Astronomy: Concepts and Terms

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.

A star with very low mass (less than 8% of that of the Sun) such that thermonuclear reactions cannot occur, its dim appearance being the result of contraction due to gravitational force and constant release of energy.

A small celestial body consisting mostly of dust and gases that moves in an elongated elliptical or nearly parabolic orbit around the sun.

One of the 88 areas into which the sky is divided for the purposes of identifying and naming celestial objects.

Grouping of millions or billions of stars, held together by gravity.

From Collins Dictionary of Astronomy
A catalog of the brightest ‘nebulae’ prepared by the French astronomer Charles Messier and printed in final form in 1784.

Faint star that suddenly erupts in brightness by 10,000 times or more, remains bright for a few days, and then fades away and is not seen again for very many years, if at all.

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.

The sun and the surrounding planets, natural satellites , dwarf planets, asteroids, meteoroids, and comets that are bound by its gravity.

Luminous globe of gas, mainly hydrogen and helium, which produces its own heat and light by nuclear reactions.

A massive star in the latter stages of stellar evolution that suddenly contracts and then explodes, increasing its energy output as much as a billionfold.

Totality of matter and energy in existence.

In astronomy, a type of star that is abnormally faint for its white-hot temperature (see mass-luminosity relation).

From Collins Dictionary of Astronomy
A short period of very rapid expansion of the Universe postulated to occur very soon after the big bang. The hypothesis of inflation in the early Universe solves a number of cosmological problems.

From Astronomy Encyclopedia
Planet not associated with our Solar System. The existence of other worlds, especially inhabited ones, has been the subject of debate since ancient times. In the 20th century, stars with irregular proper motions were suspected of having planetary companions; Barnard's Star was a famous candidate.

Force of attraction that arises between objects by virtue of their masses. The larger the mass of an object the more strongly it attracts other objects.

A graph in which the spectral types of stars are plotted against their absolute magnitudes. Stars fall into different groupings in different parts of the graph.

In physics, a device that splits a beam of light into two parts, the parts being recombined after travelling different paths to form an interference pattern of light and dark bands.

In physics, contraction or foreshortening of a moving body in the direction of its motion, proposed by H. A. Lorentz on theoretical grounds and based on an earlier suggestion by G. F. Fitzgerald; it is sometimes called the Fitzgerald, or Lorentz-Fitzgerald, contraction.

From Astronomy Encyclopedia
Experiment carried out by German-American physicist Albert Michelson (1852-1931) in 1881 and, with greater precision, by Michelson and the American chemist Edward Morley (1838-1923) in 1887. The experiment attempted to detect the motion of the Earth through the luminiferous ether.

From Collins Dictionary of Astronomy
Two mathematical expressions derived by Jan Oort that describe the effects of differential galactic rotation on the radial velocities (vr) and tangential velocities (vt) of stars at an average distance r from the Sun.

Or redshift, in astronomy, the systematic displacement of individual lines in the spectrum of a celestial object toward the red, or longer wavelength, end of the visible spectrum.

From Astronomy Encyclopedia
Radius that a body must exceed if light from its surface is to reach an outside observer. If an object collapses below this radius, its escape velocity rises to above the speed of light and the object becomes a black hole.

From Collins Dictionary of Astronomy
The single physical entity into which the concepts of space and time can be unified such that an event may be specified mathematically by four coordinates, three giving the position in space and one the time. The path of a particle in spacetime is called its world line.

The study of radiation from celestial sources in the wavelength range 91.2 to 320 nanometres, 12 to 91.2 nanometres being the extreme ultraviolet range.

In astronomy, the explosive event that marked the origin of the universe as we know it. At the time of the Big Bang, the entire universe was squeezed into a hot, superdense state. The Big Bang explosion threw this compact material outwards, producing the expanding universe seen today.

A region in space, believed to be formed when a large star has collapsed in on itself at the end of its life, with such a strong gravitational pull that not even light waves can escape from it.

From Astronomy Encyclopedia
Remnant radiation from the creation of the Universe. Early cosmologists predicted theoretically that the Universe originally began as a singularity that expanded into a small, hot ‘soup’ of radiation and elementary particles.

Radiation which consists of streams of high-energy particles from outer space, traveling at about the speed of light, most of which are thought to originate from supernovas.

Matter not visible but making up a large proportion of the mass of the universe, and sufficient to change its expansion into contraction.

From Atlas of the Universe
Before we can make any attempt to trace the history of the universe, we must look carefully at the situation we find today. As we have seen, each group of galaxies is receding from each other group, so that the entire universe is expanding.

The branch of astronomy that deals with the origin and nature of emissions from extraterrestrial sources in the gamma-ray range of electromagnetic radiation rather than in the visible range.

From Collins Dictionary of Astronomy
Galaxies must have condensed out of the gases expanding from the Big Bang, beginning at a time when the average density of the Universe was roughly the same as the current mean density of a galaxy.

From Collins Dictionary of Astronomy
Extremely weak wavelike disturbances that were predicted by Einstein's general theory of relativity. They represent the radiation associated with the gravitational force, and are produced when massive bodies are accelerated or otherwise disturbed.

Life history of a star, beginning with its condensation out of the interstellar gas (see interstellar matter) and ending, sometimes catastrophically, when the star has exhausted its nuclear fuel or can no longer adjust itself to a stable configuration.

From The Hutchinson unabridged encyclopedia with atlas and weather guide
In physics and cosmology, a mathematical theory developed in the 1980s to explain the properties of elementary particles and the forces between them (in particular, gravity and the nuclear forces) in a way that combines relativity and quantum theory.