Stars are balls of gas made incandescent by energy from nuclear reactions deep in their interiors. They come in a wide range of sizes and brightnesses, from giant dwarfs a hundredth of the Sun's diameter to dazzling supergiants hundreds of times the size of the Sun. They range in temperature from intensely hot blue-white stars (surfaces at more than 20,000°C) to cool red stars (3000°C). The Sun, which is a medium-temperature yellow star, turns out to be pretty average in all respects. Stars are born from massive clouds of gas and dust within our Galaxy. An interstellar gas cloud is termed a nebula (plural: nebulae), from the Latin for "cloud". A nebula is not uniformly distributed in space, but contains denser knots - the seeds of future stars. If the knot is dense enough it begins to contract under the inward pull of its own gravity. As it gets smaller and denser it heats up, until the temperature and pressure at the center of the shrinking blob become so great that nuclear reactions begin. The gas blob has switched on to become a true star, generating its own heat and light for millions of years. Our Galaxy has two small companion galaxies called the Magellanic Clouds. To the naked eye they appear like detached portions of the Several star-spawning clouds are well within reach of observation by amateurs. Most famous is the Orion Nebula, marking the sword in the constellation of Orion the Hunter. This nebula is visible as a hazy green glow to the naked eye; binoculars show it more clearly. At the center of the Orion Nebula is a star called theta Orionis, which small telescopes show to consist of four component stars. Energy emitted by the brightest of these four stars makes the nebula shine. But behind the bright, visible part of the cloud is an even larger, still-dark area where stars are being born at this moment. The Orion Nebula is estimated to contain enough matter to produce hundreds of stars: it is a star cluster in the making. Another famous stellar birthplace is the Tarantula Nebula in the southern constellation Dorado, which dwarfs the Orion Nebula and is in fact the largest nebula known. One celebrated group of young stars is the Pleiades cluster, popularly known as the Seven Sisters, in the constellation Taurus, the Bull. At least five members of the Pleiades can be distinguished by normal eyesight; binoculars and small telescopes bring dozens more members into view. The whole cluster is estimated to contain about a hundred stars. The brightest and youngest of these formed no more than 2 million years ago, making them extremely youthful by astronomical standards. The Pleiades is an example of a type of cluster referred to as an open cluster or galactic cluster. About a thousand of it are known to astronomers. Near the Pleiades in Taurus is a larger and older open cluster, the Hyades, which is estimated to be about 500 million years old. Being older than the Pleiades, the stars have had more time to drift apart. Eventually, most open clusters disperse completely. The Sun was probably a member of such a cluster when it was born 4600 million years ago. A different type of cluster is a globular cluster, described in Galaxies. Much larger than open clusters are stellar associations, scatterings of young stars hundreds of light years across. It is no coincidence that most of the bright stars in Orion lie at similar distances from us (the most notable exception being Betelgeuse), for they are member of such an association, centred on the Orion Nebula about 1500 light years away. Nebulae are made of a 10:1 mixture of hydrogen and helium, the primary constituents of the Universe, so, naturally enough, stars have the same composition. Stars get their energy from nuclear reactions which transform hydrogen into helium. In the reactions, four hydrogen atoms are crushed together to make one atom of helium; an uncontrolled version of the same reaction occurs in a hydrogen bomb. There are certain limits on the size of a star. A gas blob with less than about 8 per cent of the Sun's mass cannot become a star, because conditions in its interior will not become sufficiently extreme for nuclear reactions to begin. This 8 per cent limit may be considered the dividing line between a planet and a star. If the gaseous planet Jupiter in our Solar System had been about 80 times more massive than it actually is, it would have become a small star. At the other end of the scale, the largest stars have masses of about a hundred times that of the Sun. It was once thought that stars more massive than this would produce so much energy that they would literally disintegrate, but this may not be true in all cases. A few stars are known that seem to have masses greater than a hundred Suns, one example being eta Carinae. A star's most vital statistic is its mass, for this factor affects everything else about it: its temperature, its brightness and its lifetime. The stars with the least mass are, not surprisingly, the coolest; they are known as red dwarfs. A typical red dwarf such as Barnard's Star, the second-closest star to the Sun, has a mass about a tenth that of the Sun and glows a dull red with a surface temperature of about 3000°C. Even though Barnard's Star is only six light years away, it is too faint to be seen with the naked eye. Surprisingly enough, stars with the lowest mass live the longest. Their nuclear fires burn so slowly that they can survive for as much as a million million years, a hundred times as long as the Sun. The Sun itself, which by definition is of one solar mass, has a surface temperature of 5500°C, and is expected to live for about 10,000 million years. It is currently in the prime of its life. Moving up the scale, a star such as Sirius, which is twice the Sun's mass, can live for only about 1000 million years, a tenth of the Sun's age. The surface temperature of Sirius is a blue-white 11,000 °C. Larger and hotter still, the star Spica in the constellation Virgo has a mass of about 11 Suns and a surface temperature of around 24,000°C. The lifetime of this intensely hot, highly luminous star is less than 1 per cent of the lifetime of the Sun. As mentioned earlier, the Sun formed about 4600 million years ago and is now about halfway through its expected lifespan. In a few thousand million years, though, it will start to run out of