Beta Pegasi

"Scheat" redirects here. For the star in Aquarius, see Delta Aquarii.

Beta Pegasi (β Pegasi, abbreviated Beta Peg, β Peg), formally named Scheat /ˈʃiːæt/,^[12][13] is a red giant star and the second-brightest star (after Epsilon Pegasi) in the constellation of Pegasus. It forms the upper right corner of the Great Square of Pegasus,^[14] a prominent rectangular asterism.

Beta Pegasi

Location of β Pegasi (circled)
Observation data Epoch J2000      Equinox J2000
Constellation	Pegasus
Right ascension	23h 03m 46.45746s[1]
Declination	+28° 04′ 58.0336″[1]
Apparent magnitude (V)	2.42[2] (2.31 – 2.74)[3]
Characteristics
Spectral type	M2.5II–IIIe[4]
U−B color index	+1.96[2]
B−V color index	+1.67[2]
Variable type	Semi-regular[5]
Astrometry
Radial velocity (Rv)	+8.7[6] km/s
Proper motion (μ)	RA: +187.65[1] mas/yr Dec.: +136.93[1] mas/yr
Parallax (π)	16.64±0.15 mas[1]
Distance	196 ± 2 ly (60.1 ± 0.5 pc)[1]
Absolute magnitude (MV)	−1.41[7]
Details
Mass	1.7±0.3[8] M☉
Radius	109±7[9] R☉
Luminosity	1,644[8][a] L☉
Surface gravity (log g)	1.20[10] cgs
Temperature	3,606[8] K
Metallicity [Fe/H]	−0.11[10] dex
Rotational velocity (v sin i)	9.7[11] km/s
Other designations
Scheat, 53 Pegasi, HR 8775, BD+27°4480, HD 217906, SAO 90981, FK5 870, HIP 113881[4]
Database references
SIMBAD	data

Nomenclature

β Pegasi (Latinised to Beta Pegasi) is the star's Bayer designation.

It bore the traditional name of Scheat, a name that had also been used for Delta Aquarii. The name was derived from the Arabic Al Sā'id "the upper arm", or from Sa'd.^[14] In 2016, the International Astronomical Union organised a Working Group on Star Names (WGSN)^[15] to catalog and standardise proper names for stars. The WGSN's first bulletin of July 2016^[16] included a table of the first two batches of names approved by the WGSN; which included Scheat for this star (the name Skat was later approved for Delta Aquarii^[13]).

In Chinese, 室宿 (Shì Xiù), meaning Encampment, refers to an asterism consisting β Pegasi and α Pegasi.^[17] Consequently, the Chinese name for β Pegasi itself is 室宿二 (Shì Xiù èr), "the Second Star of Encampment".^[18]

Distance and properties

A light curve for Beta Pegasi, plotted from Hipparcos data^[19]

Based upon parallax measurements, Beta Pegasi is located about 196 light-years (60 parsecs) from the Sun.^[1] It is unusual among bright stars in having a relatively cool surface temperature compared to stars like the Sun. This star has a stellar classification of M2.3 II–III,^[4] which indicates the spectrum has characteristics partway between a bright giant and a giant star. It has expanded until it is 109 times as large,^[9] and has a total luminosity of 1,640 times that of the Sun.^[8] The effective temperature of the star's outer envelope is about 3,600 K,^[8] giving the star the characteristic orange-red hue of an M-type star.^[20] The photosphere is sufficiently cool for molecules of titanium oxide to form.^[21]

Johann Friedrich Julius Schmidt discovered that Beta Pegasi is a variable star, in 1847.^[22] Beta Pegasi is a semi-regular variable with a period of 43.3 days^[5] and a brightness that varies from magnitude +2.31 to +2.74 (averaging 2.42).^[3] It is losing mass at a rate at or below 10⁻⁸ times the Sun's mass per year, which is creating an expanding shell of gas and dust with a radius of about 3,500 times the Sun's radius (16 astronomical units).^[23]

Notes

1. Calculated from bolometric magnitude in the equation 10^{0.4(4.74-M_bol)}, where "M_bol" is the bolometric magnitude.