Epsilon Pegasi ( Latinised from ε Pegasi, abbreviated Epsilon Peg, ε Peg), formally named Enif , is the brightest

star A star is an astronomical object comprising a luminous spheroid of plasma (physics), plasma held together by its gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked ...

in the northern

constellation A constellation is an area on the celestial sphere in which a group of visible stars forms Asterism (astronomy), a perceived pattern or outline, typically representing an animal, mythological subject, or inanimate object. The origins of the e ...

Pegasus Pegasus ( grc-gre, Πήγασος, Pḗgasos; la, Pegasus, Pegasos) is one of the best known creatures in Greek mythology. He is a winged divine stallion usually depicted as pure white in color. He was sired by Poseidon, in his role as hor ...

. With an average

apparent visual magnitude Apparent magnitude () is a measure of the brightness of a star or other astronomical object observed from Earth. An object's apparent magnitude depends on its intrinsic luminosity, its distance from Earth, and any extinction of the object's lig ...

of 2.4, this is a second-magnitude star that is readily visible to the naked eye. The distance to this star can be estimated using

parallax Parallax is a displacement or difference in the apparent position of an object viewed along two different lines of sight and is measured by the angle or semi-angle of inclination between those two lines. Due to foreshortening, nearby objects ...

measurements from the Hipparcos astrometry satellite, yielding a value of around .

Nomenclature

''ε Pegasi'' (Latinised to ''Epsilon Pegasi'') is the star's Bayer designation. It bore the traditional name ''Enif'' derived from the

Arabic Arabic (, ' ; , ' or ) is a Semitic languages, Semitic language spoken primarily across the Arab world.Semitic languages: an international handbook / edited by Stefan Weninger; in collaboration with Geoffrey Khan, Michael P. Streck, Janet C ...

word for 'nose', due to its position as the muzzle of Pegasus. In 2016, the

International Astronomical Union The International Astronomical Union (IAU; french: link=yes, Union astronomique internationale, UAI) is a nongovernmental organisation with the objective of advancing astronomy in all aspects, including promoting astronomical research, outreac ...

organized a

Working Group on Star Names The International Astronomical Union (IAU) established a Working Group on Star Names (WGSN) in May 2016 to catalog and standardize proper names for stars for the international astronomical community. It operates under Division C – Education ...

(WGSN) to catalog and standardize proper names for stars. The WGSN's first bulletin of July 2016 included a table of the first two batches of names approved by the WGSN; which included ''Enif'' for this star. Other traditional names for the star include ''Fom al Feras'', Latinised to ''Os Equi''. In

Chinese Chinese can refer to: * Something related to China * Chinese people, people of Chinese nationality, citizenship, and/or ethnicity **''Zhonghua minzu'', the supra-ethnic concept of the Chinese nation ** List of ethnic groups in China, people of ...

, (), meaning ''

Rooftop A roof ( : roofs or rooves) is the top covering of a building, including all materials and constructions necessary to support it on the walls of the building or on uprights, providing protection against rain, snow, sunlight, extremes of temper ...

(asterism)'', refers to an asterism consisting of Epsilon Pegasi,

Alpha Aquarii Alpha Aquarii, officially named Sadalmelik (), is a single star in the constellation of Aquarius. The apparent visual magnitude of 2.94 makes this the second-brightest star in Aquarius. Based upon parallax measurements made during ...

and

Theta Pegasi θ Pegasi, Latinized as Theta Pegasi, is a single star in the equatorial constellation of Pegasus, lying about 7.5 degrees southwest of Enif. It has the traditional name Biham , and the Flamsteed designation 26 Pegasi. This object ...

. Consequently, the

Chinese name Chinese names or Chinese personal names are names used by individuals from Greater China and other parts of the Chinese-speaking world throughout East and Southeast Asia (ESEA). In addition, many names used in Japan, Korea and Vietnam are often a ...

for Epsilon Pegasi itself is (, en, the Third Star of Rooftop.)香港太空館 - 研究資源 - 亮星中英對照表
, Hong Kong Space Museum. Accessed on line November 23, 2010.

Physical characteristics

Epsilon Pegasi is a

red supergiant Red supergiants (RSGs) are stars with a supergiant luminosity class ( Yerkes class I) of spectral type K or M. They are the largest stars in the universe in terms of volume, although they are not the most massive or luminous. Betelgeuse and Ant ...

star, as indicated by the

stellar classification In astronomy, stellar classification is the classification of stars based on their stellar spectrum, spectral characteristics. Electromagnetic radiation from the star is analyzed by splitting it with a Prism (optics), prism or diffraction grati ...

of K2 Ib. It is estimated to be seven times the

Sun's mass The solar mass () is a standard unit of mass in astronomy, equal to approximately . It is often used to indicate the masses of other stars, as well as stellar clusters, nebulae, galaxies and black holes. It is approximately equal to the mass of ...

