HOME  |  RESOURCES  >  Glossary  >  Astronomical Glossary Astronomical Glossary Big Bang Theory Binary System Black Hole Brown Dwarf Centripetal Force Chandrasekhar Limit Circumstellar Disk Cluster of Galaxies Copernicus, Nicolaus Galactic Bulge Galactic Halo Galaxy Globular Cluster Intergalactic Medium Interstellar Medium Kuiper Belt Local Group Mass Function Nebula (all types) Neutron Star Nova Pulsar Quasar Red Giant Solar Mass Supernova Supernova Remnant White Dwarf Wormhole Big Bang Theory A broadly accepted theory for the origin and evolution of our universe. The theory says that the observable universe originated approximately 20 billion years ago from the violent explosion of a very small agglomeration of matter of extremely high density and temperature. Binary System A system of two stars that orbit around each other. Black Hole An invisible object in space formed when a massive star collapses from its own gravity. They contain a huge amount of mass compacted into an extremely small volume, creating a gravitational influence is so strong that nothing — not even light — can escape its grasp. Swirling disks of material — called accretion disks — may surround black holes, and jets of matter may arise from their vicinity. Top of page Brown Dwarf An object too small to be an ordinary star because it cannot produce enough energy by fusion in its core to compensate for the radiative energy it loses from its surface. A brown dwarf has a mass less than 0.08 times that of the Sun. Chandrasekhar Limit A limit which mandates that no white dwarf (a collapsed, degenerate star) can be more massive than about 1.4 solar masses. Any degenerate object more massive must inevitably collapse into a neutron star. Centripetal Force The inward, center-directed force on a body moving in a curved line around another body. Centripetal force is necessary for an object to move in a circle. Circumstellar Disk A ring of dusty material around a star. Such disks are expected to be the birthplace of planets. Top of page Cluster of Galaxies A system of galaxies containing from a few to a few thousand members which are all gravitationally bound to each other. Copernicus, Nicolaus (1473 - 1543) Polish astronomer who advanced the theory that the Earth and other planets revolve around the Sun. This was highly controversial at the time, since the prevailing model held that the Earth was the center of the universe, and all objects, including the sun, circled it. The Earth-centered perspective had been widely accepted in Europe for 1000 years when Copernicus proposed his new world view. (It should be noted, however, that the heliocentric idea was first put forth by Aristarcus of Samos in the 3rd century B.C., a fact known to Copernicus but long ignored by others prior to him.). See painting of Nicolaus Copernicus by Polish painter Jan Matejko. Galactic Bulge A thick region around the center of a galaxy that spheroidal in shape, containing warm gas and metal-rich older stars. Galactic Halo An extended region surrounding a galaxy. The halo contains globular clusters and other old stars. The halo apparently has considerable mass but relatively low luminosity, suggesting that a lot of dark matter must be present in the halo. Galaxy A huge system of about 100 billion stars, gas and dust, bound together by gravity. Our Sun is a member of the Milky Way Galaxy, which is sometimes designated by capitalization: Galaxy. There are billions of galaxies in the observable universe and they come in spiral, barred spiral, elliptical, and irregular shapes. The diameters of galaxies are generally measured in tens of thousands of light-years. The distance between galaxies within a Galactic Group or Cluster averages approximately 1 million to 2 million light-years, and the spaces between clusters of galaxies may be a hundred of times as great. Galaxies were once called "island universes." Top of page Globular Cluster A dense, rich, spherical cluster of stars, held together by their own mutual gravity, and containing up to hundreds of thousands of stars. Globular clusters are generally found in the halo of the galaxy, and contain old stars. Globular clusters are thought to be among the oldest objects in a galaxy. Intergalactic Medium The gas and dust between stars, which fills the plane of the Galaxy much like air fills the world we live in. For centuries, scientists believed that the space between the stars was empty. It wasn't until the eighteenth century, when William Herschel observed nebulous patches of sky through his telescope, that serious consideration was given to the notion that interstellar space was something to study. It was only in the last century that observations of interstellar material suggested that it was not even uniformly distributed through space, but that it had a unique structure. Interstellar Medium Simply put, the interstellar medium is the material which fills the space between the stars. Many people imagine outer space to be a complete vacuum, devoid of any material. Although the interstellar