Scheat

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

The tradition gives Scheat the nature of Mars with Mercury, and it became one of the more ill-omened stars of the catalogue: Robson reads it for extreme misfortune, and, in the lurid manner of the older sources, associates it particularly with danger of drowning and death by water. This heavy freight of doom is late compilation rather than anything observed, and belongs with the general tendency of the tradition to pile specific disasters onto the redder and more menacing-looking stars. The reliable statement is the simpler one: Scheat is a cool, luminous, variable red giant marking a corner of the Great Square, remarkable in fact as a star nearing the end of its life.

The drowning association piled onto Scheat is sometimes connected, without firm documentation, to a nineteenth-century story that a ship named for the star was lost at sea, an anecdote that if true would itself be a striking case of the tradition's imagery generating a story rather than recording one, though the tale's own origins are murky enough that it should be treated as folklore about the star rather than as an event that can be confirmed. Scheat's designation as Beta Pegasi despite occasionally outshining the constellation's Alpha star, Markab, when near visual maximum is one of several cases in this catalogue where a variable star's fluctuating brightness sits uneasily with a fixed, unchanging Bayer letter.