supergiant
/ SOO-per-jy-ant /
If a red giant is a star that has bloated to the size of Earth's orbit, a supergiant is something even more extreme — among the largest individual objects in the universe. Drop one where the Sun is and its surface might reach past the orbit of Jupiter. Supergiants are the rare, enormous, intensely luminous stars that sit across the very top of the Hertzsprung-Russell diagram.
Supergiants are the luminosity class I stars, and they come in two flavours. Red supergiants are cool (around 3,500 K) but immense, hundreds to over a thousand times the Sun's radius. Blue supergiants are smaller but searingly hot (above 20,000 K). Both are spectacularly luminous — tens of thousands to over a million solar luminosities — because either huge size, or fierce temperature, or both, push their output to extremes. They are born only from very massive stars, typically more than about 8 to 10 times the Sun's mass, and such heavyweights are rare and burn out fast, living only a few million years.
Supergiants matter because they are the stars destined to die as core-collapse supernovae, forging and scattering the heavy elements that seed planets and life. Their brilliance also makes them visible across millions of light-years, useful for probing distant galaxies. Because they are so massive and short-lived, finding a supergiant in a star cluster tells you the cluster is young. Betelgeuse and Antares are red supergiants; Rigel and Deneb are blue ones.
If Betelgeuse replaced the Sun, its surface would extend past the orbit of Mars, swallowing the inner planets whole. Yet its surface is cooler than the Sun's — it is sheer size that makes it shine roughly 100,000 times brighter.
Among the biggest stars known — and among the shortest-lived.
Supergiants are huge but not especially dense — their matter is spread so thinly that the outer layers are closer to a vacuum than to air. 'Largest' (radius) and 'most massive' are different things; the most massive stars are not always the physically biggest.