Astronomy & Astrology

Black Dwarf

Burnt-out core of an old star that no longer emits light, generally believed to follow the white dwarf stage.

Black Dwarf, a star that can no longer emit light because all of its energy sources have been exhausted.

Brown Dwarf

Type of small, starlike object that has been speculated to exist in the universe.

Brown Dwarf, type of star: a star that is smaller than a planet and has a mass equivalent to less than one-tenth of the Sun's mass.

A type of failed star.

Giant Star

A low-density star with a diameter up to 100 times greater than that of the Sun. Also called giant.

Giant and supergiant stars represent stages in the lives of stars after they have burned most of their internal hydrogen fuel. Stars swell as they move off the main sequence, becoming giants and—for more massive stars—supergiants.

Neutron Star

Rapidly spinning, extremely dense astronomical object. Neutron stars are composed primarily of neutrons.

Neutron stars are the collapsed cores sometimes left behind by supernova explosions. Pulsars are a special type of neutron star. Pulsars and neutron stars form when the remnant of a star left after a supernova explosion collapses until it is about 10 km (about 6 mi) in radius. At that point, the neutrons—electrically neutral atomic particles—of the star resist being pressed together further. When the force produced by the neutrons balances the gravitational force, the core stops collapsing. At that point, the star is so dense that a teaspoonful has the mass of a billion metric tons. Neutron stars become pulsars when the magnetic field of a neutron star directs a beam of radio waves out into space. The star is so small that it rotates from one to a few hundred times per second. As the star rotates, the beam of radio waves sweeps out a path in space. If Earth is in the path of the beam, radio astronomers see the rotating beam as periodic pulses of radio waves. This pulsing is the reason these stars are called pulsars.

Some neutron stars are in binary systems with an ordinary star neighbor. The gravitational pull of a neutron star pulls material off its neighbor. The rotation of the neutron star heats the material, causing it to emit X rays. The neutron star's magnetic field directs the X rays into a beam that sweeps into space and may be detected from Earth. Astronomers call these stars X-ray pulsars.

Pulsar

Pulsars are compact stars that give off pulses of radio waves at very regular intervals.

Sources of powerful, pulsating radio waves in space, believed to be neutron stars

A small dense star that emits brief, intense bursts of visible radiation, radio waves, and X-rays, and is generally believed to be a rapidly rotating neutron star.

Supergiant Star

Extremely large, luminous star that can be seen from vast distances across space.

A star much more massive than the Sun.

Supergiant stars have both larger diameters and larger masses than giant stars.

Spectral Class	Effective Tempterature - Star Color	Principal Characteristics
O	25,000 K - Blue star	This stage is characterized by lines of helium, oxygen, and nitrogen in the spectrum of the photosphere. O stars are extremely hot, very bright stars that emit large amounts of ultraviolet radiation.
B	11,000 K - 25,000 K - White-blue star	In this group the lines of helium in the spectrum reach a maximum intensity and then fade. The intensity of the hydrogen lines regularly increases in all the subdivisions of stage B. Type B stars are typically represented by the star Epsilon Orionis.
A	7500 K - 11,000 K - White star	This group comprises the so-called hydrogen stars. The spectra of these stars is dominated by absorption lines of hydrogen. Sirius, the Dog star, is a typical type A star.
F	6000 K - 7500 K - Yellow-white star	This group is composed of stars characterized by an elevated intensity of the H and K lines of calcium and of lines characteristic of hydrogen. A notable star in this category is Delta Aquilae.
G	5000 K - 6000 K - Yellow, solar star	This group is composed of stars with prominent H and K calcium lines and less prominent hydrogen lines. The spectra of numerous metals, in particular iron, are also present. The Sun belongs to this group, and therefore G stars are frequently called solar stars.
K	K 3500 K - 5000 K - Orange-yellow star	This group comprises stars having strong calcium lines and lines indicating the presence of other metals in their spectra.The violet light of class K stars is less intense than the stars' red light. This group is typically represented by Arcturus.
M	3500 K - Red star	This group is composed of stars whose spectra are dominated by bands resulting from the presence of metallic-oxide molecules, notably those of titanium oxide. The violet end of the spectra is less intense than that of K stars. The star Orionis is typical of this group.

Astronomers categorize stars according to the the characteristics of the light that the stars emit, which are related to the stars' temperatures. O stars are the hottest stars, and M stars are the coolest. Our Sun is a G star.

Quasar

Astronomical object that is very bright for its size and distance from Earth.

Remote high-energy astronomical object: a compact object in space, usually with a large red shift indicating extreme remoteness, that emits huge amounts of energy, sometimes equal to the energy output of an entire galaxy. Also called quasi-stellar object.

Quasars are very distant objects that are moving away from Earth at high speed. The first ones discovered were very powerful radio sources, but scientists have since discovered quasars that don't strongly emit radio waves. Astronomers believe that almost every galaxy, whether spiral or elliptical, has a huge black hole at its center.

Quasars are believed by most astronomers to be the energetic nuclei of very distant galaxies. For reasons not yet known, they have brightened so much that they mask the light from their underlying galaxies. Often they occur in extremely distant clusters of galaxies. The spectral lines of quasars display very large red shifts, which would indicate that these objects are traveling away from earth's galaxy at speeds in the range of 80 percent of the speed of light. Their apparent great speed also means that they are among the most distant of cosmological objects.

& sections from articles from this chapter, all from Encarta.

Definitions