What Is That?

Glossary Terms

aberration		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 planets 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

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

albedo		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)

altitude		1. the height above sea level 2. the angle between an objects position on the celestial sphere and the horizon

anaglyph		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

anisotropy		the variation of a physical property depending on direction

annular eclipse		a solar eclipse in which the moon does not fully cover the suns disk, allowing observers to see a thin ring of sunlight

antimatter		matter consisting of particles that have the same mass and properties as their matter counterparts, only with the opposite electrical charges

aperture		the diameter of a telescopes primary lens or mirror; the larger the aperture, the greater the telescopes light-gathering power

aphelion		the point farthest from the sun in an objects orbit

apochromatic lens		a lens with three or more elements that reduces chromatic aberration even more than an achromatic lens

apogee		the point in a satellites orbit when it is farthest from Earth

Apollo		1. the U.S. space program that sent astronauts to the moon in the 1960s and '70s 2. 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

Definition

apparition

the period of time during which a particular celestial object can be seen

archeoastronomy

the study of physical artifacts with astronomical connections

arcminute

a unit of angular size equal to 1/60 of a degree

arcsecond

a unit of angular size equal to 1/3,600 of a degree (or 1/60 of an arcminute).

asterism

a small grouping of stars in the night sky

asteroid

a small, rocky body that orbits a star

asteroid belt

the zone in which most asteroids orbit the sun, located between the orbits of Mars and Jupiter

astrometry

the study of the positions and motions of celestial objects

astronomical unit (AU)

the average distance from Earth to the sun, equal to about 93,000,000 miles (150,000,000 km)

astronomical units

a measurement used by astronomers within the solar system; one astronomical unit (AU) is the average distance between Earth and the sun (about 93,000,000 miles or 150,000,000 kilometers)

astronomy

the branch of science concerned with objects beyond Earth

astrophotography

the photography of astronomical objects

astrophysics

the branch of astronomy that deals with the physical characteristics of celestial objects

atmosphere

a gaseous envelope surrounding a moon, planet, or star

atom

the fundamental unit of matter; can consist of protons, neutrons, and electrons

atomic nucleus

the central region of an atom; can consist of protons and neutrons

attitude

the orientation of a spacecraft relative to the direction of its motion

a measurement used by astronomers within the solar system; one astronomical unit (AU) is the average distance between Earth and the sun (about 93,000,000 miles or 150,000,000 kilometers)

aurora

the emission of light when charged particles from the solar wind slam into and excite atoms and molecules in a planets upper atmosphere

aurora australis

the southern lights; see definition for aurora

aurora borealis

the northern lights; see definition for aurora

aurorae

the plural of aurora (the emission of light when charged particles from the solar wind slam into and excite atoms and molecules in a planets upper atmosphere)

autoguider

a CCD camera used to automatically guide a telescope during long-exposure photography

autumnal equinox

the time of year around September 23 when the sun crosses the celestial equator heading south

averted vision

a technique that uses the more light-sensitive rods in the eye to better see a faint object by looking at it indirectly

axis		a straight line about which an object rotates

azimuth		the angle along the horizon measured eastward from due north to the point on the horizon directly below an object

Barlow lens		a lens attached behind the eyepiece of a telescope that increases magnification

barred spiral galaxy		a spiral galaxy with a central bar consisting of stars and gas

baryonic		made up of baryons (elementary particles such as protons and electrons)

baryonic matter		"normal" matter composed of elementary particles called baryons

baryons		elementary particles such as protons and neutrons composed of three quarks

baseline		the line between two observational points or two telescopes of an interferometer

Big Bang		the giant explosion that is theorized to have created the universe 10 billion to 20 billion years ago

billion		1,000,000,000 (in American usage)

binary star		a system of two stars that orbit a common center of gravity; also known as a double star

binoculars		a small, usually hand-held instrument with two tubes that is used to magnify the view of astronomical objects; the two numbers used to describe the binoculars refer to its magnification and its aperture in millimeters, respectively

black hole		a region of space where gravity is so powerful that not even light can escape; black holes can form either from the death of high-mass stars or in the cores of galaxies

blazar		a high-energy, variable type of quasar which astronmers believe has a jet of material aimed in our direction that causes it to appear more energetic than other quasars

blueshift		a decrease in the wavelength of light coming from an object due to its motion toward Earth.

