A ring One of three Saturnian rings visible from Earth. The A ring is farthest from the planet and is separated from the B ring by the Cassini division.
absolute brightness The apparent brightness a star would have if it were placed at a standard distance of 10 parsecs from Earth.
absolute magnitude The apparent magnitude a star would have if it were placed at a standard distance of 10 parsecs from Earth.
absorption line Dark line in an otherwise continuous bright spectrum, where light within one narrow frequency range has been removed.
accretion Gradual growth of bodies, such as stars or planets, by the accumulation of gas or other, smaller, bodies.
accretion disk Flat disk of matter spiraling down onto the surface of a neutron star or black hole. Often, the matter originated on the surface of a companion star in a binary system.
active galaxies The most energetic galaxies, which can emit hundreds or thousands of times more energy per second than the Milky Way.
active optics Collection of techniques now being used to increase the resolution of ground-based telescopes. Minute modifications are made to the overall configuration of an instrument as its temperature and orientation change, to maintain the best possible focus at all times.
active region Region of the photosphere of the Sun surrounding a sunspot group, which can erupt violently and unpredictably. During sunspot maximum, the number of active regions is also a maximum.
active Sun The unpredictable aspects of the Sun's behavior, such as sudden explosive outbursts of radiation in the form of prominences and flares.
adaptive optics Technique used to increase the resolution of a telescope by deforming the shape of the mirror's surface under computer control while a measurement is being taken, to undo the effects of atmospheric turbulence.
amino acids Organic molecules which form the basis for building the proteins that direct metabolism in living creatures.
angular momentum problem The fact that the Sun, which contains nearly all of the mass of the solar system, accounts for just 0.3 percent of the total angular momentum of the solar system. This is an aspect of the solar system that any acceptable formation theory must address.
angular resolution The ability of a telescope to distinguish between adjacent objects in the sky.
annular eclipse annular eclipse Solar eclipse occurring at a time when the Moon is far enough away from the Earth that it fails to cover the disk of the Sun completely, leaving a ring of sunlight visible around its edge.
aphelion The point on the elliptical path of an object in orbit about the Sun that is most distant from the Sun
apparent brightness The brightness that a star appears to have, as measured by an observer on Earth.
association Small grouping of (typically 100 or less) stars, spanning up to a few tens of parsecs across, usually rich in very young stars.
asteroid One of thousands of very small members of the solar system orbiting the Sun between the orbits of Mars and Jupiter. Asteroids are often referred to as "minor planets."
asteroid belt Region of the solar system, between the orbits of Mars and Jupiter, in which most asteroids are found.
asthenosphere Layer of Earth's interior, just below the lithosphere, over which the surface plates slide.
astronomical unit (A.U.) The average distance of Earth from the Sun. Precise radar measurements yield a value for the A.U. of 149,603,500 km.
astronomy Branch of science dedicated to the study of everything in the universe that lies above Earth's atmosphere.
asymptotic giant branch Path on the H-R diagram corresponding to the changes that a star undergoes after helium burning ceases in the core. At this stage, the carbon core shrinks and drives the expansion of the envelope, and the star becomes a swollen red giant for a second time.
atmosphere Layer of gas confined close to a planet's surface by the force of gravity.
atom Building block of matter, composed of positively charged protons and neutral neutrons in the nucleus, surrounded by negatively charged electrons.
aurora Event which occurs when atmospheric molecules are excited by incoming charged particles from the solar wind, then emit energy as they fall back to their ground states. Aurorae generally occur at high latitudes, near the north and south magnetic poles.
autumnal equinox Date on which the Sun crosses the celestial equator moving southward, occurring on or near September 22.
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B
B ring One of three Saturnian rings visible from Earth. The B ring is the brightest of the three, and lies just within the Cassini division, closer to the planet than the A ring.
B ring One of three Saturnian rings visible from Earth. The B ring is the brightest of the three, and lies just within the Cassini division, closer to the planet than the A ring.
barred-spiral galaxy Spiral galaxy in which a bar of material passes through the center of the galaxy, with the spiral arms beginning near the ends of the bar.
baseline The distance between two observing locations used for the purposes of triangulation measurements. The larger the baseline, the better the resolution attainable.
