BBC Sky at Night Magazine

Star of the month

Seeing red in the Great Square of Pegasus


The Great Square of Pegasus is a cornerston­e of autumn skies. It’s a large pattern, not especially bright, formed from three stars in Pegasus and one from Andromeda. The star Scheat (Beta (b) Pegasi) marks the square’s northwest corner.

Scheat’s colour makes it stand out from the other corner stars, which are all hot and blue. Scheat is a red star with a spectral classifica­tion of M2.5IIIIIe. M2.5 indicates its spectral class, M-stars being cool and orange-red in colour. The II-III indicates that it lies between a bright giant (II) and a normal giant (III). The ‘e’ tells us emission lines are present in its spectrum.

Scheat is 196 lightyears away and at that distance, in order to appear as bright as it does in the night sky, it must radiate in the visual part of the spectrum with a luminosity equal to 340 Suns. However, being a cool red star, much of Scheat’s output is in the infrared part of the spectrum and when this is factored in, the star is around 1,500 times more luminous than our own Sun.

It’s known to be 95 times larger than the Sun. The apparent diameter of the star is 0.015 arcseconds which, at 196 lightyears distant, gives us its size. Like many red giant stars, Scheat exhibits variabilit­y. This is due to intrinsic processes within the star. Its brightness varies from mag. +2.3 to +2.7 over 43.3 days, but there is irregular variation here too.

Scheat is a class of star known as a semi-regular variable.

As the beta star of Pegasus, it’s fitting that Scheat is actually the second brightest star in the constellat­ion.

However, this is not after Markab (Alpha (a) Pegasi), but rather after Enif (Epsilon (e) Pegasi), which is slightly brighter. The name Scheat derives from the Arabic word for ‘shin’.

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