A supergiant is an old large star. Supergiants are rare and short-lived, but super bright in terms of stars that we can see from Earth. Because they are large and bright, you can spot them at night with your naked eye and I wrote how you can see them in the daytime through a telescope, if you know where to look.
They range in temperature from about 3,450 K to over 20,000 K (red to blue spectrum).
Rigel, in the constellation Orion, and Deneb, in Cygnus, are two blueish-white supergiants. The following chart gives you a idea of their brightness and their spectral class compared to other stars in space.
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