Evaporation of a mini black hole
Black holes get the energy to radiate Hawking radiation from their
rest mass energy.
So if a black hole is not accreting mass from outside,
it will lose mass by Hawking radiation, and will eventually evaporate.
For astronomical black holes, the evaporation time is prodigiously long
- about 1061 times the age of the Universe
for a 30 solar mass black hole.
However, the evaporation time is shorter for smaller black holes
(evaporation time t is proportional to M3),
and black holes with masses less than about 1011 kg
(the mass of a small mountain)
can evaporate in less than the age of the Universe.
The Hawking temperature of such mini black holes is high:
a 1011 kg black hole has a temperature of
about 1012 Kelvin,
equivalent to the rest mass energy of a proton.
It is not well established what an evaporating mini black hole
would actually look like in realistic detail.
The Hawking radiation itself would consist of fiercely energetic
particles, antiparticles, and gamma rays.
Such radiation is invisible to the human eye,
so optically the evaporating black hole might look like a dud.
However, it is also possible that the Hawking radiation,
rather than emerging directly,
might power a hadronic fireball
which would degrade the radiation into particles and gamma rays of
less extreme energy,
possibly making the evaporating black hole visible to the eye.
Whatever the case, you would not want to go near an evaporating mini black hole,
which would be a source of lethal gamma rays and energetic particles,
even if it didn't look like much visually.
The animation at left is a fanciful depiction
of the final moments of the evaporation of a hypothetical mini black hole.
In the final second of its existence,
the mini black hole radiates about 1000 tonnes of rest mass energy.
Such an explosion is large by human standards,
but modest by astronomical standards.
An evaporating black hole would be detectable from Earth
only if it went off within the solar system,
or at best no further away than the nearest star.
What's the cross shape?
Telescopic diffraction spikes, added for artistic effect.
Compare these beautiful Hubble Space Telescope pictures of
the Orion nebula
or of stars in the Messier 4 globular star cluster.