Twenty years on, most astronomers are now convinced that black holes are for real. Their certainty arises from knowing that objects which have succumbed to the pull of gravity - such as white dwarfs and neutron stars - are relatively common in space. Looked at in this way, black holes are simply the top end of the scale. Their pull of gravity is so strong that not even light, which travels at 300,000km per second, can escape. And since the velocity of light appears to be the speed limit of the Universe, anything falling into a black hole is trapped forever. The phrase "black hole", coined by the American theoretical physicist John Archibald Wheeler, could hardly be more appropriate.
But if black holes are invisible, how are we to detect them? It's rather like trying to find a black cat in a coal-cellar. But if the black cat is having an altercation with a white cat, the problem is solved.
The constellation of Cygnus (the Swan), which makes its first appearance on the starcharts this year, is unique in having two black hole candidates - both involved in cosmic altercations. The first, Cygnus X-1, was discovered in 1971 by the Uhuru satellite, designed to pick up the X-rays given out by violent objects in space. The hot gas in the Cygnus system, which produces the X-rays, is the equivalent of the white cat in the coal cellar. It comes from the outer layers of a supermassive star - 30 times heavier than the Sun - which is in orbit around an invisible object weighing in at 10 suns. Astronomers are certain this must be a black hole.
An even heavier black hole lies close by, identified as V404 Cygni. Here a relatively lightweight star is being whirled around an invisible object of some 10-15 solar masses.
But black holes come in all shapes and sizes. At one extreme are "mini black holes", proposed by Stephen Hawking in 1974. These tiny black holes - the size of an atom, with the mass of a mountain - have some bizarre properties. Hawking predicted these black holes should be exploding now - the result of ever-accelerating shrinkage.
What would happen if you fell into a black hole? Your fate would depend on how massive the hole was; whether it was spinning or not; whether it had an electric charge or was neutral. That fate might be mere "spaghettification" - death by being stretched into a long thin tube - or, at the other extreme, entry into another universe.
Theorists are currently working on how we might harness the properties of black holes, from rubbish disposal, through bomb-making to time travel.
The sky in May
Venus reaches its greatest brilliance on 4 May, when it hangs like a dazzling lantern in the north-west after sunset. At the beginning of the month, it sets after midnight, but by the end of May it will set just over an hour after sunset. A small telescope will show the planet looking like a miniature crescent Moon.
Also in the evening sky, for the first week of the month, is the planet Mercury. It sets about 10pm BST in early May but then draws into line with the Sun.
Mars is not visible this month, but Jupiter and Saturn are both morning objects. Jupiter rises first, at around 1am, while Saturn appears at about 3.30am mid-month.
The stars on view include two of the most familiar constellations of spring, Leo and Virgo. Virgo is the largest constellation in the northern hemisphere, and looks more like the letter Y than a maiden holding an ear of corn. The "bowl" of the Y is dotted with small fuzzy patches, a few of the galaxies that make up the giant Virgo cluster. One bright member of the cluster is the huge, ball-shaped galaxy M87, which has a powerful jet of gas spewing out of its core. The jet comes from the vicinity of a massive black hole at the galaxy's heart
Diary (all times BST)
3 May 12.48pm Full Moon
8 Asteroid Vesta at
opposition - visible
10 6.04am Moon at last quarter
17 12.46am New Moon
22 Pluto at opposition
25 3.13pm Moon at first
Heather Couper and Nigel Henbest's book 'Black Holes' is published by Dorling Kindersley on 9 May, pounds 9.95.Reuse content