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

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Orion Glossary

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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).

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