Monday 18 September 2017 - 22:50:55 Posted by  Bobby

Absolute zero:

The lowest temperature attainable in the universe. Equivalent to -273 C (-459 F). There is no upper limit on temperature.

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 the galaxy. They are believed to be powered by the frictional heating of an accretion disc as it spirals into a supermassive black hole.


Apoapsis in a solar orbit.


The farthest point in orbit from the body being orbited.


A small (up to 1,000 kilometres or 600 miles in diameter) rocky body orbiting the Sun. The vast majority 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 has 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.


A generic name for the three type of 'collapsed star' that may form at the end of a star's life: a white dwarf, a neutron star or a black hole.


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 'after of creation' or the remnant radiation from the Big Bang. It has now cooled to -270 C (-454 F), only 2.7 C above absolute zero.

Dark energy

A hypothetical form of energy that permeates all of space creating an anti-grvity force that accounts for the observed acceleration of the universe's expansion. Not to be confused with dark matter, whose gravitational effects 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 gravitational interactions with visible matter. Most of the universe's mass exists in this form.

Dark nebula

A cloud of dust and gas dense enough to block the visible light from the objects they obscure. Also known as an absorption nebula.

Dwarf galaxy

A small, faint galaxy, either 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 extremely 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 nebulae 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.


An optical phenomenon whereby a molecule absorbs an invisible high-energy ultraviolet photon and re-emits it as a visible, lower-energy photon.


See Nuclear fusion

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 Milky Way extends some fifty thousand light-years from the galactic centre.


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 and 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 a supernova or the collision of neutron stars or black holes.

Globular cluster

A distinct, densely packed sphere of stars that can approach a population density 1000 times greater than our stellar neighborhood. Globular clusters were thought to be relics of the universe's first stellar 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.


A physical force that appears to 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.


The boundary between our solar system and interstellar space - where the pressure of the solar wind equalizes with the interstellar medium. Believed to lie between 14 and 20 light hours from Earth.

Herbig-Haro Object

A distinctive class of emission nebula created by jets of material ejected by newborn stars colliding with the interstellar medium.


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 for no more than a few million years.


An unusually powerful supernova. Associated with the demise of particularly massive stars.

Index Catalogue (IC)

A two-part supplement adding 5386 astronomical objects to the New General Catalog, published in 1895 and 1908.

Interstellar medium

The extremely rarefied 'atmosphere' of space, typically consisting of 90% hydrogen, 9% helium and 1% dust.

A section of the electromagnetic spectrum invisible to human eyes, but sensed as heat or thermal radiation.


The process that produces ions - atoms that are electrically charged by the capture or loss of electrons. Atoms can be stripped of their electrons by high energy radiation (from stars, for example). Material that has been completely ionized is known as a plasma.

Irregular galaxy

Any galaxy that lacks the necessary structure to be classified as an elliptical, spiral or lenticular galaxy.

Kuiper Belt

A disc of icy bodies (KBO'S) stretching from Neptune's orbit out to at least one light day from the Sun. Its largest known resident is Pluto.

Lenticular galaxy

A galaxy shaped like a spiral galaxy with a central nucleus and a disc, but lacking spiral arms.


Electromagnetic radiation the human eye can detect. However, the term can also be applied to all electromagnetic radiation.

Light second

The distance covered by light in a vacuum in a second: 299,791 kilometres (186,282 miles)

Light minute

The distance covered by light in a vacuum in a minute: 17.9 million kilometres (11.1 million miles)

Light hour

The distance covered by light in a vacuum in an hour: 1 billion kilometres (620 million miles)

Light year

The distance covered by light in a vacuum in a year: 9.5 trillion kilometres (5.9 trillion miles)

Local Group

A largely gravitationally bound collection of approximately 40 galaxies spread over 10 million light-years, dominated by our own Milky Way and the Andromeda galaxy.


The amount of radiation emitted by a star or celestial object in a given time.


A species of neutron star with an extremely strong magnetic field.