Bok globule		a small, dark nebula thought to be a region of star formation

bolide		a brilliant meteor or fireball that explodes in mid-air

bolides		brilliant meteors or fireballs that explode in mid-air

Bose-Einstein condensate		atoms crowded close together in ultra-low temperatures that behave as if they were one fluid-like superatom

brown dwarf		a gaseous object that forms like a star but lacks the necessary mass to sustain nuclear fusion in its core; a body intermediate in mass between a star and planet

buckyball		a naturally occurring form of carbon known as C-60, its molecular structure resembles the geodesic domes once designed by Buckminster Fuller

bulge		the generally spherical, central region of a spiral galaxy

carbon star		a red giant star with much more carbon than oxygen in its surface layers

Cassegrain telescope		a reflecting telescope in which a secondary mirror reflects light back through a hole in the center of the primary mirror

cataclysmic variable		a close binary system which includes a white dwarf accreting matter from a less massive companion

catadioptric telescope		a telescope that combines the primary mirror of a reflector with a lens placed in front of the mirror that corrects for aberrations; most catadioptric telescopes for amateurs are Schmidt-Cassegrain telescopes

CCD		a silicon chip used to detect light; charge-coupled devices (CCDs) are far more efficient at collecting light than conventional film

celestial pole		the imaginary projection of Earths rotational axis onto the celestial sphere

celestial sphere		the apparent sphere of the sky; an imaginary sphere of immense radius centered on Earth often used to plot the coordinates of objects in the sky

Cepheid variable		a class of luminous stars that vary in brightness; used to calibrate distances to galaxies

Chandrasekhar limit		the maximum mass of a white dwarf star, equivalent to 1.4 solar masses

charge-coupled device		a silicon chip used to detect light; charge-coupled devices (CCDs) are far more efficient at collecting light than conventional film

Charles Messier		A French astronomer and comet hunter who discovered 13 comets independently and codiscovered a half-dozen others. While hunting for comets, Messier compiled a list of fuzzy objects that were not comets in order to avoid them. These catalog entries were later identified as star clusters, nebulae, and galaxies and became the Messier Catalog. Published in various versions beginning in 1771, the catalog grew to 103 objects by 1781. Charles Messier lived from June 26, 1730 to April 12, 1817.

chondrite		a stony meteorite containing small, round, silicate granules called chondrules

chromosphere		a layer in a stars atmosphere lying below the corona and above the photosphere

circumpolar		Circumpolar stars are permanently above the horizon from a given observing point on Earth; that is to say, they never set. At Earth's Geographical North Pole (90° north latitude), all stars in the sky are cirumpolar. On Earth's equator, no stars are circumpolar.

clock drive		a motor attached to an equatorial mount that compensates for Earths rotation and thus keeps the telescope pointing at the same area of sky

coated optics		optics treated with a thin, uniform coating that greatly reduces scattered light and thus makes the image brighter

collimation		the act of putting a telescopes optics into perfect alignment

coma		the bright shroud of gas that surrounds a comets nucleus

comet		a small piece of ice and rock that orbits a star usually in a highly elongated orbit; long-period comets have orbital periods longer than 200 years, short-period comets have orbital periods less than 200 years

comet nucleus		a solid, compact mass of rock and ice that heats up when exposed to sunlight and releases gas and dust

conjunction		a time when two or more bodies appear close together in the sky

constellation		one of the 88 patterns of stars in the sky, often named for a mythological god, hero, or animal

convection		the transfer of heat energy by moving currents of material

core		the central region of a planet, brown dwarf, star, or galaxy

corona		the outer atmosphere of the sun or a star

coronagraph		an instrument designed to block light from the solar disk, allowing the corona to be observed

coronagraphic mask		an disk-shaped instrument designed to block light from the disk of a star, allowing the region very close to a target star to be studied

coronal mass ejections		huge eruptions of gas from the solar corona; CMEs can produce geomagnetic storms and auroral displays on Earth

cosmic background radiation		microwave radiation that permeates the universe and represents the still-cooling heat generated from the Big Bang

cosmic microwave background		microwave radiation that permeates the universe and represents the still-cooling heat generated from the Big Bang

cosmic ray		an atomic nucleus (most are protons) moving at a speed approaching that of light

cosmological constant		a term in the equations of general relativity that represents a repulsive force in the universe

cosmology		the branch of science concerned with the structure and evolution of the universe

cosmos		a synonym for universe

crescent		the phase of a planet or moon during which less than half the surface is illuminated

critical density		the density of the universe that provides just enough gravity to bring the expansion to a halt after an infinite time

crust		the thin, outermost geological layer of a planet, moon, or asteroid

Damocloid

a rare type of asteroid with an elli