So far we have employed our attention to the stars — those living, breathing edifices of the universe. While a lot still remains to be said about them, let us take a shot at those that are “not visible” at all, but still have a powerful presence.

Rest assured it is not the enigmatic dark matter (about which in detail at a later stage), but the mysterious black holes that we wish to explore today. Before we proceed, let us know three basic things about them:

1. Black holes are dead or dying stars. All stars do not die in the same manner. Only massive stars become black holes.

2. Gravity, which has no nuclear reaction left to counter it, is so strong and unbridled that it does not allow light to escape.

Light would remain trapped. Keep in mind the speed of light, which in this case is its escape velocity. You know that it is a stupendous 300,000km per second.

3. A black hole naturally cannot be seen. It is recognised (found out!) from its massive gravity working on nearby stars, at least perturbing their orbits, especially that of the remaining binary. And other “nearby” stars too, if there are any in its vicinity.

A black hole is a star in its death throes i.e. it has (almost) lived out its life. It is the compressed remnant of a star whose nuclear furnace has burned out. All that is left is gravity, which without the outward flowing nuclear force, is left on its own.

That is, without a balancing force, gravity can now play havoc. And havoc is all it has the freedom to play. No light, no radiation, no nothing. Only and only gravity! And that on a massive scale.

At this point in time, the core temperature of a black hole may reach, and even exceed one billion degrees Kelvin. We will soon see how.

Although a large portion of the star (its gas) is tossed into space in the form of a supernova explosion, yet enough matter remains to turn it into a successful black hole (about supernovae at a later stage).

The German scientist, Karl Schwarzschild predicted in 1916 that when a star several times the size of the sun collapses, it is reduced to a small size disallowing any matter or material to escape its confines, not allowing even light to escape, thereby calling it a black hole. The paradox is that a black hole must continue to shrink to a stage called singularity.

A singularity is a condition where a star continues to shrink inexorably. At a radius of 9km (diameter 18km) it will become a black hole. It will continue to shrink more and more, approaching infinite density, and infinite tidal forces. Now we can safely assume that a spoonful of matter may weigh a billion tonnes.

Interestingly, when a star the size of Sun, present diameter 1,390,000km, 110 times that of Earth or smaller, runs out of fuel, the outward pressure created by thermo-nuclear forces is no longer present, the force of gravity within the star takes over crushing it to the size of Earth. Such a star is called, white dwarf.

Stars with more than three and a half times the mass of Sun have an exotic state awaiting their demise. Nothing can withstand the inward crush of these stars. Their matter is compressed into a state which approaches infinite density and infinite gravity. No radiation can escape the surface of such a body. That is why it is not visible from now on, so it is called a black hole.

Material may be pulled from another ‘nearby’ star, which may be second star of the binary pair. As the mate’s material falls into the black hole, its temperature rises first to tens of million, then to a billion degrees Kelvin. Now it produces X-rays (electromagnetic radiation of shortwave length between that of ultra-violet and gamma rays).

Black holes continue to absorb the attention of scientists, cosmologists and astronomers round the world. Much work is done theoretically, but giant strides have been taken in this regard based on basic laws of physics, as well as mathematics.

There is a lot more to black holes, but for now we should regard it as enough. Next we can talk about X-rays, Quasars — those quasi-stellar objects that look a lot like stars but in reality they are not, and supernovae.

In the next issue!

The writer is a professional astronomer and a former head of PIA Planetaria. He can be reached at astronomerpreone@hotmail.com