Epsilon Pegasi (
Latinised from ε Pegasi, abbreviated Epsilon Peg, ε Peg), formally named Enif/ˈiːnɪf/, is the brightest
star in the northern
constellation of
Pegasus.
With an average
apparent visual magnitude of 2.4,[3] this is a second-magnitude star that is readily visible to the naked eye. The distance to this star can be estimated using
parallax measurements from the
Hipparcos astrometry satellite,[13][14] yielding a value of around 690
light-years (210
parsecs).[1]
Nomenclature
ε Pegasi (Latinised to Epsilon Pegasi) is the star's
Bayer designation.
It bore the traditional name Enif derived from the
Arabic word for 'nose', due to its position as the muzzle of Pegasus. In 2016, the
International Astronomical Union organized a
Working Group on Star Names (WGSN)[15] to catalog and standardize 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 Enif for this star.[17]
Other traditional names for the star include Fom al Feras, Latinised to Os Equi.[18] In
Chinese, 危宿 (Wēi Sù), meaning Rooftop (asterism), refers to an
asterism consisting of Epsilon Pegasi,
Alpha Aquarii and
Theta Pegasi.[19] Consequently, the
Chinese name for Epsilon Pegasi itself is 危宿三 (Wēi Sù sān, English: the Third Star of Rooftop.)[20]
Physical characteristics
Epsilon Pegasi is a
red supergiant star, as indicated by the
stellar classification of K2 Ib.[21] It is estimated to be between seven and twelve[8] times the
Sun's mass. The measured
angular diameter of this star, after correction for
limb darkening, is 8.17±0.09
mas.[22] At the estimated distance of this star, this yields an enormous physical size of about 211 times the
radius of the Sun. From this expanded envelope, it is radiating roughly 9,800 times the
luminosity of the Sun at an
effective temperature of 3,965
K. This temperature is cooler than the Sun, giving it the orange-hued glow of a K-type star.[23]
Epsilon Pegasi is a
slow irregular variable star that usually has a brightness between magnitudes 2.37 and 2.45. However, it was once observed very briefly at magnitude 0.7, giving rise to the theory that it (and possibly other supergiants) erupt in massive
flares that dwarf those of the
Sun.[24][25] It has also been observed as faint as magnitude 3.5.[24]
Based on its position on the
color-magnitude diagram, Enif may have evolved from a whitish-yellow color to its current red color in the last 2,000 years, though there is currently no historical record supporting this.[8]
Pulfrich effect
Epsilon Pegasi is a fine example to observe the
Pulfrich effect. This optical phenomenon is described on page 1372 of Burnham's Celestial Handbook. According to
John Herschel: The apparent pendulum-like oscillation of a small star in the same vertical as the large one, when the telescope is swung from side to side.[28]
^Keenan, Philip C.; McNeil, Raymond C. (1989). "The Perkins catalog of revised MK types for the cooler stars". Astrophysical Journal Supplement Series. 71: 245.
Bibcode:
1989ApJS...71..245K.
doi:
10.1086/191373.
^
abcCousins, A. W. J. (1984), "Standardization of Broadband Photometry of Equatorial Standards", South African Astronomical Observatory Circulars, 8: 59,
Bibcode:
1984SAAOC...8...59C
^
abSmith, Verne V.; Lambert, David L. (June 1987), "Are the red supergiants Epsilon Peg and 12 PUP victims of mild s-processing?", Monthly Notices of the Royal Astronomical Society, 226 (3): 563–579,
Bibcode:
1987MNRAS.226..563S,
doi:10.1093/mnras/226.3.563
^Bernacca, P. L.; Perinotto, M. (1970). "A catalogue of stellar rotational velocities". Contributi Osservatorio Astronomico di Padova in Asiago. 239 (1): 1.
Bibcode:
1970CoAsi.239....1B.