Planetoid, or minor planet, small body orbiting the sun. More than 10,000 asteroids have orbits sufficiently well known to have been cataloged and named; thousands more exist.
Grouping of millions or billions of stars, held together by gravity.
In astronomy, path in space described by a body revolving about a second body where the motion of the orbiting bodies is dominated by their mutual gravitational attraction.
In astronomy, a type of star that is abnormally faint for its white-hot temperature (see mass-luminosity relation).
From Astronomy Encyclopedia (M31, NGC 224) One of the two giant spiral galaxies in the local group of galaxies, the other being our Galaxy, the Milky Way. M31 is the nearest spiral to the Milky Way, some 2.4 million l.y. away
The galaxy of which the sun and solar system are a part, seen as a broad band of light arching across the night sky from horizon to horizon.
From The Hutchinson Unabridged Encyclopedia
From The Hutchinson Unabridged Encyclopedia
The star that the planets revolve around and which gives out the heat and light energy necessary to enable living organisms to survive on Earth.
The third planet from the Sun.
The fifth planet from the Sun, and the largest in the solar system, whose orbit lies between those of Mars and Saturn.
The eighth planet from the Sun, the outermost of the four ‘gas giant’ planets; discovered in 1846 as a result of a prediction by Leverrier to explain anomalies in the observed orbit of Uranus.Ablation
A process by where the atmosphere melts away and removes the surface material of an incoming meteorite.
Accretion
The process by where dust and gas accumulated into larger bodies such as stars and planets.
Accretion Disk
A disk of gas that accumulates around a center of gravitational attraction, such as a white dwarf, neutron star, or black hole. As the gas spirals in, it becomes hot and emits light or even X-radiation.
Achondrite
A stone meteorite that contains no chondrules.
Altitude
The angular distance of an object above the horizon.
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.
Aphelion
The point in the orbit of a planet or other celestial body where it is farthest from the Sun.
Apogee
The point in the orbit of the Moon or other satellite where it is farthest from the Earth.
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)
A unit of measure equal to the average distance between the Earth and the Sun, approximately 93 million miles.
Atmosphere
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. This phenomenon is known as the Aurora Borealis in the Earth's northern hemisphere and the Aurora Australis in the Earth's Southern Hemisphere.
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.
Aurora Borealis
Also known as the northern lights, this is an atmospheric phenomenon that displays a diffuse glow in the sky in the northern hemisphere. It is caused by charged particles from the Sun as they interact with the Earth's magnetic field. Known as the Aurora Australis in the southern hemisphere.
Axis
Also known as the poles, this is an imaginary line through the center of rotation of an object.
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.
Black Moon
A term used to describe an extra new that occurs in a season. It usually refers to the third new moon in a season with four new moons. The term is sometimes used to describe a second new moon in a single month.
Blue Moon
A term used to describe an extra full that occurs in a season. It usually refers to the third full moon in a season with four full moons. Note that a blue moon does not actually appear blue in color. It is merely a coinsidence in timing caused by the fact that the lunar month is slightly shorter than a calendar month. More recently, the term has also been used to describe a second full moon in a single month.
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
A term used to describe an exceptionally bright meteor. Bolides typically will produce a sonic boom.
Celestial Equator
An imaginary line that divides the celestial sphere into a northern and southern hemisphere.
Celestial Poles
The North and South poles of the celestial sphere.
Celestial Sphere
An imaginary sphere around the Earth on which the stars and planets appear to be positioned.
Cepheid Variable
This is a variable star whose light pulsates in a regular cycle. The period of fluctuation is linked to the brightness of the star. Brighter Cepheids will have a longer period.
Constellation
A grouping of stars that make an imaginary picture in the sky.
Corona
The outer part of the Sun's atmosphere. The corona is visible from Earth during a total solar eclipse. It is the bright glow seen in most solar eclipse photos.
Cosmic Ray
Atomic nuclei (mostly protons) that are observed to strike the Earth's atmosphere with extremely high amounts of energy.
Crater
A bowl-shaped depression formed by the impact of an asteroid or meteoroid. Also the depression around the opening of a volcano.
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.
Double Asteroid
Two asteroids that revolve around each other and are held together by the gravity between them. Also called a binary asteroid.
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.
Dwarf Planet
A celestial body orbiting the Sun that is massive enough to be rounded by its own gravity but has not cleared its neighboring region of planetesimals and is not a satellite. It has to have sufficient mass to overcome rigid body forces and achieve hydrostatic equilibrium. Pluto is considered to be a dwarf planet.
Eccentricity
The measure of how an object's orbit differs from a perfect circle. Eccentricity defines the shape of an object's orbit.
Eclipse
The total or partial blocking of one celestial body by another.
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 shaped like an ellipse and is smooth and lacks complex structures such as spiral arms.
Equinox
The two points at which the Sun crosses the celestial equator in its yearly path in the sky. The equinoxes occur on or near March 21 and September 22. The equinoxes signal the start of the Spring and Autumn seasons.
Event Horizon
The invisible boundary around a black hole past which nothing can escape the gravitational pull - not even light.
Evolved Star
A star that is near the end of its life cycle where most of its fuel has been used up. At this point the star begins to loose mass in the form of stellar wind.
