The space between stars. The law that a field follows if its strength decreases with the square of the distance. Fields that follow the inverse square law rapidly decrease in strength as the distance increases, but never quite reach zero. The process by which ions are produced, typically by collisions of electrons, ions, or photons. A strangely shaped galaxy, often rich in interstellar matter, but apparently not a member of any of the major classes of spiral or elliptical galaxies.
Most elements can exist in several isotopic forms.
A common example of an isotope is deuterium, which differs from normal hydrogen by the presence of an extra neutron in the nucleus.