Two exceptions to this grouping are the so-called red giants and white dwarfs. The red giants are bright stars of comparatively large dimensions; white dwarfs are low in brightness, small, and extremely dense.



Life of a Star

Stars begin life as a large, relatively cool mass of gas in a nebula, an example of which is Orion (left). As gravity causes the gas to contract, its temperature rises, eventually becoming hot enough to trigger a nuclear reaction in its atoms. The shining of a main sequence star (middle) is caused by the massive, fairly steady output of energy from the fusion of hydrogen nuclei to form helium. The main sequence phase of a medium-sized star is believed to last as long as 10 billion years (our sun is generally considered about 5 billion years old). Stars eventually use up their energy supply, ending their lives as white dwarfs, which are extremely small, dense globes, or in the case of larger stars, as spectacular explosions called novas or supernovas. A supernova is caused by the abrupt collapse of a massive star, shown on the right within the Large Magellanic Cloud. More energy is emitted by the dying star in a few seconds than is produced by the sun in millions of years.



Evolution of Stars

This illustration depicts the stages in the lives of two stars. Stars form from a nebula (upper left), which consists of dust particles and hydrogen gas. Gravity pulls this material together into globules, which gradually expand as they convert their constituent hydrogen into helium during nuclear reactions. At its time of death, the center star explodes in a supernova, leaving behind a pulsar (blue object to the left of the supernova), while the core of the other star collapses, forming a planetary nebula (lower right).



Stellar Nursery

Stars form in dense clouds of gas and dust such as this one, called the Rho Ophiuchi dark cloud. These clouds are so dense that visible light produced by the stars is blocked by the dust. Astronomers use infrared telescopes in space to detect emissions from new stars.

A star begins its life as a large and comparatively cool mass of gas. The contraction of this gas and the subsequent rise of temperature continue until the interior temperature of the star reaches a value of about 1,000,000° C (about 1,800,000° F).

At this point a nuclear reaction takes place in which the nuclei of hydrogen atoms combine with heavy hydrogen deuterons (nuclei of so-called heavy hydrogen atoms) to form the nucleus of the inert gas helium. The latter reaction liberates large amounts of nuclear energy, and the further contraction of the star probably is halted.

When the release of energy from the deuteron-hydrogen nucleus reaction ends, contraction begins anew, and the temperature of the star increases again until it reaches a point at which a nuclear reaction can occur between hydrogen and lithium and other light metals present in the body of the star.

Again energy is released and contraction stops.

When the lithium and other light materials are consumed, contraction resumes, and the star enters the final stage of development in which hydrogen is transformed into helium at extremely high temperatures through the catalytic action of carbon and nitrogen.

This thermonuclear reaction is characteristic of the main sequence of stars mentioned above and continues until all the available hydrogen is consumed. The star gradually swells and becomes a red giant. It attains its greatest size when all its central hydrogen has been converted into helium. If it is to continue shining, its temperature at the center must rise high enough to cause fusion of the helium nuclei. During this process the star probably becomes much smaller and denser. When it has exhausted all possible sources of nuclear energy, it may contract further and become a white dwarf. This final stage may be marked by the stellar explosions known as novas. When a star sheds its outer envelope explosively as a nova or supernova, it returns to the interstellar medium elements heavier than hydrogen that it has synthesized in its interior.

Future generations of stars formed from this material will therefore start life with a richer supply of heavier elements than the earlier generations of stars.

Stars that shed their outer layers in a nonexplosive fashion become planetary nebulas, or old stars surrounded by spheres of radiating gases.

Massive stars, many times the mass of the sun, run through their cycle of evolution rapidly in astronomical time, perhaps only a few million years from birth to a supernova-type disruption. The remainder may then become a neutron star.

A limit exists for the size of neutron stars, however, beyond which such stars are gravitationally bound to keep contracting until they become a black hole, from which light radiation cannot escape.

Typical stars such as the sun may persist for many billion of years. The final fate of low-mass dwarfs is unknown, except that they cease to radiate appreciably. Most likely they become burned-out cinders, or black dwarfs.

The birth of stars is intimately connected with the presence of dust grains and molecules, as in the Orion nebula region of earth's galaxy. Here, molecular hydrogen (H²) is compressed to high densities and temperatures, dissociating the molecules. The atomic hydrogen then recollapses and forms a dense stellar core that gravitationally attracts surrounding material.

The hot core dispels the cocoon of the overlying molecules, and the new star emerges. Further gravitational heating raises the temperature until nuclear processes can occur. Stars are generally born in small groups at one edge of a large molecular cloud. Successive generations of stars eat into the edge of the cloud more and more, leaving a trail of stars of increasing ages.

The birth of stars has been observed in photographs taken of a sky region over a period of years. Modern techniques of space-based ultraviolet, infrared, and radio astronomy have further pinpointed sites of star formation and actual processes taking place.