Death of a Black Hole

This is a very Frequently asked question...

How do black holes die?

Well, some people think they last forever, and they're very nearly right. Black holes do die, but it is on a timescale of billions of years. No one has ever seen a black hole actually die, but astrophysics Stephen Hawking has proposed a theory about black hole death.

Black holes aren't black at all. In fact, they constantly emit radiation in the form of small virtual particles that are born in pairs out of photons, i.e. particles and antiparticles. At the event horizon, there is a sort of gravity well, which seperates the innards of the black hole from the outside. At this point, the energy is sufficient enough for the photons, or energy packet (also called quanta) to 'condense' into matter. Since the law of conservation of mass and energy is maintained, the particles have to be each others' anti selves.

Mostly, these particles do not last more than a few millionths of a second, quickly annihilating each other to form photons again. This is the actual Hawking radiation, the emission of photons by such a physical reaction. Unfortunately, this radiation is much too faint to be detected by even the most powerful telescope on earth.

But sometimes, it doesn't work out so well for the particle. They do not get to meet with their other halves. The anti particle gets whisked away beyond the event horizon, falling down the intense gravity into the object's core. The other particle is left partner less, and wanders off alone. Since it has a positive mass, it has a positive momentum and it can escape the gravitational field of the black hole (mind you, the event horizon is the point of no return. Anything outside that can and will escape).

The antiparticle goes and annihilates with a positive particle at the black hole's core, thereby reducing the mass of the black hole by one particle.

So what happens then?

Billions of years later, the mass of the black hole drops sufficiently to fall beneath the Chandrasekhar limit of 1.44 Solar Masses. At this stage, Pauli's exclusion principle kicks in. The particles, that have been oppressed and crushed into this tiny little space for billions of years by their own gravity, explode outwards violently. The energy released in this explosion is phenomenal. It could blow away half of the galaxy in which the black hole resides. Let's just say that if the Black hole at the centre of a galaxy a few billion light years away exploded by this method, when the light eventually reached us, it would make our nights as bright as day. That is the awesome power of an exploding black hole.

Will we be able to see one any time soon?

No. Like I said, black holes don't live forever, but they come very very close. If the theory is correct (and so far, it has never been disproved, and all evidence seems to back it), it'll be a few billion years (few hundred billion years, actually) before one does die. And even then, depending on its distance from us, it'll take even longer for the light to reach us. But, as you know, Earth isn't gonna last more than 5 billion more years, thanks to our daddy, the Sun. It's gonna become an old star, expand its size till it engulfs all four terrestrial planets (Mercury, Venus, Earth and Mars) before shrinking into a tiny white dwarf that will eventually cool off to become a hunk of helium floating in space.

More on this cheerful topic of the Earth getting fried next time... Tune in to Young-Geniuses.

Black Holes continued...

Delving deeper into black holes after my last post, I think I'll need to add a few points here.

Black holes are formed not when the whole star has a mass of 1.44 solar masses. That is the mass of the collapsed core alone. The Chandrashekhar limit (1.44 Solar Masses) is just the limit beyond which the exclusion principle cannot support the core and it collapses.

Here's what actually happens.

If the mass of the star is 24 solar masses (henceforth, I'll refer to it as Msun) or less, about 7/8ths of it is blown away by the explosion. The core suffers the original gravity of the star made much worse by its existing radius. So it collapses. Now there are three possible endings for this sort of stellar death.

1) White dwarf:

This is a type of star that is extremely hot, and extremely dense. White dwarf stars have from 0.8 Msuns up to 1.44 Msuns. When its parents star is about to die, it expands into a Red Giant, blows off its outer layers, and collapses into this beauty. Like I said, extremely dense. A teaspoon of white dwarf material can weigh upto 5 tons on earth! Gravitational force on this star can crush you into a pancake in less than a second.
Unfortunately, they haven't got the pressure required to initiate nuclear fusion. So even though they may live for a long long time, they'll run out of heat eventually, and die.

2) Neutron star:

This is what results when a White Dwarf star has more than 1.44 Msuns, but less than 3 Msuns. These are born pretty much the same way. But because of the intense Gravitational force, even the atoms are squeezed together. The electrons combine with the protons in the nucleus to form neutrons. Now, the entire star is composed of just atomic nuclei composed almost entirely of neutrons. This is very dense, and by very, I mean very. A teaspoon of this stuff can weigh a million tons on earth. That's VERY VERY heavy.
Neutron stars live on almost forever. The curvature of space-time created by Neutron stars is second to only black holes.

3)Black Holes:

When a stellar core has a mass of more than 3 Msuns, it turns into the ultimate cosmic mystery; a black hole. Now I've explained about black holes already, so I won't go into much detail, but all in all, at the event horizon, the curvature is so strong that time actually stops. Even light cannot escape the gravity of this monster.
Needless to say, a teaspoon of this substance on Earth would literally swallow it up, along with half the Solar System. So don't try bringing a black hole to Earth.

Black holes don't live forever, but they come very very close. This is because of a type of radiation called 'Hawking Radiation'. More detail on this subject will be in the next post. But according to this theory, black holes constantly emit radiation. In the end, this radiation reduces that mass of the black hole to so low that it crossed the Chandrasekhar limit on the way down. Once it's less than 1.44 Msuns, it does not need to be contained in the centre, and explodes in a magnificent display. If Sagittarius A* (that's Milky Way's very own black hole) explodes, we won't need the Sun. It'll be so bright, it'll be 'day' even at night.