Star formation begins when a dense, interstellar cloud of hydrogen and dust particles collapses inward under the influence of its own gravity. This gravitational contraction causes an increase in the cloud's density and internal temperature. The heat vaporizes the dust grains. The gases in the cloud's center begin to exert an outward pressure that stops the collapse. Stars begin to form in the middle of the cloud. As the stars begin to radiate energy derived from gravitational contraction, their gases are blown away, leaving a star cluster. As the temperature in a star's interior increases, deuterium (heavy hydrogen) is destroyed, followed by the decay of lithium, beryllium, and boron into helium. The temperature of the star's core continues to increase until it reaches a critical level at which nuclear fusion reactions begin. Once fusion begins in the star's core, contraction stops and the star begins to utilize its hydrogen at a very rapid rate, transforming it mainly to helium. In this main-sequence stage, the star radiates huge amounts of energy out into its own shell and into the surrounding space.

Single stars that are less than 1.4 times the mass of the sun remain in the main-sequence stage for a very long period. As time passes, the chemical composition of the star changes. The hydrogen in its core is converted into helium, and the central temperature slowly rises. The change in composition is accompanied by changes in the star's structure, size, and luminosity.

As the main-sequence stage comes to an end, all of the hydrogen in the core has been exhausted and the central region consists almost entirely of inert helium. Energy production begins to occur in a thin shell around the core. The core gradually increases in mass but shrinks in size because ever-increasing amounts of inert elements are fed in through the hydrogen-burning shell.

As the outer layers of the star expand and cool, the star becomes red in color. At the same time, energy from the contracting core heats the hydrogen and increases the star's luminosity. The star is now in the first red-giant stage.

The red giant has a complex structure in which different kinds of nuclear reactions are going on at different depths. While the core becomes dense and hot, the outer layers flow out, cool, and eventually surround the star to form a planetary nebula. In time the material in the planetary nebula is lost.The nucleus left behind cools to become a white dwarf.

White-dwarf stars are named for the white color of the few that were first discovered. They are characterized by low luminosity, a mass similar to that of the sun, and a radius comparable to that of the Earth. Because of their large mass and small size, such stars are dense and compact objects with average densities approaching 1 million times that of water.

Because white dwarfs have exhausted their nuclear fuel, they have no residual nuclear energy sources. When its reservoir of thermal energy is also exhausted - that is, after the star has cooled - the white dwarf stops radiating and becomes a cold and inert stellar remnant, sometimes called a black dwarf.

White-dwarf stars are occasionally found in double-star systems, in which members revolve closely around each other. In certain cases, a red giant expands into the gravitational domain of a white dwarf. The gravitational field of the white dwarf is so strong that hydrogen-rich matter from the outer atmosphere of the red giant is pulled onto the smaller star.

When a sizable quantity of this material accumulates on the surface of the white dwarf, a nuclear explosion occurs there, causing the ejection of hot surface gases. The white dwarf becomes a nova when the energy from these reactions blows off the accumulated matter in a brief but violent explosion.

The nova eruption separates the stars and interrupts the transfer of matter until, after a considerable length of time, the two stars move close together again. The nova explosion temporarily increases a white dwarf's faint luminosity to at least several thousand and sometimes to as much as 100,000 times its normal level. A nova may shine intensely for several days or occasionally for a few weeks before gradually resuming its former white-dwarf state.

Stars that become novas are nearly always too faint before eruption to be seen with the unaided eye. Their sudden increase in luminosity, however, is sometimes great enough to make them readily visible in the nighttime sky. To observers, such objects may appear to be new stars - hence the name nova, from the Latin word for "new."

Unlike a nova outburst, a supernova explosion is a cataclysmic event for a star - one that essentially ends its active (energy-generating) lifetime. For several months, supernovas may shine 10 billion times more brightly than a normal star. Supernovas are rare, occurring only about once a century in a galaxy the size of the Milky Way.

With single stars that are more than five times as massive as the sun, the sequence of events is faster. These stars can continue generating energy by fusion after they have depleted their hydrogen supplies. This is because their gravitational potential energy enables them to build up extremely high pressures deep in their interior. In this way, they can successively create elements as heavy as iron in their cores. After its main-sequence stage, such a star becomes a red supergiant.

It is thought that reactions involving iron fusion result in the catastrophic collapse of the star's core. The outer layers of the massive star are violently blown off in a supernova explosion. Unlike a nova outburst, a supernova explosion is a cataclysmic event for a star--one that essentially ends its active (energy-generating) lifetime. For several months, supernovas may shine 10 billion times more brightly than a normal star. Supernovas are rare, occurring only about once a century in a galaxy the size of the Milky Way.