biological rhythm

Biological rhythm, periodic biological fluctuation in an organism corresponding to and in response to periodic environmental change, such as day and night or high and low tide. The internal mechanism that maintains this rhythm even without the apparent environmental stimulus is a “biological clock.” When the rhythm is interrupted, the clock's adjustment is delayed, accounting for such phenomena as jet lag when traveling across time zones. Rhythms may have 24-hour (circadian rhythm), monthly, or annual cycles. See also photoperiodism.

Halley's comet

Halley's comet or Comet Halley, periodic comet named for Edmond Halley, who observed it in 1682 and identified it as the one observed in 1531 and 1607. Halley did not live to see its return in 1758, close to the time he predicted. It reappeared in 1835 when it was carefully recorded by visual observers, and in 1910, when its long tail and outbursts of dust jets were observed photographically. For its most recent return in 1985 and 1986, astronomers observed it from the ground and from space. A massive observing effort (1982–89) including visual observations, photography, and studies of the area around the nucleus, was coordinated by the International Halley Watch. Japan, the European Space Agency, and the USSR sent spacecraft to study the comet; the Vega and Giotto probes revealed a darker-than-expected nucleus 8 km (5 mi) wide and 15 km (9 mi) long, and shaped like a potato.

stellar structure

Stellar structure, physical properties of a star and the processes taking place within it. Except for that of the sun, astronomers must draw their conclusions regarding stellar structure on the basis of light and other radiation from stars that are light-years away; this light enables them to observe only the stars' surfaces. Knowledge of the processes taking place in a star and of conditions within its interior must be inferred from the laws of physics and chemistry. A star is a nearly spherical body of incandescent gas, mostly hydrogen and helium. Because it is observed to be stable, astronomers can conclude that the inward pressure of gravitation holding the star together is balanced by the outward pressure due to the energy generated by the star, and that the rate at which energy is radiated away from the star's surface is equal to the rate at which it is produced in the interior. The most important properties of a star are its size, mass, luminosity, chemical composition, and the temperature, pressure, and density at all distances from its center to its surface. These last three properties are related by the gas laws; their values decrease with distance from the star's center. Stars vary widely in size and luminosity but have masses only within the range from about 0.08 to 100 times the mass of the sun, with few exceptions; less massive bodies cannot support the energy-producing processes of a star, while more massive bodies are generally unstable. An ordinary star has a surface temperature of thousands of degrees, implying central temperatures of millions of degrees. The central pressure and density are also extremely high, but the temperature is such that the material will still remain in the gaseous state. At these temperatures, energy is produced by thermonuclear fusion (see nuclear energy), in which two or more nuclei are fused to form a single heavier nucleus. As such fusion processes proceed within the star, its chemical composition necessarily changes, with heavier elements increasing at the expense of lighter elements (see nucleosynthesis). The mass and chemical composition of the star together determine all of its other properties, e.g., size, luminosity, and temperature. Astronomers can determine the temperature and chemical composition of the star's surface from analysis of the spectrum of light from the star. Such a spectrum consists of a continuous black body spectrum produced by complex conditions within the star superimposed on which is a series of dark lines due to absorption of energy by the cooler stellar atmosphere. From such observations much is learned about the other properties and conditions within the star and thus about its stage of stellar evolution.

stellar evolution

Stellar evolution, life history of a star, beginning with its condensation out of the interstellar gas (see interstellar matter) and ending, sometimes catastrophically, when the star has exhausted its nuclear fuel or can no longer adjust itself to a stable configuration. Because a star's total energy reserve is finite, a star shining today cannot continue to produce its present luminosity steadily into the indefinite future, nor can it have done so from the indefinite past. Thus, stellar evolution is a necessary consequence of the physical theory of stellar structure, which requires that the luminosity, temperature, and size of a star must change as its chemical composition changes because of thermonuclear reactions.

neutron star

Neutron star, extremely small, extremely dense star, about double the sun's mass but only a few kilometers in radius, in the final stage of stellar evolution. Astronomers Baade and Zwicky predicted the existence of neutron stars in 1933. In the central core of a neutron star there are no stable atoms or nuclei; only elementary particles can survive the extreme conditions of pressure and temperature. Surrounding the core is a fluid composed primarily of neutrons squeezed in close contact. The fluid is encased in a rigid crystalline crust a few hundred meters thick. The outer gaseous atmosphere is probably only a few centimeters thick. The neutron star resembles a single giant nucleus because the density everywhere except in the outer shell is as high as the density in the nuclei of ordinary matter. There is observational evidence of the existence of several classes of neutron stars: pulsars are periodic sources of radio frequency, X ray, or gamma ray radiation that fluctuate in intensity and are considered to be rotating neutron stars. A neutron star may also be the smaller of the two components in an X-ray binary star.