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

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Orion Glossary

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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).