Main sequence

The main sequence is best pictured on a Hertzsprung-Russell diagram, where a star's position corresponds to its luminosity and temperature. When plotted in this fashion, 90% of all known stars will fall into a well-defined curving diagonal line known as the main sequence. A star on the main sequence as a healthy star, converting hydrogen and helium by nuclear fusion. Below the main sequence one finds white dwarfs, above it short lived, unstable supergiants. Stars are not confined to the main sequence throughout their lives. For example, our Sun will slide off the main sequence, above and to the right of its current position, as it exhausts its supplies of hydrogen and swells to become a red giant. Finally as a white dwarf it will sink to the bottom of the graph.


A measure of the amount of matter in a body. With his famous equation E=MC squared, Einstein showed that mass is equivalent to energy. In astronomy there is a careful distinction between mass and weight - weight is the force acting on an object with mass in a gravitational field.

Messier Catalogue (M)

A list of about 100 nebulous-looking astronomical objects compiled by Charles Messier between 1758 and 1781.

Milky Way

Our galaxy. The name is derived from our perception of it as a misty band of stars that divides the night sky. The Milky Way is a large spiral galaxy spanning 100,000 light-years and containing over 200 billion stars. Our solar system lies about two-thirds of the way towards the edge of its disk.

Molecular cloud

An accumulation of dust and gas markedly more dense than the interstellar medium. Such clouds can span hundreds of light-years and are the exclusive sites of star formation.


The natural satellite of a planet. There are at least 140 moons in the solar system. More specifically, moon refers to the natural satellite of Earth.


A cloud of dust and gas in space. See molecular cloud, emission nebula, reflection nebula and Dark Nebula.

Neutron star

An extremely dense stellar remnant produced in a supernova where huge gravitational forces have compressed electrons and protons producing neutrons. See also pulsar and magnetar.

New General Catalogue (NGC)

A compilation of 7,800 astronomical objects published in 1888.


A star that suddenly increases in brightness by a factor of more than a hundred. Novae occur in binary systems where one component is a white dwarf. Material from the companion star is transferred onto the white dwarf triggering explosive nuclear reactions, resulting in the increased brightness.

Nuclear fusion

The process that powers stars. Two atomic nuclei are forced together, forming a single larger nucleus and releasing energy. Most stars convert hydrogen into helium, more massive, hotter stars can fuse heavier elements.


The trajectory of one celestial body around another.

Oort Cloud

A swarm of comets thought to surround the solar system between a light week and a light year from the Sun. Its existence has been deduced from studies of long-period comet orbits, which seem to have their aphelia in this zone.

Open star cluster

A group of young stellar siblings. Born of the same molecular cloud but only loosely bound together by gravity, such clusters are fated to disperse over a period of several hundred million years.

O-class star

A massive star at the hottest extreme of the stellar spectral classification, typically massing between 30 and 100 solar masses. With a surface temperature greater than 30,000 C° (50,000 degrees F°), their light appears bluish-white and they can also be described as blue giants or blue stars.


The point in an orbit nearest to the body being orbited.


Periapsis in a solar orbit.


One of the processes that shapes molecular clouds in areas of star formation. Caused by ultraviolet radiation energizing the cloud sufficiently to overcome the gravity that binds it together.


A particle of light, the quantum unit of the electromagnetic force.


An object that orbits a star - the name is derived from the Greek planates, or 'wanderers'. By tradition, the name is reserved for large bodies and there is some controversy over where the definition of a planet should end. If Pluto was discovered today, it is unlikely it would be classified as a planet. Instead it would be relegated to the league of minor planets, bodies that include asteroids, comets, Kuiper Belt objects and Oort Cloud Objects.

Planetary nebula

A luminous shell of debris surrounding a dying star. As red giant stutter to the end of their lives, they cast off their outer layers creating an expanding cloud of dust and gas. Eventually, this mass loss exposes the star's core, which, despite no longer supporting nuclear fusion, typically has a temperature of 100,000 °C (180,000 °F). The shell of debris lights up as it fluoresces in the blaze of ultraviolet radiation pouring from the core. Planetary nebulae are transitory phenomena lasting no more than 100,000 years - fading as their central white dwarf cool. The name planetary nebula is a historical artefact: through the telescopes of early astronomers they looked like planets.


The 'fourth state of matter', an electrically conductive mixture of electrons and ions. Many astronomical objects including stars, emission nebula and accretion discs consists of plasma.

Protoplanetary disc

The disk of dust surrounding a star out of which planets might form.

