The term ``black hole'' was coined by the astrophysicist John Wheeler in 1969 to describe a kind of astrophysical object. Since then, the expression has been used very often as a metaphor, usually inappropriately. These enigmatic objects have also become stars of science fiction literature, doubtless due to their suggestive names and their peculiar properties. For those are curious about this issue, they have probably run across mysterious funnels, time tunnels and other aberrations. Many so called science popularizing works, resemble fantastic stories, more than essays to explain what is basically a simple concept.

But what are in the end, these dark portents of the heavens? Do they really exist?

To try to clean the name of this creature a little bit, let's start by clarifying that the concept of black hole (but not its overused name) was first coined by the English physicist John Mitchell in 1783, more than two hundred years ago...!

It can be said that towards the end of the seventeenth century, Isaac Newton brought sky and Earth together.

Based on planet movement studies made by Tycho Brahe and Johannes Kepler, he deduced the existence of a force that made the Sun, the Earth, the Moon and everything contained in the cosmos attract towards one another. The same law that make planets move as they do, make us keep our feet on the ground, stopping us from being ejected to space.

If we throw a ball upwards, it will ascend more and more slowly until stopping and starting its fall. The stronger the force we throw the ball with, the higher the altitude it will reach. It sounds tempting to say ``everything that ascends has to fall'' but it is not so for a simple reason: Newton's gravity force gets weaker as we go further. There is a limit velocity called escape velocity, beyond which thrown objects do not fall anymore. The escape velocity of the Earth is about 40000 km per hour. If we can make a rocket reach that velocity before running out of fuel it will no longer fall. It is thanks to this process that we can send space ships to explore the Moon and the planets.

Naturally, anyone can argue that one will not feel any lighter for being on the second floor instead of on the ground floor. What happens is that even when the Earth starts just under our feet, it spreads thousands of kilometers deep until reaching the antipodes. When we stand on the ground we are more than six thousand kilometers away from the center of the Earth, so going a few metres further does not affect our weight in any perceptible way.

The speed of light is slightly more than a thousand million kilometers per hour, and this is known since Newton's days, when Olaf Rmer measured it for the first time. If there was a star with an escape velocity that exceeded that figure, its light could not escape from itself and consequently we would not be able to see it. That idea was conceived by John Mitchell and it is what today we would call a black hole.

Two hundred years ago, hardly anything was known about the physics of the ordinary stars we see on the sky every night, so arguing about stars that can not be seen was something irrelevant. But the issue has gained interest in the last decades, as our present day knowledge of the evolution of stars could indicate that in the end black holes could really exist.

As the Sun contains more than 330000 times more material than the Earth, its escape velocity is much higher. But the size of the Sun is also much bigger, so its surface is very far from its center. The escape velocity of the Sun is sixty times higher than that of the Earth, but yet nothing compared to the speed of light.

One way to make a black hole is to take a star and add more material to it, so that its escape velocity gets higher, but this procedure is not practical. The alternative is to compact it in such a way so that its surface gets closer and closer from its center and its gravity becomes stronger. If we can make the star compressed enough, we will also have a black hole.

Of course, deflating stars is not within our reach. But stars keep inflated because they are very hot, in the same way hot air balloons are inflated . This heat comes from thermonuclear reactions that take place in their inner part, being the most common, the same that makes hydrogen bombs explode. If we wait for hydrogen (and other elements that can serve as fuel) to run out, the star will finally get cold and deflate. For a star like the Sun, this is a long process in which the star first inflates until reaching a volume tens of thousands bigger than its current size, to finally become a small star the size of the Earth (the volume of the Sun at the moment is more than a million times bigger). This kind of ``stellar corpse'' is called white dwarf.

Fortunately, there is no need to wait until the Sun runs out of fuel. There are many stars in the sky, and some of them have already become white dwarfs. In 1844, German astronomer Frieddrich Bessel discovered that Sirius, the brightest star in the sky, produced a slight zigzag movement that was hardly perceptible. He then deduced that Sirius must have been together with another star, and that both circled around each other completing one lap every fifty years. The problem was that this star could not be seen, but it was discovered eighteen years later. The little star in question (called Sirius B) is similar to the sun in quantity of material, but its size is too small for a star, approximately the same as the Earth (that is why its brightness is so tenuous that it could only be discovered with a good telescope). It was the first white dwarf discovered.

The escape velocity of a white dwarf is of some twenty million km/h, five hundred times higher than the Earth but still fifty times slower than the speed of light.

White dwarfs are extremely dense: only a spoonful of their substance would weight more than a ton on the Earth. However, it is ordinary matter made of electrons, protons and neutrons, although in a very altered state.

Indian astrophysicist Subrahamanyan Chandrasekhar calculated that the more matter there is in a white dwarf, the more it shrinks and found that there is a limit beyond which the star can no longer sustain its own weight and it shrinks until reaching a tiny size. This happens because the pressure is so high that electrons penetrate in the atomic nucleus mixing up with protons. Soviet physicist Lev Davidovich Landau demonstrated that even after this collapse there can be another kind of star corpse called neutron star. In this kind of object the whole star remains reduced to a small ball of a few kilometers of diameter, a whole star shrivelled to the size of a mountain...! When a star runs out of its combustible, if it still has more than one and a half the amount of matter of the Sun, it will not be able to exist as a white dwarf. It will continue to shrink until becoming a neutron star.

Neutron stars are very small, but if they rotate quickly and have strong magnetic fields, it is possible to detect its presence. It is believed that pulsars, objects detected with radiotelescopes (the first was discovered by Jocelyn Bell in 1967) are neutron stars in quick rotation. Many of them have been discovered in places where remains of great explosions can still be found: The stars that when running out of combustible exceed the Chandrasekhar Limit, cannot form white dwarfs and collapse, emitting great amounts of energy to space.

But neutron stars also have a limit: If the amount of matter they contain exceeds it, they also collapse. What happens then...? Do they shrink until becoming an infinitesimal point...? Before this takes place, the escape velocity becomes faster than the light and the star ``disappears''... Of course it does not disappear physically, what remains of the star continue to be there, but we can no longer see it because it can not emit any more light or heat. It has become a black hole.

Do this things really exist on the sky...? How can we be sure if we can not see them...?

We know there are stars with fifty times more matter than the Sun. Certainly, along their lives, these stars eject great amounts of gas to space, but it is difficult to imagine they can get rid of so much matter so as to become black holes.

In the same way Sirius B was discovered because of the peculiar movement of its brighter companion, there are many cases of stellar pairs in which only one star can be seen. One of these pairs, called Cygnus X-1, emits also X rays. The most likely interpretation suggests that the object we do not see in Cygnus-X is very small and part of the gases of the atmosphere of the visible star fall over it. In the same way meteorites become incandescent and smelt because of their friction with the air, gases falling on that compact object heat so much that emit X rays. In principle, this invisible companion could be a white dwarf (too weak to be seen) or a neutron star, but thanks to the study of the movement of the visible star, we know it must have more than three times the matter of the Sun. Too much weight for a white dwarf and also more than a neutron star can handle! Most probably it is really a black hole...

Translation by Elinadja Fernandes - nadinha1@lycos.com