supergiant

A supergiant is a very luminous, massive star that has evolved off the main sequence and expanded to a large radius. In stellar classification they are assigned luminosity classes I (bright supergiants) and II (less luminous, high-luminosity giants). Supergiants are among the brightest stars in galaxies, with luminosities from tens of thousands up to millions of solar luminosities and radii hundreds to thousands of times that of the Sun.

There are several types. Red supergiants are cool (approximately 3,000–4,500 K) with very large radii, found at the cool end of the Hertzsprung–Russell diagram, and include Betelgeuse and Antares. Blue and yellow (or white) supergiants are hot (above 10,000 K) and highly luminous, such as Deneb and Rigel. Cepheid variables are a subset of yellow supergiants that exhibit periodic brightness changes and are used as standard candles. Hypergiants, including luminous blue variables like Eta Carinae, represent very massive, extreme cases with powerful winds.

Formation and evolution: Supergiants originate from massive stars typically more than about 8–10 solar masses. After exhausting hydrogen in the core, they expand and cool (red supergiants) or stay hot with a compact envelope (blue/white supergiants). They experience substantial mass loss through strong stellar winds and have relatively short lifespans, on the order of tens of millions of years.

End stages: Most end in core-collapse supernovae (Type II). Depending on initial mass, the remnants are neutron stars or black holes; in the most massive cases, pair-instability or other extreme routes may occur.