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undefined, object
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in optics, an imperfect focus caused when a mirror or lens fails to bring light to a sharp focus
absolute magnitude
the apparent brightness an object would have if it were 10 parsecs (32.6 light-years) from Earth
absolute zero
the coldest theoretical temperature, equal to 0 kelvin (-459.67° F or -273.15° C)
absorption lines
dark lines in a spectrum caused by the absorption of light by atoms or molecules in a star or planet’s atmosphere
accretion disk
a disk surrounding a black hole or star in which matter gravitationally falls onto the central object
achromatic lens
a two-element lens, or doublet, that significantly reduces chromatic aberration
active galactic nuclei
the exceptionally bright cores of some galaxies, thought to be fueled by matter falling into supermassive black holes
active galaxy
a galaxy emitting unusually large amounts of energy from a compact central source
active optics
The techniques by which corrections are made to the shape of a large mirror or radio dish to adjust for minute distortions in its shape. These variations arise as a telescope is subjected to forces such as gravity and temperature changes.
adaptive optics
a system of telescopes, computers, and deformable mirrors used to compensate for atmospheric blurring
Airy disk
the bright disk-like image of a point source of light, such as a star, as seen in an optical system with a circular aperture
the percentage of light that an object reflects
altazimuth mount
a mount that enables a telescope to move freely both vertically (in altitude) and horizontally (in azimuth)
1. the height above sea level
a stereoscopic, composite image in which the right component (usually red in color) is superimposed on the left component (usually blue) to produce a three dimensional effect when viewed through correspondingly colored filters
Andromeda Galaxy
the largest member of the Local Group of galaxies; roughly twice the size of the Milky Way; also known as M31
angular size
the apparent width of an object as seen by an observer, usually expressed in degrees, arcminutes, or arcseconds
differences in physical properties depending on direction
the variation of a physical property depending on direction
annular eclipse
a solar eclipse in which the moon does not fully cover the sun’s disk, allowing observers to see a thin ring of sunlight
matter consisting of particles that have the same mass and properties as their matter counterparts but opposite electrical charges
The name given to a comet's tail when it points toward the sun. This rare event typically occurs when Earth crosses the plane of the comet's orbit and the comet is relatively close to the sun.
the diameter of a telescope’s primary lens or mirror; the larger the aperture, the greater the telescope’s light-gathering power
the point farthest from the sun in an object’s orbit
apochromatic lens
a lens with three or more elements that reduces chromatic aberration even more than an achromatic lens
the point in a satellite’s orbit when it is farthest from Earth
1. the U.S. space program that sent astronauts to the moon in the 1960s and '70s2. an asteroid with a perihelion less than 1.017 AU (and thus comes within the orbit of Earth)
apparent field of view
the angular diameter of the circle of light that the eye sees through an eyepiece
apparent magnitude
the measure of the brightness of an object as seen from Earth
the period of time during which a particular celestial object can be seen

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