Absolute Magnitude – a measurement of how bright a star would be at a distance of one parsec

Apparent Magnitude – a measurement of how bright a star appears in the night sky

Black Dwarf – the remains of a white dwarf that has cooled down to the background temperature of the universe

Black Hole – a region of spacetime from which gravity prevents anything, even light, from escaping

Brightness – the amount of light reaching us from a star; it depends on the luminosity and the distance

Brown Dwarf – a body formed by the collapse of a nebula that does not have enough mass to trigger fusion

Deuterium – an atom of hydrogen with a neutron, so with an atomic mass of 2

Fusion – the combining of two small nuclei to form larger heavier nucleus

Gamma Ray – a high energy, high frequency photon of electromagnetic radiation

Isotope – atoms of the same element that have different masses due to having a different number of neutrons

Luminosity – a measurement of the rate of energy being released by a star

Main Sequence – the band of stars that stretch form hot, luminous stars to cool stars with a low luminosity on an HR diagram

Nebula – a cloud of gas and dust in outer space

Neutrino – a neutral subatomic particle with a very small mass that rarely reacts with matter

Neutron Star – a small very dense object in space composed of neutrons

Positron – the antiparticle of an electron, it has the same mass as an electron, but a positive charge

Protostar – the stage in a star life cycle where the nebula has collapsed, but fusion has not yet begun

Red Giant – a large star with high luminosity and low surface temperature that forms as the hydrogen in the core is used up

Standard Candles – a reference star that is used to calculate the distance to other stars

White Dwarf – the final end stage for small stars after most of the nuclear fuel has been spent

(source)