Black hole: the result of a massive star collapsing. Gravity acts so strongly on black holes that nothing can escape it.
Brown Dwarf: Collection of matter that could not achieve nuclear fusion.
Protostar: cloud of gas that is beginning the process of nuclear fusion.
Main Sequence: Longest part of a star's lifespan. Stars with larger mass have shorter main sequences. Stars with smaller mas have longer main sequences.
Nebula: vast, cold cloud of interstellar gas and dust
Nuclear Fusion: the process that forms stars, when H and He combine to create heavier and heavier elements.
Neutron stars: One of the last stages of a mid-sized star. Neutron stars have a core of densely-packed neutrons.
Red giant: Stage in a star's lifespan in which it burns helium instead of hydrogen.
Solar Nebula Theory: theory that our solar system formed as a nebula of gas & dust began to collapse approximately 4.5 billion years ago
Supergiant: Another name for a red giant. Supergiants are larger than red giants.
Supernova: violent, luminous explosion or death of a very massive star
White Dwarf: Stage in which a star stops fusion. White dwarfs can have high temperatures, but they produce less light than a star in main sequence.