Glossary of Terms

Night Skies

Glossary of Astronomical Terms


Absolute Magnitude The apparent magnitude of a star viewed at a distance of 10 parsecs

Acceleration The rate of change of an object's velocity in response to a change in speed and/or direction

Active Galaxy A galaxy emitting large amounts of energy

Albedo the percentage of sunlight that a planet , asteroid, or moon reflects

Angular Diameter The angle subtended by the diameter of an object

Angular Resolution The angular size of the smallest feature that can be resolved

Annular eclipse An eclipse in which the size of the Moon (due to distance) is too small to completely cover the Sun leaving a ring of sunlight surrounding the moon at mid-eclipse

Aphelion The point in orbit farthest from the Sun

Apogee The point in orbit farthest from Earth

Apparent Brightness The magnitude of the stellar flux arriving to Earth

Arcminute One-sixtieth of a degree. Denoted by the symbol ‘

Arcsecond One-sixtieth of an arcminute. Denoted by the symbol ‘'

Asteroid One of many small, rocky-type bodies in orbit around the Sun

Astronomical Unit (AU) The average distance between the Earth and the Sun

Baseline Interferometry the distance between two telescopes whose light is combined to produce a higher resolution image

Big Bang An explosion creating Space that occurred roughly 13.7 billion years ago marking the origin of the Universe

Binary star Two stars in orbit around each other

Black Hole A compact object with a force of gravity great enough that not even light can escape its gravitational pull

Blackbody A hypothetical perfect absorber and emitter of radiation

Celestial equator A great circle on the celestial sphere 90 degrees from the poles. The parallel of Earth's equator extended into space

Celestial Poles The extension of the Earth's North and South Poles into space and the point around which our heavens appear to rotate

Celestial sphere An imaginary sphere of very large radius centered on Earth upon which is projected the heavens

Center of Mass The point between any two massive objects about which both objects rotate

Circumpolar A star that appears to rotate about one of the celestial poles very close to the pole

Cluster of Stars A group of stars formed at roughly the same time and held together by gravitational attraction

Comet A small icy, dusty asteroid-sized body in orbit around the Sun

Constellation A configuration of stars in the same region of sky

Cosmic Microwave Background The radiation field of 2. degrees Kelvin left over by the residual energy from the Big Bang

Dark Energy An unknown form of energy pervading the Universe causing its expansion speed to accelerate

Dark matter Non-luminous energy that is thought to make up approximately 90% of all matter in the Universe

Dark Nebula A cloud of interstellar gas and dust that obscures the light of the stars behind it

Differential Rotation The different rotation rate of various parts of a non-solid rotating body (for example the Sun)

Doppler Effect The apparent change in wavelength of an object due to relative motion between the source and the observer

Electromagnetic Radiation Radiation composed of light having both an electric component and a magnetic component of which examples include microwaves, gamma rays, X-rays, radio waves, visible light, and ultraviolet light

Electromagnetic Spectrum The entire range of observable electromagnetic radiation

Element A stable configuration of protons, neutrons, and electrons (such as gold or silver or nickel) that is determined by the number of protons

Energy Flux Rate of energy flow usually measured in Joules per square meter per second

Emission nebula A glowing gaseous nebula whose spectrum contains bright emission lines

Extrasolar planet An planet orbiting a star that is not the Sun

Eyepiece The magnifying lens used to observe the object in your telescope

Focal Length The distance from a lens or mirror to the point of light ray conversion

Galaxy A large assemblage of stars, gas, dust, and dark matter separated from similar objects by large expanses of empty space

Giant Molecular Cloud A vast cloud of interstellar gas and dust in which densities are high enough and temperatures low enough that atoms can combine into molectures such as neutral hydrogen. Stars are born from the collapse of parts of Giant Molecular Clouds.

Grand Unified Theory A theory that explains and links the four known forces (strong nuclear, weak nuclear, electromagnetic, and gravitational) together

Gravity The force attracting all matter to other matter

HII Region A region of ionized hydrogen in space (generally created by radiation effects from hot stars)

Habitable Zone The region around a star in which conditions are suitable for the development of planets that could harbor life

Hubble law The empirical relationship between the distance of a galaxy and the rate of its recessional velocity

Inflation A period of sudden and abrupt expansion of space during the early Universe

Interstellar medium The gas and dust in the space between stars

Kelvin A unit of temperature on the Kelvin temperature scale. Zero degrees Kelvin equals 273 degrees Celsius

Kiloparsec 1000 Parsecs or 3260 light-years

Kuiper belt A region around the Sun extending from the orbit of Pluto to about 500 AU from the sun where icy bodies orbit the Sun

Laws of physics A set of laws quantifying the natural phenomena by which we can describe the actions of the Universe

Light Curve A graph showing how the brightness of an astronomical object varies over time

