A black hole is a star that has stopped twinkling. But why?
An ordinary star is one of the simplest entities in nature: it is a sphere of gas that is by mass 73 per cent hydrogen, 25 per cent helium and 2 per cent other elements. The temperature in the centre of a star is very high - high enough to fuse nuclei of hydrogen and helium together. The nuclear fusion produces energy that is radiated from the surface of the star as heat and light.
The universe has ten times as many stars as grains of sand on Earth - 70,000 billion billion stars (7 with 22 zeros after it), to be precise. To us all stars look similar, but no two stars are the same. Astronomers classify hundreds of billions of stars in our galaxy by their luminosity, colour, size and age. To us, stars also appear changeless. But stars are born, they live for millions of years, and they die.
The birth sites of stars are the dark clouds of gas and dust in our galaxy. The clouds, which are clumps of hydrogen atoms with a sprinkling of helium, are not uniform; they contain regions differing by density (1,000 to 10 million molecules per cubic centimetre) and temperature (-263 to -173 degrees Celsius), and regions with shapes ranging from spheroids to elongated tubes. Gravity tries to pull these clouds into the smallest possible space. Compression causes the gas to become hotter. Eventually the temperature and pressure rise high enough to ignite the gas. Hydrogen starts turning into helium, which creates vast amounts of energy. A star is born. All stars shine as a result of the nuclear fusion of hydrogen into helium, which takes place within their hot, dense cores, where temperatures may reach 20 million degrees Celsius.
Our Sun is a star - in astronomers' jargon, a main sequence star. A main sequence star - and 90 per cent of stars are these - fuses hydrogen nuclei into helium nuclei at its centre. The Sun has lived 4,600 million years as a stable star, and many billion years lie ahead. After consuming its hydrogen, the Sun will begin to expand. It will change into a type of star known as a giant, and will be about 100 times brighter than it is now.
After a few thousand years, the giant Sun will completely exhaust its supply of hydrogen and will shrink into a white dwarf - no larger than Earth, but so heavy that a teaspoonful of its matter would weigh thousands of kilograms. A white dwarf is so hot that it shines white-hot. Over billions of years, the white dwarf will turn black and cold. It will now be a dead star - a black dwarf.
A heavyweight star (a star with more than eight times the mass of the Sun) has a dramatic but brief life after becoming a supergiant. It expends its fuel so extravagantly that it collapses within a few million years. It then explodes as a supernova, which ejects an enormous amount of matter and even outshines the entire galaxy for a few days. The remaining matter forms a neutron star, only about 25 kilometres across, which contains tightly packed neutrons. These neutron stars do not glow, and are so heavy that even a pinhead of their matter would have a mass of a million tonnes.
Sometimes the crushing weight of a dying star like a neutron star squeezes it into a point with infinite density. At this point, known as singularity, mass has no volume and both space and time stop. The singularity is surrounded by an imaginary surface known as the event horizon, a kind of one-way spherical boundary. Nothing - not even light - can escape the event horizon. Matter falling into it is swallowed and disappears forever. That's why scientists call these regions of space-time black holes. If an astronaut passed through the event horizon of a black hole, gravitational forces would stretch his or her body into the shape of very long spaghetti, and when this very dead spaghetti slammed into the singularity of the black hole, the astronaut's remains would be ripped apart into atoms.
The radius of a black hole is the radius of the event horizon surrounding it. This is called the Schwarzschild radius, after the German astronomer Karl Schwarzschild who in 1916 predicted the existence of a dense object into which other objects could fall, but out of which no objects could ever come (the term 'black hole' was first used in 1969 by the American physicist John Wheeler; prior to that they were known as 'collapsars' or 'frozen stars'). The Schwarzschild radius is roughly equal to three times the weight of the black hole (in solar masses). A black hole weighing as much as the Sun would have a radius of 3 kilometres; one with the mass of Earth would have a radius of only 4.5 millimetres; and one with the mass of a small asteroid would be roughly the size of an atomic nucleus. A black hole's weird effects occur within 10 Schwarzschild radii of its centre. Beyond this rather limited distance, the only effect is through the black hole's normal gravitational pull. So, contrary to popular belief, a black hole is not like a cosmic vacuum cleaner that sucks in everything around it.
Not that long ago, black holes were in the realm of science fiction, but now there is convincing evidence for their existence. This evidence is still circumstantial -there is no way black holes can be observed directly. There are at least two species of massive black holes: smaller ones (a few times as massive as the Sun) that orbit normal stars; and their supermassive siblings (weighing many million Suns) which lurk in the centres of most galaxies. Our galaxy is believed to have a relatively small black hole that is as massive as 2.6 million Suns. A black hole with a mass 100 million times that of our Sun and a radius of 25 million kilometres squats at the centre of a galaxy 130 million light years away.
In 1971 the eminent theoretical physicist Stephen Hawking, who has greatly advanced our knowledge of black holes, proposed that during the first moments of the big bang that marked the birth of the universe, some areas were forced by the turbulence to contract rather than expand. This could have crushed matter into black holes that ranged in size from a few micrometres to a metre (their masses ranged from fractions of a gram to that of a large planet). This multitude of primordial or mini black holes may still exist, including some within the solar system, or even in orbit around Earth. These black holes have not yet been detected; there is not even circumstantial evidence for their existence.
Three years later, Hawking said that 'black holes are not really black after all: they glow like a hot body, and the smaller they are, the more they glow'. He proposed a mechanism by which black holes transform their mass into both radiation and particles that leave the hole. The result is that black holes gradually evaporate. So they do not last forever. The amount of radiation, now known as the Hawking radiation, escaping from a black hole is inversely proportional to the square of its mass; that is, the smaller the black hole, the shorter its life span. A primordial black hole with the initial mass of Mount Everest (and the size of an atomic nucleus) would have a lifetime roughly equal to the age of the universe, that is, 14 billion years; but a black hole with the initial mass of the Sun would vanish after about 100 million billion billion billion billion billion billion (1 with 62 zeros after it) years.
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