Throwback Thursday: How do black holes get so big, so fast? (Synopsis)

I've never found that pair-instability explanation convincing. Presumably we just have a equation of state, relating density temperature and pressure (and internal energy). Then I'm guessing the slope of this relation reaches some critical value whereby a contraction just can't stop (i.e. the adiabatic heating from contraction doesn't raise the pressure enough to stop it). Do you a a more physical description that doesn't require elaborate model building?

@Omega Centauri #1: You're quite right that you "just" have an equation of state, which relates density, temperature and pressure.

The temperature tells you the radiation spectrum (black body), and when the temperature gets high enough, a significant fraction of that spectrum must be above 1 MeV (the Planck equation doesn't know about pair production or anything else).

Once you have multi-MeV photons in a hot dense plasma, you get pair production. The cross-section is straightforward to compute (in a first-year graduate course on field theory), and is relatively high. So most of those multi-MeV photons disappear within just a few mean free paths.

That distorts the spectrum, cutting off the mostly high-end tail. That in turn alters the naive equation of state we started with (see, it isn't "just" an equation of state: you need the microphysics to inform the macrophysics).

Cutting off the tail is equivalent to reducing the effective temperature, which reduces the pressure, which drives a density increase. You'd think that would raise the temperature back up, but it doesn't. The physics still cuts off the high-end tail, which (roughly) means that the temperature plateaus, even as the density goes up. And up.

I'm quite interested in how black holes and other phenomenal structures in the universe arise. I found it fascinating that black holes have a mass that is significantly greater than that of the sun, but between 130-250 solar masses, no black holes form due to a lack of energy and cooling down. Does the fact that massive black holes in distant galaxies and supenovas increasing in size support the evidence that our universe is expanding?

Stars pump out radiation to balance the force of gravity that is trying to compress it. Stars that are roughly 5 times the mass of the sun are capable of forming black holes. This occurs because no energy can be formed when fusing 2 iron atoms together. What results is a split second where the radiation of the star is 'turned off', the star therefore produces no outward pressure and the pressure of gravity wins causing the star to implode. The mass of the star collapses down into a smaller and smaller space leading to the formation of a black hole.

I found it fascinating that ..., but between 130-250 solar masses, no black holes form due to a lack of energy and cooling down

I find it fascinating that you made that up and found it fascinating.

It's not true.

Jodi, your post includes the truth but draws entirely incorrect conclusions from it.

Black holes don't form because fusion stops being exothermic at Fe. Novas and Supernovae occur because of that. Supernovae may achieve black hole status, but only because of their mass after the event.

And stars don't produce radiation to balance gravity. Radiation pressure exists and it does balance gravity, but only approximately, and when it doesn't the star expands or contracts. It doesn't produce the radiation to balance gravity, though.

You're mistaking the result with an intent.

The nature of science is such that new research will always be going on, disproving and proving theories and hypotheses that have been previously thought true. Therefore, rather than throwing out all of our theories as to the formation of the universe, let us use the information that has been researched already and build on it, validating it and invalidating it where necessary. So perhaps what we perceive as “back in time” could merely be what is occurring in the universe now, and we have perceived it differently. Sorry to throw a spanner in the works. How has the sun not begun to disappear as fast as the black holes have grown in the universe? (Mind you, I’m glad it hasn’t!). Are there many black holes in our galaxy that pose the kind of threat mentioned in the below-cited article?
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(See http://hermes.sussex.ac.uk/content/massive-black-holes-halt-star-birth-…).

So, we need huge concentrations of stars to get huge black holes. The problem is now to explain where these huge concentrations of stars come from, isn’t it? Do some mathematical simulations from the Big Bang give the needed concentrations?

Who are all these users from south africa with numbers in their names? They started appearing couple of months ago out of nowhere.. Bots.. or? Ethan?

@Sinisa #9: I think they're college students. A few of them have labelled the numbers (always 8 digits, starting with 1...) as "sudent number" or "student". Maybe they're being asked to read these blogs as part of a survey course, or something?

It is quit a large ask to imagine just how old the universe is, us as people can not put it in to context as the amount of time we been around is really but the tip of the the knife. I learn in one of my geology classes that the earth is approximately 3.4 billion years old and that if you were to squeeze that entire period into a year for scale purpose, that humans would only appear in the last 3 seconds of the last day of the year. This truly show us we have no understanding of just exactly how eminence time is.

@ Michael
noticed the university/student as well in some posts. Don't know, just a hunch that something is off. The style of writing, the content...can't really put my finger on it. It reminded me of people who are paid to leave comments in order to get the site ranking up.. They don't do follow-ups, even when they asked some questions. Like I said, just a feeling.. I might be very wrong, if so.. sorry students :)

Michael, thanks for that explanation. I have an addition question regarding the pair-instability supernova. From the virial theorem we would expect that the internal energy of the initial configuration to be half of the (negative) gravitational potential energy. So we would need an additional source of energy to blow the star apart. I presume this comes from fusion.