Big Bang Event that cosmologists consider the beginning of the universe, in which all matter and radiation in the entire universe came into being.
binary-star system A system which consists of two stars in orbit about their common center of mass, held together by their mutual gravitational attraction. Most stars are found in binary-star systems.
blackbody curve The characteristic way in which the intensity of radiation emitted by a hot object depends on frequency. The frequency at which the emitted intensity is highest is an indication of the temperature of the radiating object. Also referred to as the Planck curve.
black dwarf The end-point of the evolution of an isolated, low-mass star. After the white dwarf stage, the star cools to the point where it is a dark clinker in interstellar space.
black hole A region of space where the pull of gravity is so great that nothing-not even light-can escape. A possible outcome of the evolution of a very massive star.
blue giant Large, hot, bright star at the upper left end of the main sequence on the Hertzsprung–Russell diagram. Its name comes from its color and size.
blue shift Motion-induced changed in the observed wavelength from a source that is moving toward us. Relative approaching motion between the object and the observer causes the wavelength to appear shorter (and hence bluer) than if there were no motion at all.
blue supergiant The very largest of the large, hot, bright stars at the uppermost left end of the main sequence on the Hertzsprung–Russell diagram.
Bohr model First theory of the hydrogen atom to explain the observed spectral lines. This model rests on three ideas: that there is a state of lowest energy for the electron, that there is a maximum energy, beyond which the electron is no longer bound to the nucleus, and that within these two energies the electron can only exist in certain energy levels.
brown dwarf Remnant of a fragment of collapsing gas and dust that did not contain enough mass to initiate core nuclear fusion. Such objects are frozen somewhere along their pre-main-sequence contraction phase, continually cooling into compact dark objects. Because of their small sizes and low temperatures they are extremely difficult to detect observationally.
brown oval Feature of Jupiter's atmosphere that appears only at latitudes near 20 degrees N, this structure is a long-lived hole in the clouds that allows us to look down into Jupiter's lower atmosphere.
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C ring One of three Saturnian rings visible from Earth. The C ring lies closest to the planet and is relatively thin compared to the A and B rings.
C ring One of three Saturnian rings visible from Earth. The C ring lies closest to the planet and is relatively thin compared to the A and B rings.
Cassegrain telescope A type of reflecting telescope in which incoming light hits the primary mirror and is then reflected upward toward the prime focus, where a secondary mirror reflects the light back down through a small hole in the main mirror, into a detector or eyepiece.
Cassini Division A relatively empty gap in Saturn's ring system between the A and B rings, discovered in 1675 by Giovanni Cassini. It is now known to contain a number of thin ringlets.
catastrophic theory A theory that invokes statistically unlikely accidental events to account for observations.
celestial&151;coordinates Pair of quantities—right ascension and declination—similar to longitude and latitude on Earth, used to pinpoint locations of objects on the celestial sphere.
celestial sphere Imaginary sphere surrounding the Earth, to which all objects in the sky were once considered to be attached.
center of mass The "average" position in space of a collection of massive bodies, taking their masses into account. In an isolated system this point moves with constant velocity, according to Newtonian mechanics.
Cepheid variable Star whose luminosity varies in a characteristic way, with a rapid rise in brightness followed by a slower decline. The period of a Cepheid variable star is related to its luminosity, so a determination of this period can be used to obtain an estimate of the star's distance.
chaotic rotation Unpredictable tumbling motion that non-spherical bodies in eccentric orbits, such as Saturn's satellite Hyperion, can exhibit. No amount of observation of an object rotating chaotically will ever show a well-defined period.
