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In the night sky: Orion
In the night sky: Orion

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Orion Glossary

Browse the glossary using this index

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accretion disc

A flow of matter that is largely confined to a plane and that is spiralling in towards a central object.

angular size

The angle between two lines drawn from an observer’s eye to opposite sides of the object being observed.


A body made of mainly rocky materials (including metals) in orbit around the Sun, that is typically much less massive than a planet. Most asteroids are found in the asteroid belt between Mars and Jupiter. Asteroids (and comets) are often called minor planets. The largest known asteroid (Ceres) is classified as a dwarf planet.

asteroid belt

A region between Mars and Jupiter in which most asteroids reside in stable orbits. Also called main belt.


The study of the movements and relative positions of celestial bodies claimed as having an influence on human affairs and the natural  world. Astrology is not a science.


Astronomy is the scientific study of stars, space, and the physical universe as a whole.


The building block of chemistry comprising a tiny, dense, positively charged atomic nucleus (which can contain both protons and neutrons) with circulating negatively charged electrons equal in number to the number of protons in the nucleus.


Big Bang

The event which is believed to mark the origin of time and space. Consequences of the Big Bang include the fact that space is expanding, that the temperature of the Universe is falling and that elements or isotopes such as helium, lithium and deuterium have certain abundances in the Universe.

Big Crunch

An hypothesis that, if there were enough matter in the Universe, the expansion of the Universe would eventually become a contraction, leading to a big crunch. This hypothesis is no longer believed to be valid.

black hole

A region of the Universe in which gravity is so strong that not even light can escape.


The shift in the spectrum of a source of electromagnetic radiation that’s approaching the observer. Equal to the shift in wavelength of a particular feature in the spectrum divided by the original (or rest) wavelength of the same feature. A blueshift corresponds to a shortening of the emitted wavelength.

brown dwarf

Brown dwarfs are cool, faint objects, with a temperature too low for hydrogen burning to commence. They are inherently difficult to observe, but a number have been discovered.


A structural component of the Milky Way and other spiral galaxies, consisting of a thick, dense concentration of stars around the centre of the disc. Bulges are a common feature of spiral and lenticular galaxies. In the case of the Milky Way (and many other galaxies), the bulge is elongated to form a bar.


carbon burning

The process by which carbon nuclei undergo nuclear fusion to form a neon nucleus (and a new helium nculeus) in massive stars.


The lower part of the Sun’s atmosphere situated immediately above the photosphere.


A relatively small (typically a few kilometres across) body made of icy materials and rocky materials. Comets are found mainly in the Oort cloud, and also in the Kuiper Belt, and some enter the inner Solar System.


The central very hot region of a star (such as the Sun) where nuclear reactions occur during the Main Sequence lifetime.


The upper part of the Sun’s atmosphere. It is very hot, very tenuous and very extensive.

cosmological constant

A constant (often written as a capital lambda or Λ) introduced into the equations of general relativity by Albert Einstein. It was originally conceived as a property of space and time that counteracts the predicted expansion or contraction of the Universe. It is now sometimes thought of as a content of the Universe, rather than a property of space itself. See also dark energy.


dark energy

Energy of currently unknown origin that has become the dominant influence on cosmic expansion and is currently causing that expansion to accelerate. Dark energy is thought to account for about 70% of all the energy in the Universe. It is not thought to be associated with dark matter. It is related to Einstein’s cosmological constant: this constant can be thought of as an in-built property of space and time to expand, or as a mysterious substance within space and time that causes expansion. If taking this latter view, the cosmological constant can be thought of as a particular (constant) type of dark energy.

dark matter

A currently unknown form of matter that neither absorbs nor emits any detectable radiation, but can be detected and studied through its gravitational influence on directly observable (‘luminous’) matter.


Also known as ‘heavy hydrogen’. An isotope of hydrogen, the nucleus of which contains one proton and one neutron. Compare with tritium.


(of a planet). A process in which the constituents of large Solar System bodies are sorted into layers of distinct composition, with the most dense materials concentrated towards the centre, usually as a result of heating.


A structural component of the Milky Way, consisting of a disc approximately 100 000 light years in diameter and a few thousand light years thick that contains approximately 1011 stars, together with gas and dust. Similar discs are present in other spiral and lenticular galaxies.

Doppler effect

The process by which the wavelength of a wave is altered when the source of the wave is moving with respect to the observer. Motion away from an observer causes the wavelength to be perceived as longer than that with which it was emitted; motion towards an observer causes the wavelength to be perceived as shorter than that with which it was emitted.


In astrophysical terms dust referes to carbon-rich molecules and small mineral rich grains composed of elements such as oxygen, silicon, iron and magnesium.

dwarf planet

A large rocky or icy body in orbit around the Sun that does not fulfil the criteria of being classed as a planet (usually because it has not swept out a clear path on its orbit).



