A

Absolute Magnitude
a scale for measuring the actual brightness of a celestial object without accounting for the distance of the object. Absolute magnitude measures how bright an object would appear if it were exactly 10 parsecs (about 33 light years) away from Earth. On this scale, the Sun has an absolute magnitude of +4.8 while it has an apparent magnitude of -26.7 because it is so close.

Absolute Zero
the temperature at which the motion of all atoms and molecules stops and no heat is given off. Absolute zero is reached at 0 degrees Kelvin or -273.16 degrees Celsius.

Accretion
process of particles sticking together to form larger bodies; for example, solar nebular dust accreted to form chondrules, and planetesimals accreted to form planets.

Achondrite
a stony meteorite representing differentiated planetary material.

Albedo Feature
a dark or light marking on the surface of an object that may or may not be a geological or topographical feature. Albedo is the measure of the reflectivity of a planet, measured on a scale from zero to one. An albedo of zero describes a planet that absorbs all the light it receives. A planet with an albedo of one reflects all the light that shines on it.

Alpha Centauri
the closest bright star to our solar system.

Altitude
angle in degrees above the horizon.

Angstrom
abbreviated Ĺ. A unit of length equal to 10-8 cm (one-hundredth of a millionth of a centimeter). An Angstrom is on the order of the size of an atom.

Antimatter
matter consisting of particles with charges opposite that of ordinary matter. In antimatter, protons have a negative charge while electrons have a positive charge.

Antipodal point
the point that is directly on the opposite side of the planet; e.g., the Earth's north pole is antipodal to its south pole.

Apastron
the point of greatest separation of two stars, such as in a binary star system.

Aperture
the size of the opening through which light passes in an optical instrument such as a camera or telescope. A higher number represents a smaller opening while a lower number represents a larger opening.

Aphelion
the point in its orbit where a planet is farthest from the Sun.

Apoapsis
the point in orbit farthest from the planet.

Apogee
the point in orbit farthest from the Earth.

Apparent Magnitude
the apparent brightness of an object in the sky as it appears to an observer on Earth. Bright objects have a low apparent magnitude while dim objects will have a higher apparent magnitude.

Arc Second
abbreviated arcsec. A unit of angular measure in which there are 60 arc seconds in 1 arc minute and therefore 3600 arc seconds in 1 arc degree. There are 206,265 arcseconds per radian. One arc second is equal to about 725 km on the Sun.

Arc Degree
a unit of angular measure in which there are 360 arc degrees in a full circle.

Arc Minute
one 1/60 of a degree.

Asteroid
a small planetary body in orbit around the Sun, larger than a meteoroid but smaller than a planet. Most asteroids can be found in a belt between the orbits of Mars and Jupiter. The orbits of some asteroids take them close to the Sun, which also takes them across the paths of the planets.

Astrochemistry
the branch of science that explores the chemical interactions between dust and gas interspersed between the stars.

Astronomical unit (AU)
the average distance from the Earth to the Sun; 1 AU is 149,597,870 kilometers (92,960,116 miles).

Atmosphere
one atmosphere is 14.7 pounds per square inch (105 Newtons per square meter); the average atmospheric pressure at sea level on Earth. Atmosphere is also a layer of gases surrounding a planet, moon, or star. The Earth's atmosphere is 120 miles thick and is composed mainly of nitrogen, oxygen, carbon dioxide, and a few other trace gases.

Aurora
a glow in a planet's ionosphere caused by the interaction between the planet's magnetic field and charged particles from the Sun.

Aurora Borealis
the Northern Lights caused by the interaction between the solar wind, the Earth's magnetic field and the upper atmosphere; a similar effect happens in the southern hemisphere where it is known as the aurora australis.

Aurora Australis
also known as the southern lights, this is an atmospheric phenomenon that displays a diffuse glow in the sky in the southern hemisphere. It is caused by charged particles from the Sun as they interact with the Earth's magnetic field. Known as the Aurora Borealis in the northern hemisphere.

Autumnal equinox
the point on the celestial sphere where the sun crosses the celestial equator from north to south. The time when the sun is at the autumnal equinox defines the first day of autumn. This happens on about September 22 each year.

Average orbital
speed around the Sun: This is a measure of how fast a planet moves through space, in kilometers per hour.