hydrogen at its core. In search of more hydrogen to use as fuel, the nuclear reactions inside the Sun will start to move outwards, releasing more energy. Eventually, when they are surrounded by a shell of burning hydrogen, even the helium atoms in the Sun's core will enter into nuclear reactions of their own, fusing together to form carbon atoms. With all this extra energy being given off, the Sun will become much brighter than it is today and will start to swell alarmingly in size. But as the Sun's outer layers expand they will also cool, becoming redder in colour, and the Sun will turn into a red giant, similar to the bright stars Aldebaran and Arcturus. At its largest, the red giant Sun will grow to at least a hundred times its present diameter, engulfing Mercury, Venus and perhaps even the Earth within its outer layers. Needless to say, all life on our planet will long since have become extinct. Once a red giant has swollen beyond a certain size its distended outer layers drift of into space, forming a stellar smoke ring known rather confusingly as a planetary nebula, even though it has nothing to do with planets. The name was first used in 1785 by William Herschel, because they looked like the small, rounded disks of planets as seen through his telescope. Probably the best-known of all planetary nebulae is the Ring Nebula in Lyra, though it is not the easiest to see. Much larger is the Dumbbell Nebula in Vulpecula, which can be picked up in binoculars on a clear, dark night. Two small but bright planetary nebulae for amateur telescopes are NGC 6826 in Cygnus and NGC 7662 in Andromeda. At the center of a planetary nebula, the core of the former red giant is exposed as a small, intensely hot star. Once the surrounding gases of the planetary nebula have dispersed, usually after a few thousand years, the central star remains as a so-called white dwarf. A white dwarf is only about the diameter of the Earth, but contains most of the matter of the original star; only about 10 per cent of the star's mass is lost in the planetary nebula stage. White dwarfs are therefore exceptionally dense bodies. A teaspoonful of white dwarf material would have a mass of thousands of kilograms. Over thousands of millions of years, white dwarfs slowly cool off and fade into oblivion. Being so small, white dwarfs are very faint. Not one is visible to the naked eye. Both the nearby bright stars Sirius and Procyon have white dwarf companions, but Procyon's companion is too close to its parent to be distinguishable in amateur telescopes, and the companion of Sirius can be glimpsed only under the most favourable conditions. The easiest white dwarf to see is a companion of the star omicron Eridani (also known as 40 Eridani); a small telescope will show it. Of added interest is a fainter third member of this system, a red dwarf, which is also visible in amateur telescopes. Our Sun, it seems, is destined to go through the stage of being a planetary nebula before fading away as a white dwarf. But stars with several times the Sun's mass suffer a far more spectacular end. As we have seen, they first become dazzling supergiants rather than mere giants. They do not get a chance to reach the planetary nebula stage. So massive are they that the nuclear reactions at their centers continue in runaway fashion until the star becomes unstable and explodes. Such an explosion is known as a supernova. In a supernova eruption a star's brightness increases millions of times, so that for a few days the star can rival the brilliance of an entire galaxy. The shattered outer layers of the star are thrown off into space at speeds of around 5000 km per second. In AD 1054 astronomers on Earth saw a star erupt as a supernova in the constellation Taurus. The star became brighter than Venus, and was visible in daylight for three weeks. It finally faded from naked-eye view more than a year after it had first appeared. At the site of that explosion lies one of the most famous objects in the heavens: the Crab Nebula, the shattered remains of the star that erupted. The Crab Nebula is visible as a smudgy patch in amateur telescopes, but is best seen on long-exposure photographs taken with large instruments. Over the next 50,000 years or so the gases of the Crab Nebula will disperse into space, forming delicate traceries like those of the Veil Nebula in Cygnus, itself the remains of a former supernova. A star might not blow itself entirely to bits in a supernova explosion. Sometimes the central core of the exploded star is left as an object even smaller and denser than a white dwarf, known as a neutron star. In a neutron star, the protons and electrons of the star's atoms have been crushed by the tremendous forces of the supernova so that they combine to form the particles known as neutrons. A typical neutron star is a mere 20 km in diameter, but contains as much mass as one or two Suns. Being so minute, neutron stars can spin very rapidly without flying apart. Each time they spin we receive a flash of radiation like a lighthouse beam. Astronomers have detected radio pulses from several hundred such sources, which they term pulsars; one lies at the center of the Crab Nebula. The Crab Pulsar flashed 30 times per second; others pulse more slowly, down to once every four seconds. Most neutron stars are too faint to be seen optically, but the pulsar in the Crab Nebula has been seen flashing in step with the radio pulses. If the core of the exploded star has a mass of more than three Suns, then even a neutron star is not the end of it. Instead, it becomes something still more bizarre: a black hole. No force can shore up a dead star weighing more than three solar masses against the inward pull of its own gravity. It continues to shrink, becoming ever smaller and denser until its gravity becomes so great that nothing can escape from it, not even its own light. It has dug its own grave - a black hole. Since a black hole is by definition invisible, it is of only academic interest to amateur observers. However, professional astronomers have detected X-ray emissions from various locations in space which they believe come from hot gas plunging into the bottomless pits of black holes. The best-known candidate for a black hole is Cygnus X-1; it lies near a visible 9th-magnitude star in the constellation Cygnus.