VY Canis Majoris

VY Canis Majoris is a highly luminous red hypergiant in the constellation Canis Major. Its enormous, uncertainly measured radius places it among the largest known stars.

Red hypergiant: A very luminous, evolved massive star with a cool surface and an extended atmosphere. This is the stellar class used to describe VY Canis Majoris.
Stellar mass loss: The process by which a star ejects matter into its surroundings. VY Canis Majoris has shed large amounts of material into its circumstellar environment.
VY Canis Majoris nebula: The irregular cloud of gas and dust surrounding VY Canis Majoris. Its arcs and knots preserve evidence of the star's past eruptions.
Stephenson 2-18: A red supergiant candidate in the constellation Scutum, sometimes estimated to have an exceptionally large radius. Its uncertain size competes with VY Canis Majoris in lists of the largest known stars.
Stellar radius: The distance from a star's center to the boundary used to define its surface. Its unusually large estimated radius is central to VY Canis Majoris's reputation.
Stellar wind: A continuous outflow of charged particles from a star's atmosphere. A dense outflow carries gas away from the star and shapes its surrounding nebula.
Hubble Space Telescope: A space observatory that observes the universe across ultraviolet, visible, and near-infrared wavelengths. Hubble images resolved the star's complex circumstellar structures.
UY Scuti: A red supergiant in the constellation Scutum whose radius estimates have varied substantially. Its changing estimates illustrate why claims about the largest star are unstable.
Luminosity: The total energy a celestial object emits per unit time. Its high luminosity is a defining property independent of its apparent brightness from Earth.
Circumstellar envelope: Gas and dust surrounding a star, often formed from material the star has expelled. VY Canis Majoris is embedded in an extended envelope created by its mass loss.

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