Science Questions

Why don't black holes explode once they lose enough mass?

Sun, 16th Oct 2011

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Dennis Slone asked:

If it takes a very massive star collapsing to form a black hole, and Hawking's radiation eats it away, then why doesn't it blow up after enough matter is eaten away?


I was listening to one of your previous Naked Astronomy shows and they stated that eventually the black hole will disappear.† If after awhile it no longer has enough mass to sustain being a black hole why doesn't it just blow up?


We put this question to Andrew Pontzen...

Andrew -   I'm Andrew Pontzen.  I'm a research fellow at the Kavli Institute for A black holeCosmology at the University of Cambridge. 

The picture the question paints is that we have this black hole and Hawking radiation is making that black hole shrink slowly until the black hole gets to below some critical mass and then suddenly it is not a black hole anymore and an explosion follows. 

That's not quite an accurate picture because actually, there isn't critical minimum mass for black hole.  It is more to do with the density of the matter.  Itís about packing matter in really tightly and so, itís even possible to have sort of microscopic black holes. 

So what actually happens is that a black hole never stops being a black hole. Once you've formed a black hole, that object is going to stay a black hole.  But Hawking radiation does slowly eat away at the mass in a black hole and in fact, the prediction is that a black hole should radiate faster and faster as it becomes smaller and smaller.  So, right at the end, as a black hole becomes tinier and tinier, we do expect that to be sort of energetic pop at the very end of its life as the black hole shrinks away to nothing and emits a final burst of very intense radiation.  So, there is sort of a bit of an explosion right at the end, but that's not because itís reached a minimum mass.  Itís just because itís radiating away energy faster and faster.


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Hi Dennis,

Very cool question,

You need mass for an explosion. If the blackhole is "eating its own mass" there will be no mass for an explosion.

This is not to say that it may collapse when the mass is not grate enough to sustain its self.

Kind Regards

nommiiss, Fri, 11th Mar 2011

As the blackhole evaporates it will eventually reach the planck mass of 22 micrograms then disappear in a 2.2*10^10J burst of radiation.
This will only happen to primordeal blackholes as those created by the collapse of massive stars cannot evaporate as the temperature of CMBR is too high. syhprum, Fri, 11th Mar 2011

I suppose what you are trying to ask is, if there is a minimum mass for a star to become a black hole, shouldn't the black hole no longer be capable of staying a black hole once it goes under that minimum mass? The answer is that any amount of mass can be a black hole so long as that mass occupies a region within it's Schwarzchild radius. That is, it can be a black hole if it is compressed enough. It's just that it takes a very heavy star in order to sufficiently compress matter to that point. Once it reaches that point, it's "stable" (except in regard to Hawking Radiation) and can stay that way. Supercryptid, Fri, 11th Mar 2011

This is all theory, but there is ideas of them exploding.

"But what happens when the tiny black hole evaporates so small that it becomes so tightly wrapped around the structure of a fifth dimension (other than the ďnormalĒ three spatial dimensions and one time dimension)? Well, the black hole will explosively show itself, much like an elastic band snapping, emitting energy. These final moments will signify that the primordial black hole has died. What makes this exciting is that researchers believe they can detect these events as spikes of radio wave emissions and the hunt has already begunÖ"

Measuring the Emission from the Fifth Dimension.

On the other hand, what goes on inside a black hole nobody knows. You might view it as a 'opening' into 'something else'. Assuming that the universe have its own way of 'enclosing' singularities they might just 'vanish' at some point, like our expansion make new 'space' just appear without any 'explosions' as far as I know. If you imagine the gravity in a black hole to be at 'max' and then assume that gravity is 'everywhere' you could imagine that a black hole is the natural state of gravity being only 'energy'. This 'energy' have no measurable size as we expect, much in the same way as point like particles like photons, so maybe that is what we all rest on, immeasurable gravity. yor_on, Fri, 11th Mar 2011

Yes Black holes will eventually evaporate. 

The smaller the black hole is the faster it will evaporate so it will go out quite quickly in its last stages but it is not an explosion in the true sense of the word

BUT  and a very big but.  You have to put things in the correct perspective and time scale

A black hole will not evaporate until the cosmic microwave background is at a lower temperature than the evaporating black hole because all the time it is higher it will be soaking up more energy than it is emitting. 

Most black holes that are being formed from the collapse of a star are greater than 5 solar masses and they can get up to many billions of solar masses.

Let us consider the properties of a very small black hole  on this scale say 1 solar mass


a 1 solar mass black hole 

has a temperature of 61 nanokelvin  about  one ten millionth of the temperature of the CMB at the moment so it wont start evaporating for many millions of times that the universe has been in existence today and even when it does it will take  2 e +58 years to evaporate into a totally cold universe.  This is an almost unimaginably long time.

The final demise is a very small pop only managing to reach solar luminosity for the last few nanoseconds so one could only be detected if it was quite close.

Soul Surfer, Sun, 13th Mar 2011

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