Black holes draw such fascination that you don’t have to be a scientist to be mystified by their strangeness. Almost like elusive superstars (no pun intended), they attract a lot of attention; a lot has been written about them; they can only be observed indirectly; they are sometimes subjected to controversies; they seem to be at the centre of the world (for some); they draw in massive crowds and seem to have a trail of followers at all times; yet, very little is known as to what exactly goes on inside them…

As black as a black hole

As black as a black hole

My first encounter with a black hole was when I was reading about the life-cycle of stars, back in my (very) early schooldays. And this foray into the birth and death of stars came about from my learning about the age of the Earth. The Earth is about 4.5 billion years old – give or take some 40 million years. The Earth formed, along with the other planets, around the Sun. The Sun, itself, formed as a cloud of dust accumulated to form a giant, super-giant, ball of hot gas. Technically speaking the Sun is only just slightly older than the Earth. The dust cloud which coagulated to eventually form the Sun did not do so on its own accord. It is theorised that a nearby dying star exploded and the shock-waves rippling through space, as a result of that violent and tragic death, caused the dust cloud to amass into a rotating ball. As the dust-cloud got more and more compressed, it heated up. The core of that cloud formed the Sun while the outskirt of it eventually formed the planets we know of. The cloud was essentially a collection of hydrogen and helium molecules. The heart of the Sun is mostly this: a fierce furnace of hot hydrogen and helium. A dying star, as you can see, can be the source of another one; the circle of life rolls on…

And so, the Sun and its other brethren stars have a life-cycle. They are born, they burn and they breathe out their last. The birth of stars tend to be not so different from one another. What differs is mostly how they live and die. And the death of a star is closely related to its mass. Typically, less massive stars die a less-than-spectacular death; no fanfare, no highlights. Plus, they tend to live much longer than massive stars. The death of massive stars are awesome, to say the least. With supernovas and black holes as part of their flamboyant exit, these supergiant beasts do know how to make an impression in their final phase.

The Sun is not massive enough to become a black hole when it dies. It will end up as a white dwarf – which is to say, it will end up as a much smaller sphere of degenerate stuff composed mostly of electrons. But prior to this it would have ejected a cloud or nebula of gas that would most likely engulf Mercury and Venus as it expands outwards from the dead Sun. The Earth might be too far to feel the direct impact of this nebula but it would, inevitably, feel the heat and harmful radiation which would cause its own tragic death. Even if the Sun will not end up as a supernova, the nebula will nevertheless be a mesmerising sight (for those lucky enough to witness this event). The Cat’s Eye nebula, below, was formed when a star with a mass similar to the Sun’s reached its end.

Cat's Eye Nebula

Cat’s Eye Nebula © NASA

So what does it take to end up as a black hole? And what is a black hole anyway? Well, as mentioned earlier, the fate of a star depends on its mass – more specifically, on the mass left behind after it has ejected the cloud of gases at the end of its life. If what is left is massive enough then that core will be subject to its own force of gravity. As such, it will pull in on itself in all directions, shrinking and shrinking further until, well, until it collapses. Until it disappears from sight! Until it rips the very fabric of spacetime around it. Indeed, as Einstein has explained (please see Gravity), gravity can be thought of as a consequence of the geometry of space and time. What is meant by that is that a massive object, like a star, can cause spacetime around it to bend. It is as if spacetime was like a sheet of rubber; by placing a heavy object on a rubber sheet, that object causes a dent in the sheet. Likewise a star causes spacetime around it to be distorted. The more massive the star, the greater the distortion. And if you push that analogy to the extreme then there reaches a point where this dent, this distortion ends up as a perforation. The massive object rips through the sheet. What that means in real term is that the massive star (or whatever is left of it) causes spacetime around it to collapse into a hole.

Now, as you are very much aware (especially after tripping over and falling), gravity sucks. Its pull is dependent on the mass of the object exerting this force of gravity: the more massive the greater the pull – the harder the fall. So, with supermassive stars that end up as black holes, the pull of gravity is so large that nothing can escape from or fight back that pull. It’s not like firing a rocket off the surface of the Earth. The rocket has enough thrust to fight back the pull of gravity and rise up through the atmosphere. Near a black hole, in contrast, the pull is so strong that not even light can escape. That’s right, even light is sucked into that hole. And bear in mind that light is the fastest moving thing in the Universe. Nothing travels faster than light – not even the most powerful rocket you can think of. So if even light can’t escape the pull of a black hole then nothing can. As you have probably deduced by now, if light can’t escape a black hole then that hole is clearly black. For fear of repeating myself, a black hole is called a black hole for precisely that reason. A hole in spacetime where not even light can escape. It is black as black can be…


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