The plane of the Earth's orbit about the Sun. Also the path of the Sun in the sky (on the celestial sphere) during the course of a year.

electromagnetic radiation

A type of radiation that includes visible light and travels through empty space at the speed of light. All forms of electromagnetic radiation consist of wave-like patterns of electric and magnetic disturbances but interact with matter (i.e. are emitted or absorbed) as a stream of particles, called photons.

electromagnetic spectrum

The entire range of electromagnetic radiation from radio waves through microwaves, infrared radiation, light, ultraviolet radiation, and X-rays to gamma rays.

electron degeneracy pressure

An effect of quantum physics that provides the support against gravity in a white dwarf star.

elliptical galaxy

A galaxy that appears as an elliptical distribution of stars (typically old stars).

emission line

A spectral line produced when atoms of a particular type emit photons with a specific energy.

emission spectrum

A spectrum containing emission lines. A pattern of narrow bright lines at specific wavelengths, generally superimposed on a dark background. The wavelengths of the lines are characteristic of the composition of the excited gas that produces them.

event horizon

The boundary around a black hole within which the escape velocity exceeds that of light. Note that this does not consist of a solid, physical surface.


A planet orbiting a star other than the Sun. Shortened version of extrasolar planet.


The power to which a number or quantity is raised. For example, 103 is read as 'ten to the power 3' and means 10 x 10 x 10, where the exponent (or index) is '3'.


Of or pertaining to the Universe outside our own galaxy.

extrasolar planet

A planet orbiting a star other than the Sun. Also abbreviated to exoplanet.



A highly energetic form of solar activity, typically lasting a few hours, caused by the sudden release of magnetic energy in the corona.


(of a wave) The number of complete cycles of a wave that pass a certain fixed point in a unit of time. Conventionally measured in the SI unit of hertz, Hz (or, equivalently, s−1). It is equal to one over the period of the wave, and is related to the wavelength λ and the speed of a wave, ν, by ν = fλ.



A vast assembly of dark matter and luminous matter, typically tens of thousands of light years in diameter and containing billions of stars, held together by the mutual gravitational attraction of its constituents. Our own galaxy, known as the Milky Way or simply the Galaxy, is a typical spiral galaxy with a mass about a hundred billion times that of the Sun.

galaxy cluster

A gathering of galaxies in a region of space typically 12 to 15 million light years across. Some clusters have many members ('rich' clusters) but sparse clusters, with fewer than 50 members, are termed 'groups'.

gas giant

A planet considerably larger than the Earth, composed largely of hydrogen and helium. Also called giant planet (cf. terrestrial planet).

Giant Molecular Cloud

A dense cloud of cold dust and gas within which stars form, extending for many billions of kilometres and typically containing more than a million solar masses of material.

giant planet

A planet considerably larger than the Earth, composed largely of hydrogen and helium. Also called gas giant (cf. terrestrial planet).

globular cluster

Globular clusters are compact, nearly spherical, groups of many thousands or millions of stars found within the galactic halo. The stars in globular clusters are among the oldest in the Galaxy, and so have very low amounts of elements other than hydrogen and helium.


A gathering of galaxies with fewer than 50 members, bound together by gravity (cf galaxy cluster).


habitable zone

The range of distances from a star within which an Earth-like planet would be habitable.

Harvard Spectral Classification

A commonly used classification scheme for stars by which the temperature of a star is denoted by the assignment of a letter in the sequence OBAFGKM, running from hottest (O) to coolest (M)

helium burning

The process by which stars fuse three helium nuclei into one carbon nucleus via the triple alpha process.

Hertzsprung-Russell (HR) diagram

A diagnostic diagram whereby the evolutionary state of a star may be discerned via a combination of stellar temperature and luminosity. Named after its originators Ejnar Hertzsprung and Henry Norris Russell.

hot Jupiter

A giant planet, with a mass close to or greater than that of Jupiter, and which orbits very close to its parent star.

Hubble deep field

A project with the Hubble Space Telescope to obtain a series of very deep exposures to search for distant galaxies at high redshift.

Hubble’s law

The observation that, for distant galaxies, the greater a galaxy’s distance, the greater the redshift of the radiation received from that galaxy.

hydrogen burning

The process by which stars on the main sequence release energy via the conversion of hydrogen into helium in their cores.

hydrogen shell burning

The process by which, in post-Main Sequence stars, hydrogen is converted into helium in a thin shell exterior to the core.

hydrostatic equilibrium

In a star, the state at which physical forces directed inwards are exactly balanced by those directed outwards.


icy materials

A term used in Solar System studies for materials that comprise water and other substances that are solid only at relatively low temperatures, although they can be liquid on the surfaces or in the interiors of planets, or gaseous in the atmospheres of planets.


