have been fried when Star B went supernova, but the orbits I don't think would change all that much. It's like how people say Earth would continue orbiting the center of our solar system if our sun turned into a black hole, as the center of gravity would be the same.
It is worth considering that a black hole's hold on its galaxy is not as strong as a sun-like star's on its accretion disk. Our Sun is 99% of the mass of the solar system, our Sagittarius A* is ~4 million solar masses, while our Milky Way is 800 billion solar masses.
On top of that, black hole growth rate slows to nearly a stop at ~270 billion solar masses, so even if a black hole manages to meet the rare conditions to get that heavy, it would still be dwarfed by its galaxy.
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?
Here is an interesting paper that says a collapsing star sheds enough mass to not ever become a black hole or form a singularity. It's from 2014, and still controversial, but interesting, and her math seems to have been reviewed. I will follow this with interest to see how it fares peer review. Evidence of black holes is indirect at the moment, which is OK for now, but it will be interesting to see how it all pans out in the coming years.
Here’s another wacky thing about black holes. It must be possible for one black hole to contain another. Imagine for example a tremendous supermassive black with a small stellar mass one falling into it. If the small one is free falling into the big one, then a few moments after crossing the event horizon it must still be in an inertial reference frame. So, one black hole can contain another. Perhaps, black holes contain a fractal where each black hole actually has many other smaller ones inside, and those have smaller ones in turn.
Dropping a black hole into a star would actually cause its power output to increase; the radiation from matter accreting onto the black hole provides sufficient power to prevent the star's collapse. See https://en.wikipedia.org/wiki/Quasi-star for more details on the physics.
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.
BlackHoles - where does the 'extra' mass/gravity come from?
I've been watching a lot of documentaries lately, and I can't figure out how a star that _radiates_ light, collapses and suddenly light can't escape? Doesn't that mean the blackhole has more mass/gravity then the star that created it?
> For cores with a mass between about 65 and 130 times that of our sun (according to current estimates), the star is completely obliterated [via a pair-instability supernova]. Cores between about 50 and 65 solar masses pulsate, shedding mass in a series of explosions until they drop below the range where pair instability occurs. Thus there should be no black holes with masses in the 50-to-130-solar-mass range.
> The million- and billion-solar-mass supermassive black holes that anchor galaxies’ centers formed differently, and rather mysteriously, in the early universe. LIGO and Virgo are not mechanically capable of detecting the collisions of supermassive black holes.
If the star is more massive, i'd describe the black hole as orbiting the star. This matters as detecting such a situation would be more akin to detecting an invisible exoplanet than a fast-moving and bright star.
No, you need something releasing a lot of energy in the middle of a star, otherwise it collapses down on itself and stops being a star, the event horizon doesn't provide any supporting pressure.
Also, the process that compresses matter to the point it collapses to a black hole is very destructive; there is no reasonable way to form a black hole in the middle of the star.
"Black holes, if you recall, are created when stars run out of fuel and die. In dying, they collapse into a much smaller mass with such enormous gravity that it sucks in just about anything that comes light years near it."
I may be wrong, but doesn't the mass of the black hole roughly equal the mass of the star? I know some is lost during the explosion, but I thought the mass stayed roughly the same and just the volume significantly decreased.
Say you had a binary star system of two supermassive stars. The smaller one goes nova because it runs out of fuel first. The core starts to collapse, meanwhile the twin is scooping up the ejecta, and the star and the black hole start getting closer together...
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