Editing Supergiant (section)

===Variability===
[[File:Heic1323a -1243686232.jpg|thumb|right|upright=1.0|[[RS Puppis]] is a supergiant and [[Classical Cepheid variable]].]]
While most supergiants such as [[Alpha Cygni variables]], [[semiregular variables]], and [[irregular variables]] show some degree of [[Photometry (astronomy)|photometric]] variability, certain types of variables amongst the supergiants are well defined. The [[instability strip]] crosses the region of supergiants, and specifically many yellow supergiants are [[Classical Cepheid variables]].  The same region of instability extends to include the even more luminous [[yellow hypergiant]]s, an extremely rare and short-lived class of luminous supergiant.  Many [[R Coronae Borealis variables]], although not all, are [[yellow supergiant]]s, but this variability is due to their unusual chemical composition rather than a physical instability.

Further types of variable stars such as [[RV Tauri variables]] and [[PV Telescopii variable]]s are often described as supergiants. RV Tau stars are frequently assigned spectral types with a supergiant luminosity class on account of their low surface gravity, and they are amongst the most luminous of the AGB and post-AGB stars, having masses similar to the Sun; likewise, the even rarer PV Tel variables are often classified as supergiants, but have lower luminosities than supergiants and peculiar B[e] spectra extremely deficient in hydrogen. Possibly they are also post-AGB objects or "born-again" AGB stars.

The LBVs are variable, with multiple semi-regular periods and less predictable eruptions and giant outbursts. They are usually supergiants or hypergiants, occasionally with Wolf–Rayet spectra—extremely luminous, massive, evolved stars with expanded outer layers—but they are so distinctive and unusual that they are often treated as a separate category without being referred to as supergiants or given a supergiant spectral type. Often their spectral type will be given just as "LBV" because they have peculiar and highly variable spectral features, with temperatures varying from about 8,000 K in outburst up to 20,000 K or more when "quiescent".