Physics Option D, Astrophysics
Critical Density
Density for which the curvature of the universe is zero/at which it will expand forever but will reach zero/at which the universe will begin to contract after an infinite amount of time.
Stellar cluster
Stars that are gravitationally bound forming an arrangement in space.
Constellation
A collection of stars that form a recognisable group (as viewed from Earth) that need not be gravitationally bound to each other.
Pulsar
A rotating neutron star.
Chandrasekhar limit
Maximum mass a stable white dwarf can have (1.4 solar masses)
Galaxy
Large group of stars