Scheat is Beta Pegasi, a cool red giant of spectral type M2 at about 196 light years, a swollen and evolved star that varies slightly and irregularly in brightness as such giants do. It marks the northwestern corner of the Great Square of Pegasus, diagonally opposite the shared star Alpheratz. Its name derives from an Arabic word for the upper arm or the shin, marking its place on the figure of the horse. Its distinctly reddish colour sets it apart from the bluer stars of the Square, the mark of a star in a late and unstable phase of its life.
Scheat's irregular variability, unlike the strict, predictable periods of an eclipsing pair such as Algol or Alphecca, is intrinsic to the star itself, a red giant's outer layers pulsating unevenly as convection and the star's unstable, swollen structure interact, a different physical mechanism entirely from the geometric dimming that produces eclipsing-binary variability elsewhere in this set of tables. Scheat's slow, irregular pulsations place it in the broad class of semi-regular variable red giants, a category that also includes Betelgeuse in Orion, stars whose unstable outer layers swell and contract without the strict predictability of an eclipsing binary.
Sources AstronomyAndrew Fraknoi, David Morrison, and Sidney C. Wolff (OpenStax) with Almagest (Mathematike Syntaxis)Claudius Ptolemy