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Astrophysics glossary

Astrophysics glossary



Process whereby small particles of matter accumulate and create larger bodies under the influence of their mutual gravitational attraction or as a result of chance collisions.

Active galactic nucleus (AGN)

Central region of a galaxy in which considerable energy is generated by processes other than those present in normal stars. The energy generated by the nucleus may outshine all the other stars in the galaxy. Most astronomers believe that at the centre of an AGN lies a supermassive black hole.


Pair of stars bound together by mutual gravitation, orbiting their common centre of mass.
For black hole and neutron star binary see X-ray binary.

Black hole

An object with so much mass concentrated in it, and therefore such a strong gravitational pull that nothing, not even light can escape from it. One way in which black holes are believed to form is when massive stars collapse at the end of their lives.


The act of checking and therefore adjusting the accuracy of a measuring instrument, by comparison with a standard.

Crab nebula

A supernova remnant located in the constellation of Taurus. It was produced by a supernova explosion visible from Earth in 1054 AD. A pulsar at the centre of the nebula marks the neutron star corpse of the exploded massive star.


A unit of energy equal to the amount of kinetic energy an electron gains after being accelerated through an electric potential of 1 volt in a vacuum. The electron volt is about 1.60219 × 10-19 joules. keV = 103 eV.


A measure of the amount of energy or number of particles flowing through a given area of surface in a given time. Can be measured in counts made by a detector per second or the number of photons received by a detector per square centimetre per second.

Gamma ray

The most powerful form of electromagnetic radiation. A typical gamma ray is a photon with an energy greater than 100 keV.

Gamma-ray burst (GRB)

A burst of gamma rays from space. GRBs are registered about twice a day by satellites in orbit. The bursts may last from as little as a hundredth of a second up to 90 minutes. GRBs are extremely far away and must be caused by tremendous explosions. They probably are hypernovae - exceptionally violent supernovae, or mergers of neutron stars or black holes.


Hertzsprung-Russell (HR) diagram

A diagram in which the luminosities of stars are plotted against their colours or spectral types. In the conventional way in which this diagram is plotted, luminosity increases logarithmically up the vertical axis and temperature increases from right to left along the horizontal axis. Stars do not occupy all regions of the H-R diagram but form various sequences, the most important being the main sequence, the giant branch and the horizontal branch. For more information see Teacher Notes 2: Stellar Radiation and Stellar Types.


ESA's INTErnational Gamma-Ray Astrophysics Laboratory detects some of the most energetic radiation that comes from space. It is the most sensitive gamma-ray observatory ever launched. INTEGRAL is an ESA mission in cooperation with Russia and the United States.

Main sequence

The main sequence of stars on the Hertzsprung-Russell diagram, containing about 90% of all known stars. A star spends most of its life on the main sequence. The position of the star on the main sequence and the time it spends there depends mainly on the mass of the star. More massive stars are found higher on the main sequence, they also have shorter lifetimes.


An object of stellar mass that displays in miniature some of the properties of quasars. A microquasar consists of a binary system. The closest known microquasar and black hole to Earth is V4641 Sagittarii, which lies only about 1,500 light-years away.

Neutron star

An extremely dense star comprised mainly of neutrons; the endpoint of the life of a massive star which has exploded as a supernova. Under huge gravitational forces electrons have been compressed into protons and produced neutrons. A typical neutron star has about 3 times the mass of the Sun but a radius of only 10 kilometres. Fast spinning neutron stars can be observed as pulsars. If the Sun were to become a neutron star it would have a diameter of only 20 km.


A star which suddenly increases in brightness by a factor of more than hundred. Novae are close binary stars of which one component is a white dwarf star. Material from the companion star is transferred onto the white dwarf and triggers explosive nuclear reactions, resulting in the increased brightness.



The time interval between two consecutive and similar phases of a regularly occurring event. For example, the period of rotation of the Earth is the time taken to complete one revolution; the period of a variable star is the time between two successive maxima or minima on its light-curve.


A stellar source, such as a rotating single star or pair of stars, emitting electromagnetic radiation which is characterised by rapid frequency and regularity.


A Quasi-stellar extragalactic object; the highly energetic core of a remote active galaxy. Quasars are the most luminous objects in the Universe and capable of emitting over a trillion times as much energy as the Sun from an area that is just a little larger than the Solar System.

Stellar evolution

See Teacher Notes 5: Stellar Process and Evolution.


Explosion of a massive star at the end of its life. Supernova explosions are so luminous that they can outshine a galaxy. There are two types of supernovae. A supernova type I is most likely a white dwarf star in a binary system which accretes material that builds up until a nuclear explosion disrupts the star. A supernova type II is a massive star which has used up all its nuclear fuel. The star then collapses and the impact of all the material produces a shock wave which blasts the outer layers of the star out.

White dwarf

A very dense star with a mass below 1.4 solar masses that is no longer burning nuclear fuel. The Sun will one day evolve into a white dwarf with a diameter of 10 000 km.

XMM-Newton (X-ray Multi-Mirror) mission

ESA's X-ray space observatory mission, with its X-ray Multi-Module design using three telescopes each with 58 nested X-ray mirrors. Named also in honour of Sir Isaac Newton. See


Electromagnetic radiation with wavelengths between those of ultraviolet and gamma rays, approximately 0.01-10 nm (1 nm = 10-9 m). At these short wavelengths, it is more common to talk in terms of photon energies. These energies range from 0.1-100 keV.

X-ray (hard)

Higher-energy part of the X-ray spectrum ranging from approximately 5 keV to 100 keV.

X-ray (soft)

Band of low energy X-rays, between 0.1 keV and approximately 5 keV.

X-ray binary

An X-ray emitting binary star system consists of a normal star and a compact object, such as a neutron star or a black hole. Material is stripped from the normal star by the gravitational pull of the compact object. This material is accelerated to high velocities as it spirals towards the neutron star or black hole (forming an accretion disk) and heats due to friction, to temperatures (exceeding a million degrees) hot enough for X-rays to be emitted.
In the case of a high-mass X-ray binary system a neutron star orbits around a massive star. such as a giant or super-giant.
Hence terms; Black hole binary, Neutron star X-ray binary, X-ray binary pulsar and high-mass X-ray binary.

Last Update: 1 September 2019
22-Apr-2024 13:06 UT

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