Hypergiant

Lore & Background

The term hypergiant originated in 1956 when astronomers Feast and Thackeray used 'super-supergiant' for stars with absolute magnitude brighter than MV = −7. In 1971, Keenan proposed that the term be used only for supergiants showing at least one broad emission component in Hα, indicating an extended atmosphere or high mass loss rate. This Keenan criterion is now the most commonly used definition, meaning a supergiant can have higher luminosity than a hypergiant of the same spectral class.

Hypergiants form from stars with initial mass above about 25 M☉, which quickly leave the main sequence and become blue supergiants, then red supergiants, and may undergo 'blue loops' before exploding as supernovae or hypernovae. Stars above about 40 M☉ never cool enough to become red supergiants. Yellow hypergiants are generally post-red supergiant stars that have lost most of their atmospheres, while blue hypergiants can be much more luminous.

Almost all hypergiants show luminosity variations due to interior instabilities. Their lifetimes are very short—only a few million years—and they are created only in the largest, densest star formation regions. Because of their short lives, few are known despite their extreme luminosity, which allows them to be identified even in neighboring galaxies.

Reader's Guide

Hypergiants represent the most extreme stellar objects in terms of luminosity and mass loss, providing critical insights into the upper limits of stellar stability. Their proximity to the Eddington limit—where radiation pressure balances gravity—makes them natural laboratories for studying mass loss mechanisms, including continuum-driven winds and potential pseudo-photosphere formation. The Keenan criterion, requiring broad Hα emission, distinguishes hypergiants from ordinary supergiants, though this criterion is less useful for the coolest hypergiants. The evolutionary paths of hypergiants are complex: some are newly evolved from the main sequence, while others are post-red supergiant stars that have lost significant mass. This uncertainty complicates classification but underscores their role in understanding late-stage stellar evolution. Notable examples like Eta Carinae, with an estimated mass of 130 solar masses and luminosity four million times that of the Sun, may occasionally exceed the Eddington limit, producing massive outbursts. Hypergiants are also linked to luminous blue variables (LBVs) and yellow hypergiants, with some yellow hypergiants possibly being LBVs that have formed a cooler pseudo-photosphere. Their short lifetimes and rarity make them key targets for studying supernovae and hypernovae, as well as the upper mass limit for stars across cosmic time.

Did You Know?

• The term hypergiant was originally called 'super-supergiant' by Feast and Thackeray in 1956.
• Hypergiants are defined by the Keenan criterion: at least one broad emission component in Hα.
• Yellow hypergiants have a hard upper luminosity limit of about 500,000–750,000 L☉, while blue hypergiants can reach several million L☉.
• Eta Carinae, with an estimated mass of 130 solar masses, may occasionally exceed the Eddington limit, as possibly seen in outbursts from 1840–1860.