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

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Orion Glossary

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helium burning

The process by which stars fuse three helium nuclei into one carbon nucleus via the triple alpha process.

Hertzsprung-Russell (HR) diagram

A diagnostic diagram whereby the evolutionary state of a star may be discerned via a combination of stellar temperature and luminosity. Named after its originators Ejnar Hertzsprung and Henry Norris Russell.

hot Jupiter

A giant planet, with a mass close to or greater than that of Jupiter, and which orbits very close to its parent star.

Hubble deep field

A project with the Hubble Space Telescope to obtain a series of very deep exposures to search for distant galaxies at high redshift.

Hubble’s law

The observation that, for distant galaxies, the greater a galaxy’s distance, the greater the redshift of the radiation received from that galaxy.

hydrogen burning

The process by which stars on the main sequence release energy via the conversion of hydrogen into helium in their cores.

hydrogen shell burning

The process by which, in post-Main Sequence stars, hydrogen is converted into helium in a thin shell exterior to the core.

hydrostatic equilibrium

In a star, the state at which physical forces directed inwards are exactly balanced by those directed outwards.


icy materials

A term used in Solar System studies for materials that comprise water and other substances that are solid only at relatively low temperatures, although they can be liquid on the surfaces or in the interiors of planets, or gaseous in the atmospheres of planets.


The angle i between the orbital plane of an astronomical body and a reference plane. For a solar system body the reference plane is the ecliptic. (The inclination of the orbit of a binary star is the angle between the orbital plane and the plane of the sky, so that a face-on binary has i = 0 and an edge-on (eclipsing) system has i close to 90°).

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