An embryonic star, not yet massive enough to initiate the nuclear fusion of hydrogen.


A rotating neutron star that emits a sweeping beam of high-energy electromagnetic radiation.


The very bright, very distant core of an extremely powerful active galaxy. The word quasar is derived from quazistellar radio source, so-called because this class of object was first identified through its radio emissions.


The transmission of energy or matter. See also electromagnetic radiation.

Radiation pressure

The pressure exerted on an object by a stream of photons.

Red dwarf

A small and relatively cool star with a diameter and mass of less than one-third that of the Sun. Red Dwarf comprise the vast majority of stars.

Red giant

An aging star that has exhausted all the hydrogen in its nucleus and burns increasingly heavy elements as fuel to ward off collapse.

Reflection nebula

A cloud of dust that reflects or scatters the light of nearby stars. Reflection nebulae often take on a blue tint as blue light is scattered more efficiently by dust than red light.

Ring galaxy

A galaxy surrounded by a distinct rim of very bright stars. Thought to be formed when spiral galaxies are pierced by a head on collision with another galaxy.


Any body in orbit about another body.

Solar mass

A measure used to express the mass of stars and larger objects. It is equal to the mass of our Sun, or 1.98 trillion trillion tonnes.

Solar wind

A stream of plasma flowing from the Sun at up to 3.2 million kph (2 million mph). It prevades the entire solar system up to the heliopause.

Spiral galaxy

A typical spiral galaxy has a spherical central bulge of older stars surrounded by a flattened disc containing a spiral pattern of young, hot stars. The Milky Way is a spiral galaxy.


A massive ball of hydrogen and helium bound together by gravity and shining with the light of nuclear fusion. Gravity provides the energy that drives fusion and fusion provides the power that stops the star from further gravitational collapse. The balancing act determines a star's allotted life span - the larger a star, the greater its gravity, the greater its gravity the faster it burns and the faster it burns the shorter its life. The most massive hypergiants burn for a brief, if spectacular, few million years while they're smaller cousins measure their lives in billions of years. Once a star has exhausted its fuel it is at the mercy of gravity and will collapse (always dramatically, see Planetary nebula and Supernova) into either a white dwarf, a neutron star or a black hole, depending on its mass.

Starburst galaxy

A galaxy experiencing an intense burst of star formation. Most starbursts are precipitated by interaction - or even collision - with other galaxies.

Star formation

The process by which stars gravitationally coalesce from molecular clouds.

Star formation region
A molecular cloud that is in the process of collapse, forming new stars.

Stellar wind

The solar wind emanating from a star other than our Sun.

The star at the centre of our solar system. Occasionally referred to as Sol.

Supermassive black hole:
A black hole with a mass in the range of millions or billions of solar masses. Most, if not all, galaxies are thought to harbour a supermassive black hole at their core.

The explosive demise of a star. Supernova coming two types: Type I and Type II. A Type I supernova accompanies the collapse of an existing white dwarf made unstable by the accretion of material from a nearby stellar companion. A Type II supernova occurs at the end of a massive star's life as it exhausts its nuclear fuel and starts to collapse under its own weight. As the successive layers of the star collapse into each other a shock wave is produced that blasts the outer layers of the star out into space at speeds in excess of 50 million kph (30 million mph). Depending on the size of the star, either a neutron star or a black hole is left at the centre of the conflagration.

Supernova Remnant (SNR)

An expanding shell of dust and gas - the debris of a supernova explosion, mixed together with swept up interstellar matter.

T-Tauri star

A class of very young, flaring stars on the verge of becoming normal stars fuelled by nuclear fusion.

Ultraviolet radiation

Electromagnetic radiation with a shorter wavelength than violet light.


Our bubble of space and time and everything it contains.

White dwarf

The dense, cooling ember of a star that has exhausted its nuclear fuel and collapsed under the force of its own gravity. White dwarfdom awaits all but the most massive stars.

Wolf-Rayet star:
A massive star (over 25 solar masses) on the verge of supernova. Wolf-Rayets are characterized by a powerful stellar wind that strips off the outer layers of the star, leaving its core exposed.



High-energy electromagnetic radiation. Less energetic than gamma rays, but more so than ultraviolet radiation.

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