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

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Orion Glossary

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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.

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