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

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Orion Glossary

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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°).


The name given to a theory that describes the behaviour of the Universe at times between about 10-35 s and 10-32 s after the big bang. During this interval it is proposed that distances within the Universe increased by an extraordinarily huge factor. Inflation predicts that we live in a spatially flat universe.

interstellar medium

The very thin gas and tiny specks of dust that lie between the stars.

iron group

A subset of heavy metallic elements, such as iron, chromium and nickel that are the endpoints of nuclear fusion reactions in massive stars.


Atoms with the same number of protons in their nuclei but different numbers of neutrons are called isotopes. Because they have the same number of protons, they have the same atomic number and are atoms of the same chemical element. But because of the different number of neutrons, they differ in mass number.