In the depths of space and the hearts of galaxies lurk monsters: holes in space that drag passers-by to certain doom if they venture too close. That's the popular image of black holes, but these ravenous cosmic beasts are proving to be even more fascinating – and fearsome – than their reputation suggests.

Dark visions
The concept of an object so massive that not even light can escape the pull of its gravity was first mooted way back in 1783. Geologist John Michell wrote in a letter to the Royal Society that if a star were massive enough, "a body falling from an infinite height towards it would have acquired at its surface greater velocity than that of light… all light emitted from such a body would be made to return towards it by its own proper gravity".

That insight went neglected for more than a century, because physicists came to believe that light could not be deflected by gravity. However, Einstein's 1915 theory of general relativity predicted that such deflection could in fact occur – a prediction subsequently borne out by experiment. That meant the light-capturing bodies suggested by Michell were actually possible – although Einstein himself was reluctant to accept that such a weird object could really exist.

The term "black hole" was coined by the quantum physicist John Wheeler, who also gave us "wormhole". Theoretical physicists spent decades demonstrating that black holes really were consistent with Einstein's ideas and working out how they should behave. And then the hunt was on to find one.

Hunting black holes
Given that black holes are black, as is space, you might expect them to be rather hard to spot. But in fact there are several ways astronomers can search for them.

For instance, black holes exert a powerful gravitational pull on nearby stars. This pull, and the black hole's existence, can be inferred by looking at the stars' movements. In some cases stars are found to be orbiting an invisible partner, and if calculations show that partner has more than a certain mass, it is probably a black hole.

A black hole's intense gravity also tends to attract gas and dust, which forms an "accretion disc" around it. Friction in the disc heats up the material, causing it to release vast amounts of radiation, which telescopes can detect. Models suggest that accretion discs could reach the size of a solar system and glow as brightly as a star.

Another giveaway is that light from stars that lie behind a black hole as seen from Earth should be deflected by its gravity. This process is called called gravitational lensing, and the measurements of the deflection of light can again be used to infer the existence of the hole.

This might all sound like rather circumstantial evidence, but most (not all) astronomers now agree that the evidence is strong enough to accept that black holes exist. And they are getting closer to imaging the elusive beast directly. In recent years, they have found evidence of matter vanishing in the region of a suspected black hole, suggesting that it has been swallowed – and powerful telescopes may be able to take direct pictures of the traces of a black hole within the next few years.

Follow the heat
There may be another way of spotting them. It sounds like a contradiction: everyone "knows" that black holes do not allow anything, even light, to escape. But 30 years ago Stephen Hawking suggested that they should release heat.

Even in empty space, pairs of particles – one made of matter, the other antimatter – can pop into existence for an instant, before annihilating each other and disappearing. If this happens close to a black hole's event horizon, one partner may be sucked into the black hole while the other escapes. From the perspective of an outside observer, the black hole has emitted a particle.

This has never been observed in the real world, but researchers have developed working models of event horizons and computer simulations suggest it should happen.

And if Hawking radiation does exist, black holes, cosmic superpowers though they are, should slowly evaporate away.

How to make a black hole
Black holes form when the most massive stars collapse in on themselves. As gravity pulls their outer layers inwards, the star's density gets higher and higher. Eventually its gravitational field becomes so intense that even light being emitted by the star is affected, bending back towards its surface rather than being radiated directly outwards.

Once the star has passed a critical point, all of the light is completely bent back, with none escaping into the rest of the universe.

The final collapse is a messy, chaotic event that can take up to a day to occur. This may cause spectacular bursts of gamma rays or supernova explosions. But in some cases at least, it may happen without any accompanying fireworks, in which case the stars would seemingly vanish without trace.

There are other ways black holes can form, at least in theory. For instance, tiny black holes could be formed when high-energy cosmic rays collide with molecules in Earth's upper atmosphere. (The fact that this hasn't had catastrophic effects on Earth, if it happens at all, is one reason that researchers at the CERN particle physics laboratory near Geneva, Switzerland, are so confident that scare stories about black holes being produced by their Large Hadron Collider are baseless.)

One shape, many sizes
The process of collapse destroys every characteristic of the original star except its mass, spin and electric charge: everything else is radiated away as gravitational waves. The resulting hole is said to "have no hair" – to bear no trace of its former existence. So black holes can vary only in terms of these three attributes – most obviously, in their masses.

Black holes vary enormously in size, from Goliaths with the mass of a million stars to the literally microscopic. Astronomers group them into four classes:


■Supermassive black holes weigh at least a 100,000 times as much as our sun. They are often found in the centres of galaxies, but it is unclear how they grow so large: the largest known to exist has the mass of 18 billion suns. It has been suggested that there is an upper limit, that no black hole can have a mass greater than 50 billion suns.
■Intermediate black holes are the black sheep of the family. Thought to have masses hundreds or thousands of times that of our sun, until recently there was little evidence that they existed. However, certain bright X-ray sources and mysterious runaway stars have made the case much stronger. The middleweight black holes could be formed when runaway stars crash into, and merge with, several stars in succession.
■Stellar-mass black holes have a mass several times that of our sun. The largest known to exist has the mass of 33 suns, while the smallest is only 3.8 times the sun's mass.
■Micro black holes are hypothetical. Far smaller than a star, they would fall prey to Hawking radiation and evaporate rapidly, so we should not expect to find any now. However, they could have been formed just after the big bang, when the cosmos was extremely hot and dense. Such ancient objects are called primordial black holes and would have come in a wide range of sizes, from micro to supermassive. Only the largest primordial black holes could have survived to the present day.

