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  Stars are huge compared to the Earth; the Sun, a relatively small star, could contain one million planet Earths within its volume.

Stars are born when a cloud of primordial hydrogen begins to coalesce. This happens for several reasons. If there is a slightly greater density of matter in one part of the cloud, then gravitational attraction will cause other matter to drift towards it. Also, there will be light from surrounding sources but not from within the cloud. Light exerts a perceptible force upon small particles, and thus the cloud would condense.

Once the process has started, it speeds up, as the central core of matter becomes denser. The hydrogen atoms are forced into close proximity, and eventually gravity is strong enough to cause fusion of two hydrogen atoms into helium. This fusion releases energy in the form of radiation, containing some visible light. The gas cloud has become a star.

The star is now in its first phase. What happens to it next is entirely determined by how much matter the original cloud contained. It will burn hydrogen by fusion for millions of years—our Sun is in this phase. But eventually the hydrogen will run out, leaving the star with helium, which it cannot burn. Then the star will begin to collapse, since previously radiation pressure had balanced the force of gravity, and now only gravity remains.

If the star is small, about the mass of our Sun, it will not collapse far. Its outer layers will expand, enveloping and destroying any planets it may possess. It is now a red giant, and when the gas envelope dissipates, the small remaining core will be a white dwarf. White dwarf matter is very dense by Earth standards, weighing several tons per teaspoonful.

A heavier star will collapse much further, until the helium begins to fuse. The star will have a kind of rebirth, called a nova. Depending upon the size of the star, this cycle of collapse and ignition may continue, forming many other elements. All elements that we know today were formed in the interiors of stars. Eventually the star will begin to form iron and the elements heavier than iron. However, energy is absorbed rather than released when these heavy elements are formed, and thus their formation hastens the star\'s collapse. The star collapses swiftly, until the collapse is brought to an abrupt halt as a neutron core is formed. All the particles in the star have been forced into a single conglomerate with the density of a nucleus, and will not collapse further. This causes a huge explosion as the collapse rebounds, and about half of the matter of the star is lost to surrounding space. This explosion is known as a supernova, and for the few weeks that it burns, a supérnova will be brighter than its entire galaxy.

The usual remnant is a neutron star, an incredibly dense object. One teaspoonful of neutron star matter weighs several million tons. But an even heavier star will collapse further than a neutron star, to form the enigmatic black hole, from which not even light can escape. JJ

See also nuclear fission/fusion; relativity.



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