ASTR 103 - Astronomy

Glossary - A

absolute

Intrinsic or characteristic to the body, not dependent on distance or position of the observer or the body.

absolute brightness

The intrinsic brightness or luminosity, a measure of the rate at which radiant energy is emitted.

absolute magnitude

The apparent magnitude a celestial body would have if placed at a distance of 10 pc (32.6 ly) from the Sun.

absolute zero

Temperature at which molecules and atoms have a zero average kinetic energy. It corresponds to 0 K on the Kelvin scale, -273.16^o C on the Celsius (centigrade) scale, and -459.69^o F on the Fahrenheit scale.

absorption (of electromagnetic energy)

The conversion of electromagnetic energy to some other form of energy by matter when electromagnetic radiation is incident upon matter.

absorption line

The absence or reduction of radiation at one or several adjacent wavelengths in a continuous spectrum.

absorption spectrum

Continuous spectrum interrupted by absorption lines or a continuous spectrum having a number of discrete wavelengths missing or reduced in intensity.

acceleration

The time rate of change in a particular direction of the velocity, which can be either the time rate of change of speed or direction, or both.

acceleration of gravity

The acceleration imparted to a body by the gravitational field of some small body, planet, star, galaxy, or cluster of galaxies; a measure of the intensity of a gravitational field.

accretion

The accumulation of matter in one location, for example, through the action of gravity.

accretion disk

A flattened, roughly circular disk of gas or plasma attracted to a star, black hole, or other massive body, often by mass transfer during binary evolution of stars.

accretion theory

The formation of a planet, star, or other body by the addition of small pieces of matter to it.

active galactic nucleus

 

active galaxy

Galaxies with active nuclei that are very luminous and emit both thermal and nonthermal radiation.

active region

Regions on the Sun, and presumably other stars, that are the centers of intense magnetic fields, plages, sunspots, flares, prominences, etc.

airglow

Faint, visible and infrared emission in Earth's upper atmosphere caused by solar radiation.

albedo

The fraction of incident light reflected by a body.

alpha particle

⁴He₂ nucleus of the helium atom possessing two protons and two neutrons.

altazimuth mounting

Mounting an arrangement of a telescope permitting it to be rotated horizontally and vertically.

altitude

The height above the surface of a planet. Also, the name of the astronomical coordinate measuring the angular distance above the horizon.

amino acid

Constituents of proteins containing the amino (NH₂) and carboxyl (COOH) groups of compounds.

amplitude

Maximum displacement from the equilibrium position in wave motion or light curve, etc.

angstrom (A)

 

angular momentum

Measure of the quantity of rotation possessed by a spinning body about an axis or external point.

angular size

The angle subtended by an object at a given distance.

angular velocity

The rate at which a body rotates as expressed by an angular change of position per unit time.

annihilation

 

annular eclipse

Solar eclipse in which the Moon's disk does not quite cover the Sun's disk, leaving a bright ring of sunlight around the Sun.

antapex (of the solar motion)

Point on the sky from which the Sun appears to be receding relative to its stellar neighbors. It is exactly opposite to the apex.

antimatter

Matter identical in behavior to ordinary matter but containing the oppositely charged or neutral counterparts of ordinary matter. Example: the proton carries a positive charge; the antiproton carries an equal but negative charge.

aperture

Diameter of a telescope objective or radio dish or antenna structure.

apex (of the solar motion)

Point on the sky in the constellation Hercules toward which the Sun appears headed relative to its closest stellar neighbors.

aphelion

Point in the orbit of a body orbiting the Sun where it is farthest from the Sun.

apogee

Point in orbit at which any body circling Earth is farthest from the Earth.

apparent

Not intrinsic or a characteristic of a body, but depends on the distance of position of the observer.

apparent brightness

The brightness as perceived by an observer at some distance from the radiating body (see also inverse-square law of light).

apparent magnitude

Apparent brightness of a celestial body based on a logarithmic scale of brightness.

apparent solar day

 

apparent solar time

 

association

A physical grouping of young stars, such as the O and B associations or Tauri associations.

A star

Stars of spectral type A with surface temperatures of about 7,500 to 10,000 K in whose spectrum the Balmer lines of hydrogen attain their greatest strength.

asterism

The configuration of stars or "catch figure" used to identify a constellation. Example: the Big Dipper is the asterism for Ursa Major.

asteroid

One of thousands of small rocky-material bodies revolving in orbits, chiefly between Mars and Jupiter in the asteroid belt.

astrology

Pseudo-science that claims human events can be predicted from the positions that the Sun, Moon, and planets occupy in the zodiac at different times.

astrometric binary

Double star having an invisible companion whose presence is inferred from the wobbly motion of the visible component.

astrometry

Division of astronomy dealing with precise determinations of the positions, motions, and parallaxes of astronomical bodies.

astronomical unit (AU)

The mean distance between Earth and the Sun (about 150 million kilometers).

astronomy

Science that deals with the Universe, its radiation and their descriptions, radiations, movements, structures, physical and chemical characteristics, origin, evolution, and arrangement.

astrophysics

Field of astronomy concerned with the physical, chemical, and thermal properties of celestial bodies accessible to direct observation and also inferred from theory.

asymptotic branch

 

atmosphere

The outermost gaseous medium or layers surrounding a planet or a star. Important to its understanding are its thermal structure and chemical composition.

atmospheric refraction

The refraction of light entering the Earth's atmosphere from the near-vacuum of space; it is best toward the zenith.

atom

The smallest unit of a chemical element possessing the properties of the element (see also subatomic particles).

atomic mass unit (amu)

Standard unit that corresponds to one twelfth of the mass of the carbon isotope ¹²C₆ (= 1.6604 x 10^-24 g).

atomic number

Number assigned to an atom corresponding to the number of protons in its nucleus.

atomic weight

Mass of an element in atomic mass units.

aurora

Sporadic visible emission in the upper atmosphere most prominent at high latitudes in both hemispheres caused by an influx of atomic particles from the Sun.

autumnal equinox

(see also equinoxes).

azimuth

Horizontal arc from the north point of the horizon measured clockwise to the object's position projected on the horizon.