Axial inclination
axial inclination is the angle at which a planet's axis of rotation is tilted, with respect to that planet's own orbit. On Earth, as well as other planets, this tilt is responsible for the seasons.

Axis
also known as the poles, this is an imaginary line through the center of rotation of an object.

Azimuth
the angular distance of an object around or parallel to the horizon from a predefined zero point.

B

Bar
a unit of measure of atmospheric pressure. One bar is equal to 0.987 atmospheres, 1.02 kg/cm2, 100 kilopascal, and 14.5 lbs/square inch.

Barycenter
the center of mass of a system of bodies; e.g., the center of mass of the solar system.

Basalt
a general term for dark-colored, igneous rocks composed of minerals that are relatively rich in iron and magnesium.

Big Bang
the theory that suggests that the universe was formed from a single point in space during a cataclysmic explosion about 18 billion years ago. The force of the explosion accounts for the current expansion of the universe.

Binary Stars
a system of two stars that revolve around a common center of gravity.

Black Hole
the collapsed core of a massive star. Stars that are very massive will collapse under their own gravity when their fuel is exhausted. The collapse continues until all matter is crushed out of existence into what is known as a singularity. The gravitational pull is so strong that not even light can escape.

Blueshift
a shift in the lines of an object's spectrum toward the blue end. Blueshift indicates that an object is
moving toward the observer. The larger the blueshift, the faster the object is moving.

Bolide
an exploding meteorite.

C

Caldera
a large, basin-shaped volcanic depression that is more or less circular in form. Most volcanic calderas are produced by collapse of the roof of a magma chamber due to removal of magma by voluminous eruptions or subterranean withdrawal of the magma, although some calderas may be formed by explosive removal of the upper part of a volcano.

Carbonaceous chondrite
a type of primitive chondrite with evidence of nebular processes.

Celestial equator
the intersection of the earth's equatorial plane with the celestial sphere.

Celestial pole
the north and south celestial poles are points on the celestial sphere where earths axis of rotation intersects the celestial sphere.

Celestial sphere
an imaginary sphere centered on the earth on which all of the stars are imagined to be projected.

Cepheid
a pulsating variable star. This type of star undergoes a rhythmic pulsation as indicated by its regular pattern of changing brightness as a function of time. The period of pulsation has been demonstrated to be directly related to a Cepheid's intrinsic brightness making observations of these stars one of the most powerful tools for determining distance known to modern day astronomy.

CGS
centimeter-Gram-Second (abbreviated cm-gm-sec or cm-g-s). The system of measurement that uses these units for distance, mass, and time.

Chondrite
a meteorite containing chondrules and other components produced in the solar nebula.

Chondrule
small, glassy spheres commonly found in meteorites.

Chromatic aberration
an optical lens defect causing color fringes, because the lens material brings different colors of light to focus at different points.

Chromosphere
the layer of the solar atmosphere that is located above the photosphere and beneath the transition region and the corona. The chromosphere is hotter than the photosphere but not as hot as the corona.

Circumpolar Star
a star that never sets but always stays above the horizon. This depends on the location of the observer. The further South you go the fewer stars will be circumpolar. Polaris, the North Star, is circumpolar in most of the northern hemisphere.

Coma
a spherical cloud of material surrounding the head of a comet. This material is mostly gas that the Sun has caused to boil off the comet's icy nucleus. This gas shines both by reflected sunlight and light emitted by excited molecules. A cometary coma can extend up to a million miles from the nucleus.

Comet
a chunk of frozen gasses, ice, and rocky debris that orbits the Sun. A comet nucleus is about the size of a mountain on earth. When a comet nears the Sun, heat vaporizes the icy material producing a cloud of gaseous material surrounding the nucleus, called a coma. As the nucleus begins to disintegrate, it also produces a trail of dust or dust tail in its orbital path and a gas or ion tail pointing away from the Sun. Comet comas can extend up to a million miles from the nucleus and comet tails can be millions of miles long. There are thought to be literally trillions of comets in our solar system out past Neptune and Pluto, but only once per decade or so does one become near and bright enough to see easily without binoculars or a telescope.

Conjunction
an event that occurs when two or more celestial objects appear close close together in the sky.

Constellation
a grouping of stars that make an imaginary picture in the sky. There are 88 constellations.

