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A hypergiant (luminosity class 0 or Ia+) is a star with an enormous luminosity showing signs of a very high rate of mass loss. The word "hypergiant" is commonly used as a loose term for the most luminous stars found, even though there are more precise definitions. In 1956, the astronomers Feast and Thackeray used the term super-supergiant (later changed into hypergiant) for stars with an absolute magnitude brighter than ''M''V = −7 (''M''Bol will be larger for very cool and very hot stars, for example at least −9.7 for a B0 hypergiant). In 1971, Keenan suggested that the term would be used only for supergiants showing at least one broad emission component in Hα, indicating an extended stellar atmosphere or a relatively large mass loss rate. The Keenan criterion is the one most commonly used by scientists today.〔 〕 Additionally, hypergiants are expected to have characteristic broadening and red-shifting of their spectral lines producing a distinctive shape known as a P Cygni profile. The use of hydrogen emission is not helpful for defining the coolest hypergiants, and these are largely classified on luminosity since mass loss is almost inevitable for the class. Many astronomers do not use the term hypergiant, except occasionally for specific well-defined groups such as the yellow hypergiants, so it is common to see the term RSG (red supergiant) or B(e) supergiant (blue supergiant with emission spectra) being used to refer to stars that this article defines as hypergiants. There is an MKK luminosity class 0 (zero) for hypergiants, but this is rarely seen in published spectral classifications. More commonly, hypergiants will be classed as Ia-0, Ia+, or even just Iae based solely on the observed spectra. As noted, red supergiants rarely receive these extra spectral classifications. Initial observation of a highly luminous star is insufficient for it to be defined as a hypergiant. That requires the detection of the spectral signatures of atmospheric instability and high mass loss. So it is quite possible for non-hypergiant supergiant stars to have the same or higher luminosity as a hypergiant of the same spectral class. ==Formation== Stars with an initial mass above about quickly move away from the main sequence and increase somewhat in luminosity to become blue supergiants. They cool and enlarge at approximately constant luminosity to become a red supergiant, then contract and increase in temperature as the outer layers are blown away. They may "bounce" backwards and forwards executing one or more "blue loops", still at a fairly steady luminosity, until they explode as a supernova or completely shed their outer layers to become a Wolf–Rayet star. Stars with an initial mass above about are simply too luminous to develop a stable extended atmosphere and so they never cool sufficiently to become red supergiants. The most massive stars, especially rapidly rotating stars with enhanced convection and mixing, may skip these steps and move directly to the Wolf–Rayet stage. This means that stars at the top of the Hertzsprung–Russell diagram where hypergiants are found may be newly evolved from the main sequence and still with high mass, or much more evolved post-red supergiant stars that have lost a significant fraction of their initial mass, and these objects cannot be distinguished simply on the basis of their luminosity and temperature. High-mass stars with a high proportion of remaining hydrogen are more stable, while older stars with lower masses and a higher proportion of heavy elements have less stable atmospheres due to increased radiation pressure and decreased gravitational attraction. These are thought to be the hypergiants, near the Eddington limit and rapidly losing mass. The yellow hypergiants are thought to be generally post-red supergiant stars that have already lost most of their atmospheres and hydrogen. A few more stable high mass yellow supergiants with approximately the same luminosity are known and thought to be evolving towards the red supergiant phase, but these are rare as this is expected to be a rapid transition. Because yellow hypergiants are post-red supergiant stars, there is a fairly hard upper limit to their luminosity at around , but blue hypergiants can be much more luminous, sometimes several million . Almost all hypergiants exhibit variations in luminosity over time due to instabilities within their interiors, but these are small except for two distinct instability regions where luminous blue variables (LBVs) and yellow hypergiants are found. Because of their high masses, the lifetime of a hypergiant is very short in astronomical timescales: only a few million years compared to around 10 billion years for stars like the Sun. Hypergiants are only created in the largest and densest areas of star formation and because of their short lives, only a small number are known despite their extreme luminosity that allows them to be identified even in neighbouring galaxies. The time spent in some phases such as LBVs can be as short as a few thousand years. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「hypergiant」の詳細全文を読む スポンサード リンク
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