Galaxies are fundamental constituents of the Universe. They are groups of approximately 105–1013 stars that are gravitationally bound and take part in the general expansion of the Universe. Galaxies have diameters ranging from 10,000 to 200,000 light-years and they possess widely varying gas and interstellar dust contents. The distances between galaxies are typically 100–1000 times their diameters. They represent 108 overdensities above the mean stellar density of the local Universe. In total, galaxy masses range from 106 to 1014 solar mass (M⊙).

Their component stars vary from ∼0.1 M⊙ brown dwarfs that do not undergo thermonuclear fusion, to rapidly evolving stars of at least ∼50 M⊙ and possibly as massive as 100 M⊙. The stars' evolutionary state ranges from protostars undergoing contraction to begin thermonuclear reactions, to main-sequence dwarf stars that fuse hydrogen to helium in their cores, through to red giant stars with expansive gaseous atmospheres. The stellar end-products are white dwarfs, neutron stars and black holes. The specific evolutionary path of a star is governed by its mass. The most massive stars evolve over millions of years, whilst the lowest mass stars can evolve for billions of years.

At least 300,000 years after the Big Bang start of the Universe, structures that would become galaxies began to condense out of primordial hydrogen and helium.