Convection Zone
a layer in a star in which convection currents are the main mechanism by which energy is transported outward. In the Sun, a convection zone extends from just below the photosphere to about seventy percent of the solar radius.

Convection
the physical upwelling of hot matter, thus transporting energy from a lower, hotter region to a higher, cooler region. A bubble of gas that is hotter than its surroundings expands and rises. When it has cooled by passing on its extra heat to its surroundings, the bubble sinks again. Convection can occur when there is a substantial decrease in temperature with height, such as in the Sun's convection zone.

Corona
the outermost layer of the solar atmosphere. The corona consists of a highly rarefied gas with a low density and a temperature greater than one million degrees Kelvin. It is visible to the naked eye during a solar eclipse.

Coronal Mass Ejections
are huge bubbles of gas threaded with magnetic field lines that are ejected over the course of several hours.They are often associated with solar flares and prominence eruptions.

Cosmic Ray
atomic nuclei (mostly protons) that are observed to strike the Earth's atmosphere with extremely high amounts of energy.

Cosmic String
a tubelike configuration of energy that is believed to have existed in the early universe. A cosmic string would have a thickness smaller than a trillionth of an inch but its length would extend from one end of the visible universe to the other.

Cosmology
a branch of science that deals with studying the origin, structure, and nature of the universe.

Crater
a bowl-shaped depression formed by the impact of an asteroid or meteoroid. Also the depression around the opening of a volcano.

Crystalline
indicates a rock is composed of mineral crystals rather than glass. In general, when igneous melts cool very fast they form glass (like obsidian), but when they cool slower, mineral crystals have an opportunity to grow.

D

Dark Matter
a term used to describe matter in the universe that cannot be seen, but can be detected by its gravitational effects on other bodies.

Declination
the angular distance of an object in the sky from the celestial equator.

Density
the amount of matter contained within a given volume. Density is measured in grams per cubic centimeter (or kilograms per liter). The density of water is 1.0, iron is 7.9, and lead is 11.3.

Differentiated
when a (partially) molten body has been divided into two or more fractions of dissimilar compositions. In the case of the Earth, iron-nickel metal was differentiated from silicate material to form the planet's core.

Disk
the surface of the Sun or other celestial body projected against the sky.

Doppler Effect
the apparent change in wavelength of sound or light emitted by an object in relation to an observer's position. An object approaching the observer will have a shorter wavelength (blue) while an object moving away will have a longer (red) wavelength. The Doppler effect can be used to estimate an object's speed and direction.

Double Star
a grouping of two stars. This grouping can be apparent, where the stars seem close together, or physical, such as a binary system.

Double Asteroid
two asteroids that revolve around each other and are held together by the gravity between them. Also called a binary asteroid.

Dust grains
not the dust one finds around the house, which is typically fine bits of fabric, dirt, or dead skin cells. Rather interstellar dust grains are much smaller clumps, on the order of a fraction of a micron across, irregularly shaped, and composed of carbon and/or silicates. Dust is most evident by its absorption, causing large dark patches in regions of our Milky Way Galaxy and dark bands across other galaxies.

E

Eclipse
the total or partial blocking of one celestial body by another.

Ejecta
material from beneath the surface of a body such as a moon or planet that is ejected by an impact such as a meteor and distributed around the surface. Ejecta usually appears as a lighter color than the surrounding surface.

Electromagnetic Spectrum
the entire range of all the various kinds or wavelengths of electromagnetic radiation, including (from short to long wavelengths) gamma rays, x-rays, ultraviolet, optical (visible), infrared, and radio waves.

Electromagnetic Radiation
radiation that travels through vacuous space at the speed of light and propagates by the interplay of oscillating electric and magnetic fields. This radiation has a wavelength and a frequency.

Electron Flux
the rate of flow of electrons through a reference surface. In cgs units, measured in electrons s-1, or simply s-1.

Electron
a negatively charged elementary particle that normally resides outside (but is bound to) the nucleus of an atom.

Electron Volt
Abbreviated eV. A unit of energy used to describe the total energy carried by a particle or photon. The energy acquired by an electron when it accelerates through a potential difference of 1 volt in a vacuum. 1 eV = 1.6 x 10-12 erg.

Ellipse
an ellipse is an oval shape. Johannes Kepler discovered that the orbits of the planets were elliptical in shape rather than circular.

Elliptical Galaxy
a galaxy whose structure s