A black hole's spin and charge can also affect its behaviour. For example, spin may cause some black holes to fire off violent jets of matter. And as described in the next section, it might also cause them to reveal their deepest secret.

The anatomy of a black hole
Despite copious attempts to model what happens inside a black hole, no one knows for sure. The prevailing model of a black hole's interior suggests that its heart is a region of infinite density known as a singularity.

If you find the idea of infinite density puzzling, don't worry: this paradoxical-sounding concept arises because the laws of physics as we know them break down at this point. Until we have a theory that effectively integrates quantum mechanics and gravity, theoretical physicists are likely to remain almost as puzzled as everyone else about what goes on at the heart of a black hole – although that hasn't stopped them from trying to work it out.

Because singularities break the known laws of physics so spectacularly, Roger Penrose and others proposed the "cosmic censorship hypothesis", which states that all singularities must be enclosed by an event horizon. This isn't a physical barrier but a point of no return: objects that pass beyond it can never escape the black hole (but see below to understand how quantum mechanics undermines that idea). Thus the singularity is effectively hidden from the rest of the universe: we should never see a "naked" singularity.

The cosmic censorship hypothesis has never been proven, and over the years there have been several attempts to show that naked singularities really can exist. For instance, some have suggested that charged, fast-spinning black holes might be persuaded to reveal their singularities – and others have shown that this wouldn't work.

Destroying a black hole
Every time a black hole "releases" a particle of Hawking radiation, it should decrease in mass. Over billions of years, even the most massive black hole would shrink and eventually disappear. And this leads to a massive problem.

If you know a black hole's mass, electric charge and rate of spin, you know literally everything there is to know about it. To fully describe a star, on the other hand, you would need information about every single constituent particle. So a vast amount of information apparently vanishes when the hole forms. This information cannot simply escape the hole, because that would involve travelling faster than light.

If the black hole existed forever, the information might be "locked away" inside it. But if the black hole ultimately evaporates, as Hawking radiation would dictate, the information is utterly destroyed, and the laws of quantum mechanics do not allow that. This is the black hole information paradox.

Many proposed solutions involve rethinking black holes using string theory. These solutions lead to strange but physically plausible consequences: for instance, that an object thrown into a black hole would exist in two places at once, or that the singularity would be a "fuzzball" of subatomic strings.

The paradox could also be resolved if black holes do not include a true singularity, or if, as Stephen Hawking has suggested, the Hawking radiation contains the information, albeit in a mangled and unreadable state. It has even been suggested that black holes could actually be wormholes: gateways to other universes.

When holes collide
Despite the popular image of black holes as monsters lurking in wait to catch the unwary, at least some have been observed speeding through space. This raises the possibility that they could collide with each other, if the conditions are right.

If they did, computer simulations suggest that they would merge to form a single, larger black hole. Three-way mergers have also been successfully simulated.

Such mergers could give themselves away by their effect on the shapes of the black holes' parent galaxies, and in infrared and ultraviolet afterglows.

No collisions have been observed directly, but astronomers have found several pairs of black holes that are very close to each other, including some that are orbiting each other and some that seem to be on course for a collision.

Living with a black hole
The neighbourhood of a black hole can be a busy place. As previously mentioned, the black hole can accumulate a mass of dust called an accretion disc, but this is just the start.

Matter has been seen spiralling into a black hole, and the black hole's gravity can cause individual light photons to temporarily go into orbit around it.

On a larger scale, many black holes fire out huge jets of energetic matter, powered by magnetic fields. In one case, these jets have been shown to produce energetic bubbles 300,000 light years across.

Perhaps surprisingly, simulations suggest that stars can form in the vicinity of a black hole – though stars that venture too close may self-destruct.

And as we might expect, some unlucky stars get swallowed by black holes. Some black holes do this conspicuously, releasing outbursts of gamma rays and X-rays every time they feed, while others are "closet eaters" that emit very little radiation at feeding time.

Galaxies and black holes
Astronomers generally agree that enormous black holes lurk at the centre of most galaxies, and have identified plausible candidates in many galaxies, including the neighbouring dwarf galaxy M32 – and our own Milky Way.

The Milky Way's central black hole has been closely studied. At the moment it is on a starvation diet, having not eaten any large clumps of matter for several decades, but if it gets another large meal it could flare up again.

There have also been claims that there is a second, smaller black hole at the heart of our galaxy, but the evidence at present is inconclusive. It's also been suggested that the bigger black hole ate its baby brother.

When galaxies collide, their central black holes may collide as well. There have been hints that these collisions could eject one or both of the black holes, sending them hurtling across intergalactic space.

It has been suggested that these black holes must be there if galaxies are to form, and even that they directly seed galaxy formation. However, some galaxies seem to lack them, so the case is far from closed as yet.

The cosmic connection
Even if black holes aren't responsible for forming galaxies, they are still extremely important to our understanding of the universe as a whole.

They may have been responsible for mysterious cosmic "blobs" that littered the early universe. They may also be the power source behind both the incredibly luminous quasars and the most high-energy cosmic rays. And black holes evaporating explosively could also help reveal extra spatial dimensions.

And despite their formidable nature, they might even be put to humanity's service, acting as the ultimate particle accelerators. Theoreticians have even suggested that they could be used to power interstellar spacecraft.

It's a long shot, but black holes might just help our descendants explore the universe, as well as to understand it.