Light-gathering power A measure of the light-gathering ability of a telescope

Light pollution Light from Earth-based sources that inhibits the ability to collect light from astronomical sources

Light-year The distance traveled by light in a vacuum during one year

Local Group The cluster of Galaxies of which the Milky Way is a member

Lookback time The time into the past that we are viewing when we view a distanct object. If the light left the object 5 million years ago and took that long to travel to Earth, the lookback time is 5 million light-years

Luminosity The rate of electromagnetic flux radiated by a star

Magnification The factor by which the apparent angular size of an object is increased by the telescope

Main Sequence The stars on the Hertzsprung-Russell diagram that are shining via the conversion of hydrogen to helium in their cores

Mare (plural maria) Latin for sea; it is a large, open, fairly crater-free area of the moon darker than its surroundings

Mare basalt A type of lunar rock commonly found in the mare basins

Mass A total measure of the amount of material in an object

Mass-luminosity Relationship A relationship between the amount of mass within and the luminosity of a star

Megaparsec One million parsecs

Metal In stars, any element other than hydrogen or helium. On earth, an element that is a good conductor of electricity

Meteorite A Meteoroid fragment that has survived passage through the Earth's atmosphere and hits the ground

Meteoroid A small rock in interplanetary space

Milky Way Galaxy The barred-spiral type galaxy in which the Earth resides

Molecule A combination of two or more atoms

Neap tide A tide that occurs when the moon is at first-quarter or third-quarter phase

Near-earth objects An object whose orbit lies within the orbit of Mars

Neutron star A old stellar remnant composed almost entirely of neutrons

New Moon The phase of the moon where the dark side of the Moon faces Earth due to the Moons location between the Earth and the Sun

Nonthermal radiation Radiation that is not generated by heat

OB Association A group of hot, young massive O and B type stars

OBAFGKM The order of spectral classes in temperature sequence (from hot to cool)

Objective Mirror The primary light-collecting mirror of an objective telescope

Observable Universe The portion of the Universe that we can see because it is within our observable light horizon

Occultation The eclipsing of an astronomical object by another astronomical object

Oort Cloud A cloud of comet-like material surrounding the Sun at distances of roughly 50,000 AU

Parallax The apparent motion and displacement of an object in the sky due to Earth's relative motion around the Sun

Parsec A unit of distance equaling 3.26 light years

Perigee The near-Earth point in orbit

Perihelion The near-Sun point in orbit

Photometry The study and measurement of light intensities

Photon A discrete unit of electromagnetic energy

Planetary Nebula A luminous shell of gas ejected from an old, low-mass star (like the Sun).

Precession (of Earth) A slow, conical motion of the Earth's axis of rotation caused by the gravitational pull of the Moon and the Sun on the Earth's equatorial bulge

Progenitor Star The origin star of a supernova explosion

Protoplanet Moon-sized object formed by the accumulation of planetesimals in the Early Solar System

Pulsar A pulsating radio source associated with a rapidly spinning neutron star

Quasar A distant, luminous object with a large redshift and a star-like appearance

Radial Velocity The component of an object's velocity parallel to the line of sight

Red Giant A large, cool, old and luminous star

Redshift The amount of reddening of light from a distant and receeding source

Retrograde Motion The apparent backwards or westward motion of a planet with respect to the background stars

Sagitarrius A* The powerful radio source at the center of the Milky Way believed to be the black hole at the center of the Galaxy

Sidereal Month The period of the Moon's rotation around the Earth with respect to the background stars

Sidereal Period The orbital period of one object about another with respect to the background stars

Standard Candle An astronomical object of known intrinsic brightness used to aid in determining distances to related objects

Stellar Parallax The apparent displacement of a star's location due to Earth's motion around the Sun

Sunspot A temporary cooler region in the solar atmosphere caused by distorted solar magnetic field lines

Supernova A stellar outburst and death in which a star temporarily increases its luminosity by roughly one million times the original luminosity

Supernova Remnant The shell of material accelerated away from the star during the supernova explosion

Tangential Velocity That component of an object's velocity perpendicular to the line of sight

Theory A hypothesis that has withstood experimental and/or observational testing

Tidal Force A gravitational force whose strength and/or direction tends to distort an object

Transit An event during which one astronomical body crosses in front of another

UBV photometry A system for determining the surface temperature of a star by measuring the star's brightness in each of the three (ultraviolet (U), blue (B), and visible (V)) filters

Van Allen belts Two doughnut-shaped regions surrounding Earth where charged particles are trapped by the Earth's magnetic field

Velocity The speed and direction of an object's motion

Visible light Electromagnetic radiation detectable by the human eye

Visual binary A binary star pair in which each star can be resolved in a telescope

Weight The force with which gravity acts on a body

Zenith The point on the celestial sphere directly overhead the observer

Zodiac A band of 12 constellations in the sky centered on the ecliptic