Fireball
An extremely bright meteor. Also known as bolides, fireballs can be several times brighter than the full Moon. Some can even be accompanied by a sonic boom.
Flare Star
A faint red star that appears to change in brightness due to explosions on its surface.
Galactic Nucleus
A tight concentration of stars and gas found at the innermost regions of a galaxy. Astronomers now believe that massive black holes may exist in the center of many galaxies.
Galaxy
A large grouping of stars. Galaxies are found in a variety of sizes and shapes. Our own Milky Way galaxy is spiral in shape and contains several billion stars. Some galaxies are so distant the their light takes millions of years to reach the Earth.
Gravitational Lens
A concentration of matter such as a galaxy or cluster of galaxies that bends light rays from a background object. Gravitational lensing results in duplicate images of distant objects.
Gravity
A mutual physical force of nature that causes two bodies to attract each other.
Hypergalaxy
A system consisting of a spiral galaxy surrounded by several dwarf white galaxies, often ellipticals. Our galaxy and the Andromeda galaxy are examples of hypergalaxies.
Inferior Planet
A planet that orbits between the Earth and the Sun. Mercury and Venus are the only two inferior planets in our solar system.
International Astronomical Union (IAU)
An international organization that unites national astronomical societies from around the world and acts as the internationally recognized authority for assigning designations to celestial bodies and their surface features.
Ionosphere
A region of charged particles in a planet's upper atmosphere. In Earth's atmosphere, the ionosphere begins at an altitude of about 25 miles and extends outward about 250.
Kelvin
A temperature scale used in sciences such as astronomy to measure extremely cold temperatures. The Kelvin temperature scale is just like the Celsius scale except that the freezing point of water, zero degrees Celsius, is equal to 273 degrees Kelvin. Absolute zero, the coldest known temperature, is reached at 0 degrees Kelvin or -273.16 degrees Celsius.
Kepler's First Law
A planet orbits the Sun in an ellipse with the Sun at one focus.
Kepler's Second Law
A ray directed from the Sun to a planet sweeps out equal areas in equal times.
Kepler's Third Law
The square of the period of a planet's orbit is proportional to the cube of that planet's semi major axis; the constant of proportionality is the same for all planets.
Kiloparsec
A distance equal to 1000 parsecs.
Kirkwood Gaps
Regions in the main belt of asteroids where few or no asteroids are found. They were named after the scientist who first noticed them.
Kuiper Belt
A large ring of icy, primitive objects beyond the orbit of Neptune. Kuiper Belt objects are believed to be remnants of the original material that formed the Solar System. Some astronomers believe Pluto and Charon are Kuiper Belt objects.
Light Year
An astronomical unit of measure equal to the distance light travels in a year, approximately 5.8 trillion miles.
Luminosity
The amount of light emitted by a star.
Lunar Eclipse
A phenomenon that occurs when the Moon passes into the shadow of the Earth. A partial lunar eclipse occurs when the Moon passes into the penumbra, or partial shadow. In a total lunar eclipse, the Moon passes into the Earth's umbra, or total shadow.
Lunar Month
The average time between successive new or full moons. A lunar month is equal to 29 days 12 hours 44 minutes. Also called a synodic month.
Lunation
The interval of a complete lunar cycle, between one new Moon and the next. A lunation is equal to 29 days, 12 hours, and 44 minutes.
Magellanic Clouds
Two small, irregular galaxies found just outside our own Milky Way galaxy. The Magellanic Clouds are visible in the skies of the southern hemisphere.
Magnetic Field
A condition found in the region around a magnet or an electric current, characterized by the existence of a detectable magnetic force at every point in the region and by the existence of magnetic poles.
Magnetic Pole
Either of two limited regions in a magnet at which the magnet's field is most intense.
Magnetosphere
The area around a planet most affected by its magnetic field. The boundary of this field is set by the solar wind.
Mass
A measure of the total amount of material in a body, defined either by the inertial properties of the body or by its gravitational influence on other bodies.
Matter
A word used to describe anything that contains mass.
Meridian
An imaginary circle drawn through the North and South poles of the celestial equator.
Minor Planet
A term used since the 19th century to describe objects, such as asteroids, that are in orbit around the Sun but are not planets or comets. In 2006, the International Astronomical Union reclassified minor planets as either dwarf planets or small solar system bodies.
Nebula
A cloud of dust and gas in space, usually illuminated by one or more stars. Nebulae represent the raw material the stars are made of.
Neutrino
A fundamental particle produced by the nuclear reactions in stars. Neutrinos are very hard to detect because the vast majority of them pass completely through the Earth without interacting.
Neutron Star
A compressed core of an exploded star made up almost entirely of neutrons. Neutron stars have a strong gravitational field and some emit pulses of energy along their axis. These are known as pulsars.
Nova
A star that flares up to several times its original brightness for some time before returning to its original state.
Open Cluster
A collection of young stars that formed together. They may or may not be still bound by gravity. Some of the youngest open clusters are still embedded in the gas and dust from which they formed.
Opposition
The position of a planet when it is exactly opposite the Sun in the sky as seen from Earth. A planet at opposition is at its closest approach to the Earth and is best suitable for observing.
Orbit
The path of a celestial body as it moves through space.