regions are more devoid of matter than any vacuum artificially created on earth, there is matter in space. These regions have very low densities and consist mainly of gas (99%) and dust. In total, approximately 15% of the visible matter in the Milky Way is composed of interstellar gas and dust. Top of page Kuiper Belt A region in our outer solar system where many short-period comets (possessing orbits of less than 200 years) originate. This region begins beyond Neptune’s orbit and encompasses an estimated distance of between 30 and 100 astronomical units. There may be as many as 100 million Kuiper belt comets. Local Group A family of roughly thirty galaxies nearest to us, in which the galaxy we’re in — the Milky Way — belongs. Mass Function A calculation of how many stars in a region and how massive the stars are, which shows how the cloud of gas that created the stars collapsed and broke up into smaller pieces. Top of page Nebula A huge cloud of dust and gas in intergalactic space, from which new stars are created. The gas in nebulae is mostly hydrogen. There are many types of Nebula:   • STELLAR NURSERY A stellar nursery is a nebula in which star formation is occurring (stars are formed from gas). These nebulae are frequently illuminated by ultraviolet light which is emitted from the newborn stars. One example of a stellar nursery is the Eagle Nebula. • PLANETARY NEBULA Planetary nebulae has nothing to do with planets. A planetary nebula is a nebula formed from by a shell of gas which was ejected from an extremely hot star like a red giant or super giant. As the giant star explodes, the core of the star is exposed. Their often round appearance led to the name. See the Hubble image of Planetary Nebula Ngc 6826. • EMISSION NEBULA An emission nebula is a nebula that glows as it emits light energy. Reddish light is produced when electrons and protons combine, forming hydrogen atoms. Emission nebulae are formed when energetic ultraviolet light from a very hot star excites a cloud of hydrogen gas; the UV radiation ionizes the hydrogen (it strips electrons from the hydrogen atoms). The free electrons combine with protons, forming hydrogen and red light. • REFLECTION NEBULA A reflection nebula is a nebula that glows as the dust in it reflects the light of nearby stars. These nebulae are frequently bluish in color because blue light is more efficiently reflected than red light. • RING NEBULA A ring nebula is a huge cloud of dust and gas that is shaped like a ring. • SOLAR NEBULA A cloud of gas and dust that collapses and forms a solar system. Top of page Neutron Star Collapsed star composed mainly of neutrons. Pulsars are young, fast-spinning neutron stars. Nova A white dwarf star in a binary system that brightens suddenly by several magnitudes as gas pulled away from its companion star explodes in a thermonuclear reaction Pulsar Collapsed star composed mainly of neutrons. Pulsars are young, fast-spinning neutron stars. Top of page Quasar The brilliant core of a distant young active galaxy with outer regions that are often too faint to be visible. They have the brightest type of active galactic nucleus, believed to be powered by a super massive black hole. The word “quasar” is derived from quasi-stellar radio source, because this type of object was first identified as a kind of radio source. Quasars also are called quasi-stellar objects (QSOs). Thousands of quasars have been observed, all at extreme distances from our galaxy. Red Giant A red giant is a relatively old star whose diameter has swollen enormously — expanding to a size of up to a hundred times the diameter of our sun. It's temperature has also cooled appreciably and it is near the end of its life cycle. Our Sun will become a red giant star in about 5 billion years. Solar Mass A unit of mass equivalent to the mass of the Sun. Supernova (plural: Supernovae) (a) The death explosion of a massive star, resulting in a sharp increase in brightness followed by a gradual fading. At peak light output, these type of supernova explosions (called Type II supernovae) can outshine a galaxy. The outer layers of the exploding star are blasted out in a radioactive cloud. This expanding cloud, visible long after the initial explosion fades from view, forms a supernova remnant (see next definition below). (b) The explosion of a white dwarf which has accumulated enough material from a companion star to achieve a mass equal to the Chandrasekhar Limit. These types of supernovae (called Type Ia) have approximate the same intrinsic brightness, and can be used to determine distances. Top of page Supernova Remnant The gaseous debris, rich in heavy elements, thrown off by a supernova White Dwarf A collapsed core of a normal star such as the sun after it has lost its outer layers Wormhole An energetic structure with two openings in different parts of the multiverse connected by a tunnel that allows two-way traffic ... in which it is speculated that they may be safe shortcuts through space. Top of page TM & © 2005 Teka Luttrell. All rights reserved.