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.

Ursa Major and Ursa Minor

Ursa Major and Ursa Minor [Lat.,=the great bear; the little bear], two conspicuous northern constellations. Known to many peoples from ancient times, these constellations have had various names; the configuration of the seven brightest stars has been called the Bear, Septentriones (the seven plowing oxen), the Plow, Charles's Wain, and the Wagon. Ursa Minor was once known as Cynosura (from the Greek for "dog's tail"). In the United States part of Ursa Major is called the Big Dipper (or the Drinking Gourd) and part of Ursa Minor, the Little Dipper. Four of the seven bright stars in the Big Dipper form the bowl and three the handle; five of these stars are of second magnitude. The middle star in the handle of the Big Dipper is Mizar (Zeta Ursae Majoris). A fainter star, Alcor, which appears to be near Mizar, was observed from ancient times. These two stars are sometimes called a double star, but since they do not revolve around a common center of gravity they are not true doubles. Mizar itself is, however, a visual binary star and was the first to be recognized as such—by G. B. Riccioli in 1650. It was also the first spectroscopic binary to be discovered; this observation resulted from studies of the spectrum of the brighter component of Mizar, which revealed it as a binary consisting of a pair of stars of almost equal brightness. The two end stars in the bowl of the Big Dipper are known as the Pointers. A line extending through them to about five times the distance between them leads to the polestar (Polaris, or the North Star). Polaris is at the extreme end of the Little Dipper. Including Polaris there are three stars in the handle of the Little Dipper and four forming the bowl. The handles of the two Dippers extend in opposite directions, and when one bowl is upright the other is inverted. Ursa Major reaches its highest point in the evening sky in April and Ursa Minor its highest point in June. However, for observers in the middle and northern latitudes of the Northern Hemisphere both constellations are circumpolar and thus are visible throughout the year.