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Last updated on April 23, 2014 at 16:13 EDT

Universe

Doppler Effect -- The Doppler effect is the apparent change in frequency or wavelength of a wave that is perceived by an observer moving relative to the source of the waves. For waves, such as sound waves, that propagate in a wave medium, the veloc...

Double Star -- When two stars are so nearly in the same direction as seen from Earth that they appear to be a single star to the naked eye but may be separated by the use of telescopes, they are referred to as a double star. There are two different...

Drake Equation -- The Drake equation (also known as the Green Bank equation) is a famous result in the speculative fields of xenobiology and the search for extraterrestrial intelligence. This equation was devised by Dr. Frank Drake in the 1960s in ...

Eclipse -- An eclipse occurs when an astronomical body such as a planet, or satellite gets between a source of light (e.g. the Sun) and another body. For instance, Jupiter eclipses its moons when it gets between them and the Sun. -- Lunar eclipses ...

Ecliptic -- The ecliptic is the geometric plane that contains the orbit of the Earth. The orbits of most planets in the Solar System lie very close to it. Seen from the Earth, this is the great circle on the celestial sphere that contains the diffe...

Electromagnetic Radiation -- Electromagnetic radiation is a combination of oscillating electric and magnetic fields propagating through space and carrying energy from one place to another. Light is a form of electromagnetic radiation. The theoretic...

Electromagnetic Spectrum -- The electromagnetic spectrum describes the various types of electromagnetic radiation based on their wavelengths. Radio, representing wavelengths from a few feet to well over a mile, is at one end of the spectrum. Gamma ...

Elliptical Galaxy -- In astronomy, one of the main classes of galaxy in the Hubble classification and characterized by a featureless elliptical profile. Unlike spiral galaxies, elliptical galaxies have very little gas or dust and no stars have rece...

Emission Nebula -- In astronomy, emission nebulae are clouds of ionized gas, emitting light of various colors. The most common source for ionization are high-energy photons emitted from a nearby young, hot star. Usually, a young star will ionize pa...

Escape Velocity -- An escape velocity is the minimum speed at which an object without propulsion can move away from a source of a gravitational field indefinitely if there is no friction. This definition may need modification for the practical prob...

Event Horizon -- The event horizon is a boundary beyond which information will never reach an observer. An event horizon is a mathematical construct and not a physical object and a person passing through an event horizon will not notice any odd beh...

Extraterrestrial Life -- Extraterrestrial life is life beyond planet Earth (apart from humans travelling in space, and living organisms they bring along or send). There are many questions about extraterrestrial life, including: -- Does it exist...

Extrasolar Planet -- An extrasolar planet is a planet orbiting around a star other than the Sun. Extrasolar planets were first discovered in the 1990s as a result of improved telescope technology, CCD and computer-based image processing which allowed f...

Galaxy -- Stars are almost always found in collections called galaxies, together with gas, dust, and large amounts of dark matter detected by its gravitational effects. These are all held together by gravitational attraction and orbit a common centre. ...

Gamma-Ray Burst -- In astronomy, Gamma-ray bursters (GRBs) are flashes of gamma rays that last from seconds to hours, the longer ones being followed by several days of X-ray afterglow. They occur at random positions in the sky several times each da...

A gas giant is a generic astronomical term invented by the science fiction writer James Blish to describe any large planet that is not composed mostly of rock or other solid matter.

General Relativity -- General Relativity is the common name for the theory of gravitation published by Albert Einstein in 1915. According to general relativity the force of gravity is a manifestation of the local geometry of spacetime. Although the...

Geocentric Model -- The geocentric model of the cosmos is a paradigm which places the Earth at the center of the universe. Common in ancient Greece, it was believed by both Aristotle and Ptolemy. Most Greeks assumed that the Sun, Moon, stars, and plane...

Geodesic -- In mathematics and specifically in differential geometry, a geodesic is a path that furnishes shortest paths between any points on it that are close enough together. The most familiar examples are the straight lines in Euclidean geometr...

Globular Cluster -- A globular cluster is a cluster of stars that is spherical in shape and extremely dense towards its core. Globular clusters are usually composed of hundreds of thousands of old stars, similar to the bulge of a spiral galaxy but ...