# How long does it take a black hole to gain seven billion solar masses?

## Main Question or Discussion Point

We don't see matter rushing to a center point as though down a drain anywhere in the universe. Black holes' accumulation of matter must (except perhaps rare moments like a collision with another black hole) happen over a long period of time. Assuming a star, destined to be a black hole, began at a reasonable mass, say seven times our sun, how long would it take it to reach the billion times that mass at which black holes have been estimated today?

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We don't see matter rushing to a center point as though down a drain anywhere in the universe.
You're right we don't ever see that per se, but we do see the effects of that in quasars, high-mass x-ray binaries, etc. Numerous cases are fairly conclusive that 'lots of matter down the drain' is what's happening.

Black holes' accumulation of matter must (except perhaps rare moments like a collision with another black hole) happen over a long period of time.
Very true.

Assuming a star, destined to be a black hole, began at a reasonable mass, say seven times our sun, how long would it take it to reach the billion times that mass at which black holes have been estimated today?
That would take far longer than the age of the universe. But if you were able to start with a star a few hundred times the mass of our sun (difficult to do that, and uncertain if its possible) it would take just about the age of the universe (and therefore would be possible).

This is an active question in research. One of the weird things is that we see quasars (super-massive black-holes) very early in the universe.... which means they formed somehow, and very rapidly.

So am I right to say that most stellar Black holes have low probability at the current age of the universe ? sorry to hijack this thread.

So am I right to say that most stellar Black holes have low probability at the current age of the universe ?
Low probability of what?

Chronos
Gold Member
Stellar mass black holes are not deemed particularly improbable. Hugely massive stars are relatively rare, but still exist in large numbers. These goliaths burn up their fuel and go supernova in millions, as opposed to billions of years for average mass stars such as the sun. And only massive stars are capable of going supernova. Conventional wisdom is stars in excess of about 8 solar masses are doomed to become stellar mass black holes. This, however, is not always the case. For example, a paper in 2005 reported a neutron star with a progenitor mass >40 solar masses [see arXiv:astro-ph/0509408v3]. Obviously, the star found a way to shed nearly all its mass before settling down as an unassuming, ordinary sized neutron star. This is, perhaps, the result of an assymetric explosion, or processes during the supernova phase not yet understood. Perhaps, stellar mass black holes do not even arise from supernova. They may be the result of other processes, such as neutron star mergers in binary systems.
As far as supermassive black holes go, there is at least some evidence they may have skipped the whole star thing and formed directly from primordial gas clouds in the early universe. See, for example, http://www.universetoday.com/11734/never-a-star-did-supermassive-black-holes-form-directly/

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So you are saying that in order for the super massive black holes we see today to have accumulated the mass they have, they must have started out big? And why eight times bigger than the resultant black hole? Would you say all initially formed black holes had to have had original solar masses eight times as large?

if a single star originally formed a black hole it was likely nowhere near a billion solar masses but only a few hundred solar masses. The earliest it could have formed would be after the formation of stars began, so lets say the to gain the amount of weight we see today must have taken it less than 14 billion years, to gain 18 billion solar masses, so it was eating more than a sun a year?

Some good questions being asked here. Is there enough time since the Universe began to create Black holes as massive as 4 Million solar masses eg Sagittarius A* or the largest known black hole of 18 Billion solar masses?

Drakkith
Staff Emeritus
Wouldn't this depend on the availability of material to suck up into the black hole? IE a black hole near the galaxy center would probably accumulate mass much quicker than one at the outer edge or in a much less dense area.

Chronos
Gold Member
Many of the supermassive holes responsible for quasars are billions of light years from us, hence formed billions of years ago. At least some of them apparently formed in the first billion or so years after the big bang. Explaining how they came to be so massive in such a short time is a mystery. The universe was more crowded back then, but, it still is difficult to explain such massive bodies at such early times. How ever it happened, it happened in a hurry.

The 8 solar mass thing is the lower mass threshold for a star undergo core collapse. It is generally believed core collapse events are the most likely route to stellar mass black holes. But, the whole thing is rather complicated and involves a fair amount of guess work.

While I tend to shy from citing Wiki as a source, they have a pretty good write up here:
http://en.wikipedia.org/wiki/Stellar_evolution
I have fewer reservations about Astronomy Online, which also has nice article on the subject: http://astronomyonline.org/stars/blackhole.asp

Anyone have any idea of whether or not there is even the remotest of possibilities (well okay, remotest within SOME reason, ie. not me quantum tunneling through a wall after one try) that super-massive blackholes were originally primordial blackholes during the pre-inflation era, and with inflation expanded to garguantuan size? It is especially puzzling to me, like Chronos said that quasers existed when they did. Not only that, the fact that they ONLY existed then.

I may sound like an utter moron, if so excuse me I've not heard anything about this or anything but just pondered it as something interesting to think about.

Does anyone know if time dilation would slow the rate at which a BH can increase in mass, at least through known mechanisms like accretion or collision? This was one of the reasons I posted the question "time (seen by distant observer) for matter to fall to the event horizon of black hole". Or, am I thinking about this incorrectly?

Jonathan, that's my point exactly. We see things enter a black hole very, very slowly due to time dilation. Also, black holes undergo tremendously long rests where they are not feeding at all -- even the ones from the first billion years of the universe behave this way, even though matter was denser then.
And will everyone please look up at Tanalorn's concise re-phrasing of the question?