Glosariusz - I

Inferior planet is an archaic term that is sometimes used to refer to the planets Mercury and Venus. The name originated from the fact that they orbit closer to the Sun than the Earth and hence, in the geocentric cosmology of Ptolemy, both appeared to travel with the Sun across the sky. This is in contrast to so-called superior planets, such as Mars, that appeared to move independently of the Sun.

Interstellar medium is the matter that exists in the space between the stars in a galaxy. This medium mainly consists of hydrogen and helium, but it is enhanced by traces of other elements contributed by matter expelled from stars.

Interstellar reddening is an effect produced by the incremental absorption and scattering of electromagnetic energy from interstellar matter, an effect known as extinction. This effect causes the more distant objects such as stars to appear redder and dimmer than expected. It is not to be confused with the separate phenomenon of red shift.

Isochrones are curves on the Hertzsprung-Russell diagram that represent the evolutionary positions of stars having the same age but differing masses. This is in contrast to an evolutionary track, which is a plot of stars having the same mass but differing ages. In fact, multiple evolutionary tracks can be used to build isochrones by putting curves through equal-age points along the tracks. When the mass of a star can be determined, an isochrone can be used to estimate the star's age.