From the article:

When you reach iron-nickel-and-cobalt in the core — the most stable elements (on a per-nucleon basis) — there is no more fusion that can occur, since you’d actually lose energy by making heavier elements.

I somewhat dislike this phrasing. Additional fusion can occur, but because its endothermic it doesn't 'hold the star up' against collapse. Better phrasing: "when you reach iron-nickel-and-cobalt in the core — the most stable elements (on a per-nucleon basis) — then additional fusion no longer 'pushes back' against infalling matter. Additional fusion no longer holds the core up against gravity. Further nuclear reactions don't stop the core from collapsing, they actually assist in the collapse...and so it collapses."

@Omega Centauri #13: You're right; and Ethan is *MUCH* better qualified to get into this level of detail (I'm a particle physicist with a primary interest in e+/e- physics at B Factories -- one particle at a time, thank you very much :-) ).

I think the key point here is that you don't have a massive ~150 Msol star blowing up ab initio. It starts out as a gigantic H/He star doing fusion as you'd expect.

As it burns through CNO, Si, and heavy elements you eventually get to a state where the core d/p/T equation of state gets high enough that the fusion gammas have a significant fraction above 1 MeV (pair threshold). So you start getting the runaway collapse I described above.

But even while that's going on, the other layers of the star are still fusing, so there's still energy input there which can drive the final explosion.

It was at one time considered impossible to get a star formed heavier than around 60-90 solar masses. The idea was that they would collapse into a black hole before any of it could manage to reach stable fusion.

I'm a bit twitter and bisted about it, because my second undergrad paper/discussion was about supermassive stars (intended to be supermassive black holes, but I picked stellar objects), which got marked down for "not talking about black holes" despite my abstract saying what it was about, and the title of the piece we had to select was from a list, that list choice I took being "Supermassive stellar objects". Unknown to me, the primary lecturer marking believed it was impossible and I'd heard years ago about evidence for a star at least 120 solar masses, maybe 180, in a single object, so went with that.

Another lecturer did believe it was possible, but they'd assigned a different topic heading and so was not asked to mark the paper.

The marking included the statement "You did not mention supermassive black holes" and in response to my pointing out it wasn't ABOUT black holes, but luminous stars was *replied* with "But another lecturer agreed with the marking!" and "But B is a good mark!". Neither counter the facts, the latter merely indicating that either the comment was pointless as it had no effect on the marking, or that my marking could have been B+ or better if they'd bothered with the abstract.

When they engaged in normalising the marks of the year-end exams (and getting the QUESTION WRONG in at least one paper) and when I (the only one bothered to complain) told them the error, they were professing surprise and on the complaint itself "You don't even have a degree! We have PhD's!!!" ensured that I lost all inclination to have respect for someone merely because they gained a qualification or position.

I already HAD that impression, but only for a few lecturers, one of whom berated us in lab for testing a fourier analysis on our whistle, seeing who could get a sharper peak, rather than twanging rulers to prove the harmonic law, with "You can't take your vocal chords out and measure them!" then stalked off as we looked at each other, perplexed and thinking "I thought you used your lips not your vocal chords?".

But either they closed ranks or they really didn't think things through.

I *did* get them to change their process, but too late for me. They no longer took the physics with astrophysics people out and renormalised the smaller group, which left me and one other (who was damn smart) being the only two in Quantum Optics and Laser Physics. I could have gotten the same mark by just putting my name on the paper...

As you can see, still twitter and bisted :-)

I enjoyed reading this. I am always fascinated by the universe, and it is so interesting to read that there are supermassive black holes at the centres of quasars many billions of times the Sun’s mass. It is difficult to wrap my mind around just how vast and massive the universe really is.
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I've always wondered what happens at the center of a black hole. . . 13242033

I read the solution and I found it really interesting how the black holes form. There is so much more that goes on than people imagine. It is just another example of some of the magical phenomenons that occur in our universe. 15019838

The universe is truly a remarkable and complex place, one we might never really understand or even begin to understand. I did find this very interesting but I am curious about the entire process that happens in star forming regions that results in such big stars ?
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If black holes are really greater in size at the beginning of their lifespan and slowly decrease in size, does this mean our perception of black holes is incorrect? Are black holes really the beginning point for galaxies and slowly decrease in size until the galaxy no longer exists?
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does this mean our perception of black holes is incorrect?

Depends on what your perception of black holes is and how the fact you brought up changes it.

As to them being the beginning point of galaxies, no, they aren't losing mass and creating the rest of the galaxy from that mass.

Please explain to me how black holes work. I thought they consumed everything nearby?