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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.
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.
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).