supergiant

Supergiant (or supergiant star) is a term generally used for a very bright giant star. Such stars are post-main-sequence stars that were massive and hot during their main-sequence, e.g., O-type stars. The term generally means brighter than a bright giant. The very brightest supergiants are termed hypergiants. The term supergiant is not consistently defined, but is applied to stars in this general range:

Since brightness is the criteria, the radii can overlap with "non-super giant" stars. Qualifications used for some of the brightest:

• bright supergiant - brighter than typical.
• luminous supergiant - even brighter.
• extremely luminous supergiant - brightest of all.

Supergiants can be of various spectral classes and can be red, blue, or yellow. Examples of supergiants:

• Betelgeuse (a red supergiant, aka RSG).
• Canopus (a yellow supergiant, aka YSG).
• Rigel (a blue supergiant, aka BSG or OB supergiant).

Blue supergiant characteristics:

They are post-main-sequence OB stars.

An extreme helium star (EHe) is a rare type of supergiant nearly devoid of hydrogen. Their formation is a topic of research interest.