What is "hypergiant"

n. (context star English) A star that is extremely massive and even more luminous than a supergiant.

A hypergiant ( luminosity class 0 or Ia) is among the very rare kinds of stars that typically show tremendous luminosities and very high rates of mass loss by stellar winds. The term hypergiant is defined as luminosity class 0 (zero) in the MKK system. However, this is rarely seen in the literature or in published spectral classifications, except for specific well-defined groups such as the yellow hypergiants, RSG (red supergiants), or blue B(e) supergiants with emission spectra. More commonly, hypergiants maybe classed as Ia-0 or Ia, but red supergiants rarely receive these extra spectral classifications. Astronomers are mostly interested in these stars because they relate to understanding stellar evolution, especially with star formation, stability, and their expected demise as supernovae.