. The measured

angular diameter The angular diameter, angular size, apparent diameter, or apparent size is an angular distance describing how large a sphere or circle appears from a given point of view. In the vision sciences, it is called the visual angle, and in optics, it is ...

of this star, after correction for

limb darkening Limb darkening is an optical effect seen in stars (including the Sun), where the central part of the disk appears brighter than the edge, or ''limb''. Its understanding offered early solar astronomers an opportunity to construct models with such ...

, is . At the estimated distance of this star, this yields an enormous physical size of about 211 times the

radius of the Sun Solar radius is a unit of distance used to express the size of stars in astronomy relative to the Sun. The solar radius is usually defined as the radius to the layer in the Sun's photosphere where the optical depth equals 2/3: :1\,R_ = 6.957\t ...

. From this expanded envelope, it is radiating roughly 9,800 times the

luminosity of the Sun The solar luminosity (), is a unit of radiant flux ( power emitted in the form of photons) conventionally used by astronomers to measure the luminosity of stars, galaxies and other celestial objects in terms of the output of the Sun. One nominal ...

at an

effective temperature The effective temperature of a body such as a star or planet is the temperature of a black body that would emit the same total amount of electromagnetic radiation. Effective temperature is often used as an estimate of a body's surface temperature ...

of . This temperature is cooler than the Sun, giving it the orange-hued glow of a K-type star. 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 The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...

. It has also been observed as faint as magnitude 3.5. The

spectrum A spectrum (plural ''spectra'' or ''spectrums'') is a condition that is not limited to a specific set of values but can vary, without gaps, across a continuum. The word was first used scientifically in optics to describe the rainbow of colors i ...

shows an overabundance of the elements

strontium Strontium is the chemical element with the symbol Sr and atomic number 38. An alkaline earth metal, strontium is a soft silver-white yellowish metallic element that is highly chemically reactive. The metal forms a dark oxide layer when it is ex ...

and barium, which may be the result of the

s-process The slow neutron-capture process, or ''s''-process, is a series of reactions in nuclear astrophysics that occur in stars, particularly asymptotic giant branch stars. The ''s''-process is responsible for the creation (nucleosynthesis) of approximat ...

nucleosynthesis Nucleosynthesis is the process that creates new atomic nuclei from pre-existing nucleons (protons and neutrons) and nuclei. According to current theories, the first nuclei were formed a few minutes after the Big Bang, through nuclear reactions in ...

in the outer atmosphere of the star. It has a relatively high

peculiar velocity Peculiar motion or peculiar velocity refers to the velocity of an object relative to a ''rest frame'' — usually a frame in which the average velocity of some objects is zero. Galactic astronomy In galactic astronomy, peculiar motion refers to ...

of .

Evolution

Epsilon Pegasi has exhausted its core

hydrogen Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic, an ...

and expanded away from the

main sequence In astronomy, the main sequence is a continuous and distinctive band of stars that appears on plots of stellar color versus brightness. These color-magnitude plots are known as Hertzsprung–Russell diagrams after their co-developers, Ejnar Her ...

. It is almost certainly on the horizontal branch fusing

helium Helium (from el, ἥλιος, helios, lit=sun) is a chemical element with the symbol He and atomic number 2. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas and the first in the noble gas group in the periodic table. ...

in its core. It is sufficiently massive that it may die in a

core-collapse supernova A supernova is a powerful and luminous explosion of a star. It has the plural form supernovae or supernovas, and is abbreviated SN or SNe. This transient astronomical event occurs during the last evolutionary stages of a massive star or when ...

, or it may shed its outer layers and leave behind an unusual oxygen–neon

white dwarf A white dwarf is a stellar core remnant composed mostly of electron-degenerate matter. A white dwarf is very dense: its mass is comparable to the Sun's, while its volume is comparable to the Earth's. A white dwarf's faint luminosity comes ...

Pulfrich effect

Epsilon Pegasi is a fine example to observe the

Pulfrich effect The Pulfrich effect is a psychophysical percept wherein lateral motion of an object in the field of view is interpreted by the visual cortex as having a depth component, due to a relative difference in signal timings between the two eyes. Overvie ...

. This optical phenomenon is described on page 1372 of '' Burnham's Celestial Handbook''. According to

John Herschel Sir John Frederick William Herschel, 1st Baronet (; 7 March 1792 – 11 May 1871) was an English polymath active as a mathematician, astronomer, chemist, inventor, experimental photographer who invented the blueprint and did botanical wor ...

: ''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''.

References

{{DEFAULTSORT:Epsilon Pegasi Pegasi, Epsilon Pegasi, 08 K-type supergiants Pegasus (constellation) Enif Slow irregular variables 8308 206778 107315 BD+09 4891 Horizontal-branch stars

Nomenclature

Physical characteristics

Evolution

Pulfrich effect

See also

References