Electronic device used for data acquisition, composed of many tiny pixels, each of which records a buildup of charge to measure the amount of light striking it.
chromatic aberration The tendency for a lens to focus red and blue light differently, causing images to become blurred.
closed universe Geometry that the universe as a whole would have if the density of matter is above the critical value. A closed universe is finite in extent, and has no edge, like the surface of a sphere. It has enough mass to stop the present expansion, and will eventually collapse.
cold dark matter Class of dark-matter candidates made up of very heavy particles, such as supersymmetric relics.
collecting area The total area of a telescope that is capable of capturing incoming radiation. The larger the telescope, the greater its collecting area, and the fainter the objects it can detect.
color index A convenient method of quantifying a star's color by comparing its apparent brightness as measured through different filters. If the star's radiation is well described by a black-body spectrum, the ratio of its blue intensity (B) to its visual intensity (V) is a measure of the object's surface temperature.
color-magnitude diagram A way of plotting stellar properties, in which absolute magnitude is plotted against color index.
coma An effect occurring during the formation of an off-axis image in a telescope. Stars whose light enters the telescope at a large angle acquire comet-like tails on their images. The brightest part of a comet, often referred to as the "head."
comet A small body, composed mainly of ice and dust, in an elliptical orbit about the Sun. As it comes close to the Sun, some of its material is vaporized to form a gaseous head and extended tail.
comparative planetology The systematic study of the similarities and differences among the planets, with the goal of obtaining deeper insight into how the solar system formed and has evolved in time.
condensation nuclei Dust grains in the interstellar medium which act as seeds around which other material can coagulate. The presence of dust was very important in causing matter to clump during the formation of the solar system.
condensation theory Currently favored model of solar system formation which combines features of the old nebular theory with new information about interstellar dust grains, which acted as condensation nuclei.
conservation of mass and energy A fundamental law of modern physics which states that the sum of mass and energy must always remain constant in any physical process. In fusion reactions, the lost mass is converted into energy, primarily in the form of electromagnetic radiation.
contact binary A binary star system in which both stars have expanded to fill their Roche lobes and the surfaces of the two stars merge. The binary system now consists of two nuclear burning stellar cores surrounded by a continuous common envelope.
continuous spectrum Spectrum in which the radiation is distributed over all frequencies, not just a few specific frequency ranges. A prime example is the black-body radiation emitted by a hot, dense body.
convection Churning motion resulting from the constant upwelling of warm fluid and the concurrent downward flow of cooler material to take its place.
convection zone Region of the Sun's interior, lying just below the surface, where the material of the Sun is in constant convective motion. This region extends into the solar interior to a depth of about 200,000 km.
Copernican revolution The realization toward the end of the sixteenth century that Earth is not at the center of the universe.
core hydrogen burning The energy burning stage for main sequence stars, in which the helium is produced by hydrogen fusion in the central region of the star. A typical star spends up to 90% of its lifetime in hydrostatic equilibrium brought about by the balance between gravity and the energy generated by core hydrogen burning.
corona One of numerous large, roughly circular regions on the surface of Venus, thought to have been caused by upwelling mantle material causing the planet's crust to bulge outward.
corona One of numerous large, roughly circular regions on the surface of Venus, thought to have been caused by upwelling mantle material causing the planet's crust to bulge outward.
corona The tenuous outer atmosphere of the Sun, which lies just above the chromosphere, and at great distances turns into the solar wind.
coronal hole Vast regions of the Sun's atmosphere where the density of matter is about 10 times lower than average. The gas there streams freely into space at high speeds, escaping the Sun completely.
cosmic distance scale Collection of indirect distance-measurement techniques that astronomers use to measure the scale of the universe.
cosmic evolution The collection of the seven major phases of the history of the universe, namely galactic, stellar, planetary, chemical, biological, cultural, and future evolution.
cosmic microwave background The almost perfectly isotropic radio signal that is the electro-magnetic remnant of the Big Bang.
cosmological principle Two assumptions which make up the basis of cosmology, namely that the universe is homogeneous and isotropic on sufficiently large scales.
cosmological redshift The component of the redshift of an object which is due only to the Hubble flow of the universe.
crater Bowl-shaped depression on the surface of a planet or moon, resulting from a collision with interplanetary debris.
critical density The cosmic density corresponding to the dividing line between a universe that recollapses and one that expands forever.
critical universe Geometry that the universe would have if the density of matter is exactly the critical density. The universe is infinite in extent, and has zero curvature. The expansion will continue forever, but approach an expansion speed of zero.
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