Glossary Of Terms Flashcards
Accretion disc
A ring-like structure formed by material spiralling into a gravitational source, for example, a black hole.
Active galaxy
A galaxy that emits more energy than can be accounted for by its normal components: stars, dust and gas.
Active Galactic Nucleus (AGN)
The core of an active galaxy. The energy generated in an AGN may outshine all the other stars in galaxy. They are believed to be powered by the frictional heating of an accretion disc as it spirals into a supermassive black hole.
Aphelion
Apoapsis in a solar orbit.
Apoapsis
The farthest point in an orbit from the body being orbited.
Asteroid
A small (up to 1,000 kilometres in diameter) rocky body orbiting the Sun. The vast majority found are found in the Asteroid Belt between Mars and Jupiter.
Big Bang
The moment 13.7 billion years ago when the universe exploded into existence and began expanding.
Binary system
A pair of objects (two stars for example) bound together by mutual gravitation.
Black dwarf
The cold ashes of a Sun-sized star that had evolved into a white dwarf and subsequently cooled to such a degree that it no longer radiates heat. Black dwarfs are theoretical objects as the time taken for a white dwarf to cool is longer than the current age of the universe.
Black hole
A concentration of mass with a gravitational field so strong that - within a certain radius - nothing, not even light, can escape. At the end of their lives, particularly massive stars collapse under their own weight to form black holes.
Bok globule
A dark, dense cloud of dust and gas in which star formation is taking place.
Brown dwarf
Stunted stars with insufficient mass to initiate and sustain hydrogen nuclear fusion.
Collapsar
A generic name for the three types of ‘collapsed star’ that may form at the end of a star’s life: a white dwarf, a neutron star or a black hole.
Comet
A ‘dirty snowball’ of dust and ice left over from the formation of the solar system. Comets are classified according to their orbital period. ‘Short period’ comets complete their orbit in under 200 years, ‘long period’ comets can take millions of years. See also Kuiper Belt and Oort Cloud.
Cosmic background radiation
The ‘afterglow of creation’ or the remnant radiation from the Big Bang. It has now cooled to -270C, only 2 degrees above absolute zero.
Dark energy
A hypothetical form of energy that permeates all of space creating an anti-gravity force that accounts for the observed acceleration of he universe’s expansion. Not to be confused with dark matter, whose gravitational effect work in the normal attractive direction.
Dark matter
Matter that cannot be detected by its emitted radiation, although it’s presence can be inferred by its gravitation interactions with visible matter. Most of the universe’s mass exists in the form.
Dark nebula
A cloud of dust and gas dense è nought o block the visible light from the objects they obscure. Also known as an absorption nebula.
Dwarf galaxy
A small, faint galaxy,neither irregular or elliptical in structure.
Electromagnetic radiation
The most familiar type of electromagnetic radiation is light, but visible light is but a brief archipelago in an ocean of wavelengths. The full electromagnetic spectrum runs from trembly energetic gamma rays, to low power waves, via X-rays, ultraviolet, visible and infrared light and microwaves. Electromagnetic radiation can also be described as a stream of particles known as photons.
Elliptical galaxy
A galaxy that appears spherical or American-football shaped with no specific, internal structure.
Emission nebula
A cloud of gas that shines with its own light. Usually such nebulae absorb ultraviolet radiation and re-emit it at visible wavelengths. Examples of emission nebular include planetary nebulae and supernova remnants.
Event horizon
The point of no return surrounding a black hole. Anything crossing this boundary is effectively lost from the universe.
Absolute zero
The lowest temperature attainable in the universe. Equivalent to -273C. There is no upper limit on temperature.
Fluorescence
An optical phenomenon whereby a molecule absorbs an invisible high energy ultraviolet photon and re-emits it as a visible, lower energy photon.
Galactic cluster
A collection of dozens to thousands of galaxies bound together by gravity.
Galactic halo
A spherical region around a spiral galaxy that contains dim stars and globular clusters. The radius of the halo surrounding the a Milky Way extends some 50 000 light years from the galactic centre.
Galaxy
A gravitationally bound system consisting primarily of dust, gas and stars. Galaxies range in scale from hundreds to hundreds of thousands of light years and are classified according to their appearance: Spiral, Elliptical, Lenticular, Ring or Irregular.
Gamma ray
The most energetic form of electromagnetic radiation.
Gamma ray burst
A brief but immensely powerful burst (measured in minutes at most) of gamma rays from space. Believed to be triggered by supernovae or the collision of neutron stars or black holes.
Globular cluster
I distinct, densely packed sphere of stars that can approach a population density one thousand times greater than our stellar neighbourhood. Globular clusters were thought to be relics of universe’s first stroller generations, but recent observations of starburst galaxies have revealed globular clusters in the making.
Gravitational lens
A massive object that magnifies or distorts the light of objects lying behind it.
Gravity
A physical force that operas you exert a mutual attraction between all masses, proportional to the mass and distance of the objects. Einstein’s Theory of General Relativity explains gravity as the curvature of spacetime.
Heliopause
The boundary between our solar system and interstellar space - where the pressure of the solar wind equalises with the interstellar medium. Believed to blue between 14 to 20 light hours away from Earth.
Herbig-Haro Object
A distinctive class of emission nebula crated by jets of material ejected by newborn stars colliding with the interstellar medium.
Hypergiant
The most massive and luminous stars. The largest examples are over 100 times more massive than our sun and millions of times more luminous. They burn brightly but briefly - surviving no more than a few million years.
Hypernova
An unusually powerful supernova. Associated with the demise of particularly massive stars.
Index Catalogue (IC)
A two-part supplement adding 5 386 astronomical objects to the New General Catalogue (NGC), published in 1895 and 1908.
Interstellar medium
The extremely rarefied ‘atmosphere’ of space, typically consisting of 90 percent hydrogen, 9 percent helium and 1 percent dust.
Infrared
A section of the electromagnetic spectrum invisible to the human eyes, but sensed as heat or thermal radiation.
Ionization
The process that produces ions - atoms that are electrically charged by the capture of loss of everyone’s. Atoms can be still red of their electrons by high energy radiation (from stars, for example). Material that has been completely ionised is known as plasma.