The angle i between the orbital plane of an astronomical body and a reference plane. For a solar system body the reference plane is the ecliptic. (The inclination of the orbit of a binary star is the angle between the orbital plane and the plane of the sky, so that a face-on binary has i = 0 and an edge-on (eclipsing) system has i close to 90°).


The name given to a theory that describes the behaviour of the Universe at times between about 10-35 s and 10-32 s after the big bang. During this interval it is proposed that distances within the Universe increased by an extraordinarily huge factor. Inflation predicts that we live in a spatially flat universe.

interstellar medium

The very thin gas and tiny specks of dust that lie between the stars.

iron group

A subset of heavy metallic elements, such as iron, chromium and nickel that are the endpoints of nuclear fusion reactions in massive stars.


Atoms with the same number of protons in their nuclei but different numbers of neutrons are called isotopes. Because they have the same number of protons, they have the same atomic number and are atoms of the same chemical element. But because of the different number of neutrons, they differ in mass number.


Kuiper Belt

The region of the Solar System, beyond the orbit of Neptune, containing many icy-rocky bodies, including dwarf planets (such as Pluto) with relatively low inclination orbits. The Kuiper Belt is sometimes called the Edgeworth-Kuiper Belt and is a sub-set of trans-Neptunian objects.


Local Group

The grouping of galaxies in the neighbourhood of our galaxy. It contains about 40 known galaxies including the Milky Way and all the other galaxies within about three or four million light years.

local supercluster

The supercluster of galaxies centred on the rich Virgo cluster of galaxies and including the Local Group as an outlying constituent.


The amount of power emitted by a luminous object such as a star or galaxy, in the form of light or other electromagnetic radiation. It is related to the observed brightness of an object by the relationship: luminosity is proportional to brightness multiplied by distance squared. Luminosity may be measured in the unit of watts.


magnitude scale

An historical, logarithmic scale devised to represent the observed brightness of astronomical objects such as stars.

main sequence

The portion of the lifetime of a star during which it produces energy via the fusion of hydrogen to helium in its core. A star will remain on the Main Seqeunce for the majority of its total lifetime.


Extra-terrestrial object that is observed by the electromagnetic radiation it emits as it enters the Earth's atmosphere. 'Shooting stars' are visible meteors.


An extra-terrestrial rock (generally a fragment of an asteroid) that has fallen to the Earth’s surface.


near Earth asteroid

An asteroid that has an orbit that brings it close to the orbit of the Earth (defined as any asteroid that comes within 1.3 astronomical units from the Sun). Abbreviated to NEA.

neon burning

The process by which massive stars fuse a neon and helium nucleus together to produce a magnesium nucleus, releaseing energy in the process.

neutron star

When a high-mass star runs out of nuclear fuel, it will undergo a type II supernova explosion and its core will collapse to form a neutron star. A neutron star has a mass similar to that of the Sun, but a radius of only about 10 km. A neutron star is supported against gravity by an effect of quantum physics known as neutron degeneracy pressure. See also white dwarf, black hole.

nuclear fusion

The fusing together of two atomic nuclei to make a larger nucleus. There is a decrease in the total mass of the nuclei in a fusion reaction, so this process results in the release of large amounts of energy, according to Einstein’s equation E = mc2.

nuclear reactions

Processes in which two or more nuclei are involved, resulting the creation of different nuclei from those originally present. See also nuclear fusion.


The process by which the nuclei of elements (other than hydrogen) are formed. There are believed to be three sites (or epochs) where (or when) nucleosynthesis occurs (or has occurred). Light nuclei, such as deuterium, helium and lithium, were formed in the early Universe when the Universe was between about 100 s and 1000 s old. Nuclear fusion inside the cores of stars is responsible for the formation of more helium nuclei, and also for the formation of other nuclei up to those with a mass around that of iron. Supernovae explosions are responsible for the formation of more massive nuclei.


Oort cloud

A spherical cloud of icy bodies surrounding the Solar System and extending up to one-third of the way to the nearest star. It is sometimes called the Öpik-Oort cloud.

oxygen burning

The process by which, towards the end of their lives, massive stars fuse oxygen nuclei together to form silicon.



A particle of light or other electromagnetic radiation. Monochromatic light consists of photons that each have exactly the same amount of energy, called a quantum of energy. Photons have no mass or electric charge.


The luminous 'surface' of the Sun (or of other stars). The Sun’s photosphere is about 500 km thick.


