absorption line: Wavelengths at which the intensity of radiation is less than it is at neighboring wavelengths.
absorption nebula: Gas and just seen in silhouette
accelerating universe: The model for the Universe based on observations late in the 1990s that the expansion of the Universe is speeding up over time, rather than slowing down in the way that gravity alone would modify its expansion.
accretion disk: Matter that an object has taken up and that has formed a disk around the object.
active galactic nucleus (AGN): A galaxy with an exceptionally bright nucleus in some part of the spectrum; includes radio galaxies, Seyfert galaxies, quasars, and QSOs.
active galaxy: A galaxy radiating much more than average in some part of the spectrum, revealing high-energy processes; radio galaxies and Seyfert galaxies are examples.
active region: Regions of the Sun where sunspots, plages, flares, etc., are found.
active sun: The group of solar phenomena that vary with time, such as active regions and their phenomena.
AGN: Active galactic nucleus.
albedo: The fraction of light reflected by a body.
allowed states: The energy values that atoms can have by laws of quantum theory.
alpha particles: A helium nucleus; consists of two protons and two neutrons.
alt-azimuth: A two-axis telescope mounting in which motion around one of the axes, which is vertical, provides motion in azimuth, and motion around the perpendicular axis provides up-and-down (altitude) motion.
altitude: (a) Height above the surface of a planet, or (b) for a telescope mounting, elevation in angular measure above the horizon.
amino acid: A type of molecule containing a chain of carbon atoms and the group NH2 (the amino group). Amino acids are fundamental building blocks of life.
Amor asteroids: A group of asteroids with semi major axes greater than Earth’s and between 1.017 A.U. and 1.3 A.U.; about half cross the Earth’s orbit.
angstrom: A unit of length equal to 10-8 cm.
angular momentum: An intrinsic property of a system corresponding to the amount of its revolution or spin. The amount of angular momentum of a body orbiting around a point is the mass of the orbiting body times its (linear) velocity of revolution times its distance from the point. The amount of angular momentum of a spinning sphere is the amount of inertia, an intrinsic property of the distribution of mass, times the angular velocity of spin. The conservation of angular momentum is a law that states that the total amount of angular momentum remains constant in a system that is undisturbed from outside itself.
angular resolution: See resolution.
angular velocity: The rate at which a body rotates or revolves expressed as the angle covered in a given time (for example, in degrees per hour).
anisotropy: Deviation from isotropy; changing with direction.
annular eclipse: A type of solar eclipse in which a ring (annulus) of solar photosphere remains visible.
anthropic principle: The idea that since we exist, the Universe must have certain properties or it would not have evolved so that life would have formed and humans would have evolved.
antimatter: A type of matter in which each particle (antiproton, antineutron, etc.) is opposite in charge and certain other properties to a corresponding particle (proton, neutron, etc.) of the same mass of the ordinary type of matter from which the Solar System is made.
antiparticle: The diameter of the lens or mirror that defines the amount of light focused by an optical system.
aperture: The diameter of the lens or mirror that defines the amount of light focused by an optical system.
aperture synthesis: The use of several smaller telescopes together to give some of the properties, such as resolution, of a single larger aperture.
aphelion: For an orbit around the Sun, the farthest point from the Sun.
Apollo asteroids: A group of asteroids, with semimajor axes greater than Earth’s and less than 1.017 A.U., whose orbits overlap the Earth’s.
apparent magnitude: The brightness of a star as seen by an observer, given in a specific system in which a difference of five magnitudes corresponds to a brightness ratio of one hundred times; the scale is fixed by correspondence with a historical background.
archaebacteria: A primitive type of organism, different from either plants or animals, perhaps surviving from billions of years ago.
association: A physical grouping of stars; in particular, we talk of O and B associations.
asterism: A special apparent grouping of stars, part of a constellation.
asteroid: A “minor planet,” a non-luminous chunk of rock smaller than planet-size but larger than a meteoroid, in orbit around a star.
asteroid belt: A region of the Solar System, between the orbits of Mars and Jupiter, in which most of the asteroids orbit.
astrometric binary: A system of two stars in which the existence of one star can be deduced by study of its gravitational effect on the proper motion of the other star.
astrometry: The branch of astronomy that involves the detailed measurement of the positions and motions of stars and other celestial bodies.
Astronomical Unit: The average distance from the Earth to the sun.
astrophysics: The science, now essentially identical with astronomy, applying the laws of physics to the Universe.
Aten asteroids: A group of asteroids with semimajor axes smaller that 1 A.U.
atom: The smallest possible unit of a chemical element. When an atom is subdivided, the parts no longer have properties of any chemical element.
atomic clock: A system that uses atomic properties to provide a measure of time.
atomic number: The number of protons in an atom.
atomic weight: The number of protons and neutrons in an atom, averaged over the abundances of the different isotopes.
aurora: Glowing lights visible in the sky, resulting from processes in the Earth’s upper atmosphere and linked with the Earth’s magnitude field.
aurora australis: The southern aurora.
aurora borealis: The northern aurora.
autumnal equinox: Of the two locations in the sky where the ecliptic crosses the celestial equator, the one that the Sun passes each year when moving from northern to southern declinations.
A.U.: Astronomical Unit.
azimuth: The angular distance around the horizon from the northern direction, usually expressed in angular measure from 0° for an object in the northern direction, to 180° for an object in the southern direction, around to 360°.