Historically categorized by their shape, what is a massive, gravitationally bound system that consists of stars and stellar remnants, an interstellar medium of gas dust, and some dark matter?
Galaxy Galaxy A galaxy is a gravitationally bound system of stars, stellar remnants, interstellar gas, dust, and dark matter. The word galaxy is derived from the Greek "" (), literally "milky", a reference to the Milky Way. Galaxies range in size from dwarfs with just a few hundred million () stars to giants with one hundred trillion () stars, each orbiting its galaxy's center of mass. Galaxies are categorized according to their visual morphology as elliptical, spiral, or irregular. Many galaxies are thought to have supermassive black holes at their centers. The Milky Way's central black hole, known as Sagittarius A*,
Dark matter common black holes, neutron stars, faint old white dwarfs and brown dwarfs, collectively known as massive compact halo objects (MACHOs), which can be hard to detect. However, multiple lines of evidence suggest the majority of dark matter is not made of baryons: Candidates for non-baryonic dark matter are hypothetical particles such as axions, sterile neutrinos, weakly interacting massive particles (WIMPs), gravitationally-interacting massive particles (GIMPs), or supersymmetric particles. The three neutrino types already observed are indeed abundant, and dark, and matter, but because their individual masses—however uncertain they may be—are almost certainly tiny, they can only supply a small fraction of