Large rocky or gaseous bodies in orbit around a star. According to the International Astronomical Union, a planet must meet the following criteria: it is large enough to become spherical under the influence of its own gravity; it orbits a star; it has swept out a clear path on its orbit around the Sun; it is not a satellite of another body.

planetary embryo

A massive body thought to have formed from the accretion of planetesimals, and which themselves combined to form planets, during the formation of the Solar System. A few planetary embryos remain in the form of the largest asteroids and trans-Neptuinan objects.

planetary nebula

The expanding dust and gas thrown out by a star of similar mass to the Sun as it reaches the end of its life.


Small bodies (a few klm in size), that formed from aggregation of dust grains in the early solar nebula and are large enough to attract each other by gravity.


Bodies formed in the early solar system from the accretion of planetary embryos and planetesimals, that are large enough to have undergone differentiation.


A collapsing fragment of a molecular cloud that eventually becomes a star.


A rapidly spinning neutron star. As the neutron star spins on its axis, beams of radio emission sweep around the sky like the light from a lighthouse. On Earth, these beams are detected as pulses of radio emission.


radial velocity

The component of an astronomical object's relative velocity in the line of sight of an observer, i.e. in a radial direction towards or away from the observer.

red giant

A large, cool star that is going through subsidiary stages of nuclear reactions, having exhausted its hydrogen fuel supply.

red giant branch

A region on the Hertzsprung-Russell diagram in which red giants (luminous but cool stars in the later stages of their evolution) are found.


A shift of a spectral line to redder (longer) wavelengths. There are two types of redshift discussed in this course: (1) Doppler shift – resulting from motion of the emitting object away from the observer. This is the case for some nearby galaxies, but this is not the underlying physical cause of most cosmological redshifting. (2) Cosmological redshift – resulting from expansion of the Universe (see the Hubble constant, Big Bang) which stretches the wavelength of the light, making the light redder.

rocky materials

Materials, such as minerals and metals, that are present in Solar System bodies that require high temperatures in order to melt (cf. icy materials).



An object in orbit around a larger one, e.g. a ‘moon’, or an artificial space probe orbiting a planet.

silicon burning

The process of silicon burning produces elements such as sulfur, argon and calcium via the fusion of silicon nuclei at the end of the life of a massive star.

size of the observable Universe

That part of the wider Universe that has been able to send us light signals since the beginning of the Universe. The Universe as a whole is larger, perhaps infinitely larger, than the observable part of it.

solar nebula

The hypothetical cloud of gas and dust within which the Sun and other constituents of the Solar System formed.

Solar System

The system comprising the Sun and all the bodies (planets and their satellites, dwarf planets, comets and asteroids) that orbit around it.


A display (such as a graph or a photograph) of the distribution of light or other types of radiation versus the wavelength (or frequency or energy) of the radiation. It indicates the intensity of light at each different wavelength. A spectrum may be a continuous spectrum or may show emission lines (emission spectrum) or absorption lines (absorption spectrum).

speed of light

The speed at which light travels. It is equal to approximately 300 million metres per second (3 × 108 m s−1). Often denoted by the letter c as in Einstein's famous equation, E = mc2.


A luminous gaseous body that is gravitationally bound and that is capable, or was capable in the past, of sustaining itself against gravitational collapse by thermonuclear reactions. Until the very late stages of their evolution, stars are composed mostly of hydrogen and helium, which are the most abundant elements in the Universe.

star clusters

Star clusters are physically compact groupings of tens to millions of stars which formed simultaneously in the same region of space.


A star, several times more massive than the Sun, after it has exhausted its hydrogen nuclear fuel supply.

supermassive black hole

A term used to describe black holes, implying that they have very much more mass than would be expected from a black hole that originated as the remnant of a single star.


A dramatic stellar explosion, produced when a star several times the mass of the Sun has exhausted its nuclear fuel.


terrestrial planet

A planet similar in size to the Earth, composed of rocky materials (cf. giant planet).

thermal pulses

Thermal pulses are brief bursts of enhanced energy production in solar mass stars due to the instabilities inherent in helium shell burning.

triple-alpha process

A sequence of nuclear fusion reactions that occurs in the cores of high-mass stars. In this sequence, three helium nuclei are converted into a carbon nucleus.



A periodic (regularly repeating) disturbance that transports energy from one place to another, characterised by its wavelength, frequency (or period) and amplitude.


The distance between one part of the wave profile, (e.g. the peak of the wave) at a particular instant in time, and the next identical part of the wave profile at the same instant in time. Wavelength λ is related to frequency f and speed v of a wave by v = fλ.

white dwarf

A small hot star, left behind when a red giant throws off its outer layers as a planetary nebula. A white dwarf has a mass similar to that of the Sun, but a radius similar to that of the Earth. A white dwarf is supported against gravity by an effect of quantum physics known as electron degeneracy pressure.

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