Glossaire beginning with I

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Inclination

Angle between the plane of an orbit and the ecliptic.

Interstellar (matter)
Gas and dust filling the Milky Way between the stars. This is the material from which stars, planets and comets form.
Isotope
Atomic nucleus containing a different number of neutrons than the main nucleus, the number of protons remaining the same. Example: 3He, is the rare isotopic nucleus of helium, with one neutron and 2 protons, instead of 2 neutrons and 2 protons for the abundant species 4He.