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A hypergiant (or hypergiant star) is a term for the brightest extreme of giant stars, e.g., on the order of absolute magnitude -7, and up to millions of solar luminosities. All are presumed to be post-main-sequence stars that were early during their main-sequence, e.g., they were an O-type star. In general, hypergiants that are currently O-type and B-type are termed blue hypergiants and A-type through K-type, yellow hypergiants, though some astronomers use other spectral features as well to distinguish them. Example blue hypergiants:
Yellow hypergiants:
A star cool enough to be M-class is often termed a red supergiant even if it fits the above hypergiant magnitude.