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This glossary covers words that you will come across on this website.

Brown Dwarf

These are sub-stellar objects with a lower mass than necessary to maintain hydrogen-burning nuclear fusion reactions in thier cores. However, they have fully convective surfaces and interiors.


The bulge of a spiral galaxy is the central concentration of stars.

Cepheid Variable

A Cepheid is a member of a particular class of variable stars. It is usually a Yellow Giant or Supergiant that is pulasting. The relationship between a Cepheid variable's luminosity and variability period is quite precise. Cepheids are viable as the standard candle, meaning that they are the foundation of the Extragalactic Distance Scale.


1. The splitting of light into its separate colours by means of a prism or diffraction grating.

2. A phenomenom where by electromagnetic waves of different frequencies travel at different speeds through certain medium such as ionized gases (plasmas). ( note: all electromagnetic waves should travel at the speed of light [3*10^8]) Most interstellar matter is ionized hydrogen, and is therefore dispersive.

Doppler Effect

A change in the observed frequency of a wave, as of sound or light, occurring when the source and observer are in motion relative to each other, with the frequency increasing when the source and observer approach each other and decreasing when they move apart. The motion of the source causes a real shift in frequency of the wave, while the motion of the observer produces only an apparent shift in frequency. Also called Doppler shift.

Electromagnetic Spectrum

The complete spectrum of Electromagnetic radiation for longest wavelengths to shortest wavelengths.

Radio waves (10^5) - (10^-3)
Infrared waves (10^-3) - (10^-6)
Visible Light (4 - 7 [* 10^-7])
Ultraviolet waves (10^-7) - (10^-9)
X-rays (10^-9) - (10^-11)
Gamma Rays (10^-11) - (10^-14)

Elliptical Galaxy

This is a galaxy with a smooth circular or elliptical appearance. It has no spiral arms, and little or no interstellar gas or dust. Its symbol is given as E.

Synonyms: elliptical, ellipticals

A Galaxy is a massive system of stars, gas, dust and dark matter all bound together by gravity. The name is from the Greek root galaxias [γαλαξίας], meaning "milky," a reference to the Milky Way galaxy.

Galaxy Names

The letters indicate the catalogue listing of the galaxy. Galaxies are listed in several different catalogues. The most common catalogue is ‘NGC’, which stands for New General Catalogue. Other catalogues include M (Messier), ESO (European Southern Observatory), IRAS (Infrared Astronomical Satellite), MrK (Markarian), and UGC (Uppsala General Catalogue).

Hubble Constant

The constant which determines the relationship between the distance to a galaxy and its velocity of recession due to the expansion of the Universe. After many years in which the Hubble constant was only know to be somewhere between 50 and 100 Kms-1Mpc-1, (kilometres per second per megaparsec). Recent research favours a value towards the lower ed of this range.

Hubble’s Law

The law that governs the expansion fo the Universe. The recession velocity of a galxay is proportional to its distance away from the observer. The law was presented by E.P. Hubble in 1929.


The process of converting an atom or molecule into an ion by adding or removing charged particles.

Lenticular galaxy

This type of galaxy has a disk and a bulge, similiar to spiral galaxies, but with no obvious spiral arms. In Hubble's classification system they are identified as S0 galaxies.

Synonyms: Lenticular, lenticulars

The amount of energy a body radiates per unit time.


A measure of the brigthness of a star. The lower the value, the brighter the object.


The morphology of a galaxy is a description of how it appears visually (i.e. how it looks).


A unit of stellar distance. A parsec is the distance at which one astronomical unit subtends an angle of one second of arc. (one parsec = 3.2616 light years = 206265 astronomical units = 30.857[*10^12]km)


Energy in the form of waves or moving sub-atomic particles emitted by an atom or other body as it changes between energy states. Radiation can be classified as ionizing or non-ionizing, depending on its effect on atomic matter.

Recessional velocity

A term used to describe the rate at which an object is moving away, typically from Earth. This term is enerally applied in reference to distant Galaxies.


When electromagnetic radiation emmitied or reflected by an object is shifted towards the less energetic (red) end of the spectrum. This is known as the Doppler Effect. Redshift is also defined as an increase in the wavelength of E-M radiation corresponding to a drop in frequency. A decrease in wavelength is called ' Blueshift '.


Conditions or values that vary over a continuum. (plural of spectrum)

Spectral line

When an electron makes a transition between two energy levels in an atom, emission or absorption lines arise in a spectrum. A drop to a lower level releases a photon of energy, and a jump to a higher level requires an input of energy.


The method of obtaining the spectra of clestial bodies, from which we can determine their composition and motions. Originally it was the study of the interaction between radiation and matter as a function of wavelength.


1. A range of electromagnetic energies arranged in order of wavelength or frequency.

2.The colours produced when visible light passes through a spectroscope.

Spiral galaxy

A spiral galaxy consists of a central bulge of older stars, and a disk of younger stars, arranged in spiral arms, spread out from the central region. In Hubble's classification this is given by the symbol, S.

Synonyms: spiral, spirals
Standard candle

Many methods of measuring interstellar distances rely on a Standard candle, which is an astronomical object that has a known luminosity.

Type Ia Supernova

A category of cataclysmic variable stars that results from the violent explosion of a white dwarf star.

White Dwarf

Also called a degenerate dwarf, a White Dwarf is a small star composed of electro-degenerate matter. White dwarfs are very dense and their luminosity comes from the emission of stored heat.