Once a large enough star expends the fuel needed to sustain equilibrium, how fast will that star collapse into a black hole? Days, weeks, years?
I’m trying to picture what it would look like to witness that.
If the star is massive enough it can collapse directly to form a black hole without a supernova explosion in less than half a second. A black hole can also form via the collapse of a neutron star into a black hole if the neutron star accretes so much material from a nearby companion star, or merges with the companion star that it gets pushed over the neutron star mass limit and collapses to become a black hole. This process could take a long time, maybe a million years or more depending on how quickly it accretes the material. Once the neutron star is over the mass limit, which is at a mass of about 3 solar masses, the collapse to a black hole occurs in less than a second.
Wow, that’s fast. So if it was observable, it would essentially just vanish before your eyes. The matter on far outside of the star’s surface would be traveling pretty fast when it hit the point of singularity.
Yeah, since the speed is proportional to gravitational force and that much matter creates a honkin’ huge little g.
Incredibly fast indeed compared to what I might have intuitively imagined!
However, it may be useful to more precisely define what one means by “collapse into a black hole”. It could only meaningfully refer to the observed formation of an event horizon. It cannot meaningfully be said to refer to the stabilization of the internal structure of the black hole in its final form. For one thing, if the black hole was particularly massive, the distance to the central singularity might be considerable and the travel time to the singularity could be considerable despite the immense gravitation. For another thing, time probably doesn’t mean what we think it does inside the event horizon, since there is good reason to believe, per the Einstein field equations, that beyond the EH the radial spatial dimension and the time dimension switch places. This is also a useful way of picturing why there is no “out” direction to escape from inside a black hole; “out” is now your past, and the singularity is your future.
So IOW, half a second or a second is the time it would take for the collapsing object to disappear behind an event horizon, but says nothing about what may continue to happen inside.
While a black hole will “ring” some after forming (the mathematics describing this are actually exactly the same as the ringing of a spherical bell), the ringing dies off exponentially, on a timescale corresponding to the diameter of the hole. A stellar-mass black hole is only a few kilometers across, so that process will only take a matter of tens of microseconds.
And the formation of a black hole wouldn’t generally look like “Poof, now it’s gone”. The formation of a black hole is an extremely energetic event, and that extreme energy will usually (except in the case of smaller black holes merging) manifest in some extremely obvious way. You’ll typically be too distracted by the supernova to notice the black hole itself. And supernovas usually stay near peak brightness for times on order of several months.
The exception is the events behind short-duration gamma ray bursts. Those are believed to be cases where all of the radiating energetic matter (what’s producing the gamma rays) gets engulfed in the event horizon within a few seconds, and so the radiation does abruptly stop. But you need a fairly rare set of circumstances for that to happen.
It is fast:
The outer edges of the core collapse inward at 70,000 meters per second, about 23% the speed of light. In just a quarter of a second, infalling material bounces off the iron core of the star, creating a shockwave of matter propagating outward. This shockwave can take a couple of hours to reach the surface. SOURCE
You’ll never see the black hole form. It is the core at the center of a star and well hidden by the supernova around it when it forms.
That said, there are hypermassive black holes like TON 618. These are much, much, much bigger than stellar mass black holes. A stellar mass black hole might be the size of a large city. TON 618 could fit our solar system inside it 11 times. Something like 66 billion solar masses (that’s more than all the stars in our galaxy).
Hopefully, someone else here can explain better but it is believed these form in a different fashion.
The exact process by which even supermassive black holes (a few million solar masses, like the one at the center of our Galaxy) form is one of the big open questions in astronomy. They could start as a single hole that just eats a lot of stuff, or as a result of mergers of other smaller holes, or from a whole bunch of stuff that collapses together to make a black hole that starts off large, or any combination of those, but nobody’s yet been able to construct a model that actually works (i.e., gets you to the holes we have now, within the lifespan of the Universe).
I suppose we could speculate about what you would see if you could see a black hole come into being.
I think you would just see it wink into existence. There will be that moment when the event horizon forms. If you could see it happen, it would seem instantaneous. One instant it is not there, the next it is.
What would be really fascinating is if you had a super-highspeed, high resolution camera pointed at the singularity as it formed. I can’t imagine pulling that off but it would be cool if we could.
One way to “see” the formation is to look for the abrupt termination of the neutrino signal from the collapsing star.
99% of the energy released in a core-collapse supernova exits the star via neutrinos. The energy spectra of all neutrino “flavors” leaving the star as a function of time are rich with information about the dynamics of the interior of the event. For a supernova in our galaxy, current and upcoming detectors would record thousands of neutrino events. It is expected that in the case of black hole formation the neutrino signal will exhibit a sharp cutoff as the event horizon develops.
Spin off question:
From reading this cite:
The outer edges of the core collapse inward at 70,000 meters per second, about 23% the speed of light. In just a quarter of a second, infalling material bounces off the iron core of the star, creating a shockwave of matter propagating outward. This shockwave can take a couple of hours to reach the surface.
As the wave passes through, it creates exotic new elements the original star could never form in its core. And this is where we get all get rich. All gold, silver, platinum, uranium and anything higher than iron on the periodic table of elements are created here. A supernova will then take a few months to reach its brightest point, potentially putting out as much energy as the rest of its galaxy combined.
the stellar collapse happens in seconds, the rebound happens in hours, What is the mechanism for the light lingering for months?
This article discusses that
The basic answer is that the collapse and rebound create a lot of radioactive nuclei that heat the ejecta hot enough that the ejecta glows for some time.
This seems like an error in your cite. I think they meant 70,000 kilometers per second.
I wish megameters was used more often, it would make errors like this less common.