Glossary -- A

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absolute magnitude

the brightness of an object that would be measured by an observer if the object was 10 parsecs away. It is a measure of the object's luminosity.

absorption line spectrum

dark lines in a continuous spectrum that are produced by cool, thin (low-pressure) gases. Made by electrons jumping up farther from the nucleus.

acceleration

a change in motion = (the velocity change)/(time interval of change). It involves a change in the speed (increase or decrease) OR direction OR both speed and direction.

accretion disk

disk of gas that forms around a massive object as material spirals onto the massive object. Accretion disks around white dwarfs, neutron stars, and black holes form when material is drawn off a nearby normal or giant star. Accretion disks around neutron stars and black holes can be hot enough to radiate X-rays.

active galaxy

luminous galaxy that produces most of its energy from a very compact source at its center. It has a non-thermal continuous spectrum. The energy is coming from an accretion disk of gas around a supermassive black hole at the nucleus of the galaxy.

adaptive optics

a technique that compensates for atmospheric turbulence by quickly adjusting the light path in the optics. This removes seeing effects and enables the telescope to achieve much better resolution, closer to its theoretical resolving power.

altitude

position on the celestial sphere that is the number of degrees an object is above the nearest horizon. Varies from 0° at horizon to 90° at zenith. Vertical position of an object.

angular momentum

a measure of the amount of spin or orbital motion an object has. It is proportional to the mass of the object multiplied by its radius multiplied by its spin or orbital speed.

angular diameter

see angular size.

angular size

the apparent size of an object measured by the angle between two lines of sight along each side of an object. An object's actual linear diameter can be found from the angular size if the distance is already known. The linear diameter = (2p/360°) × (its distance × angular size in degrees).

annular eclipse

a type of solar eclipse that happens when the Sun and Moon are exactly lined up but the Moon is too far away from the Earth to totally block the Sun's surface. A ring (annulus) of sunlight is seen around the dark Moon (contrast with total solar eclipse).

aphelion

point in an object's orbit around the Sun that is furthest from the Sun.

apparent magnitude

the apparent brightness of an object measured by an observer at an arbitrary distance away.

arc minute

a small angle unit = 1/60th of a degree.

arc second

a tiny angle unit = 1/3600th of a degree = 1/60th of an arc minute.

asteroid

boulder to mountain-sized piece of rock remaining from the early solar system. The largest asteroid is only 1000 kilometers across but most are much smaller.

astrology

a non-scientific belief system in which the positions of the planets among the stars are thought to hold the key to understanding what you can expect from life.

astronomical unit

(A.U.): average distance between the Earth and the Sun (149.6 million kilometers). Used for interplanetary distances.

astronomy

a discipline that uses the scientific method to understand the physical universe (usually beyond the Earth's atmosphere).

astrophysics

a branch of astronomy that deals with the physical properties and interactions of celestial bodies---the application of the principles of physics to celestial bodies and phenomena.

aurora australis

aurorae seen in the southern hemisphere.

aurora borealis

aurorae seen in the northern hemisphere.

aurorae

light displays produced by molecules and atoms high up in an atmosphere. The gas particles are excited by collisions with solar wind particles that were deflected by the planet's magnetic field toward the magnetic poles of the planet.

autumnal equinox

specific moment in the year (on September 22) when the Sun is directly on the celestial equator, moving south of the celestial equator.

azimuth

position on the celestial sphere that is the number of degrees along the horizon away from the exact north point. Exact North = 0°, exact East = 90°, exact South = 180°, exact West = 270°, exact North = 360° (or 0°). Horizontal position of an object.

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last update: 06 August 1999