57 Pegasi is a

variable Variable may refer to: * Variable (computer science), a symbolic name associated with a value and whose associated value may be changed * Variable (mathematics), a symbol that represents a quantity in a mathematical expression, as used in many ...

binary star system in the northern constellation of

Pegasus (constellation) Pegasus is a constellation in the northern sky, named after the winged horse Pegasus in Greek mythology. It was one of the 48 constellations listed by the 2nd-century astronomer Ptolemy, and is one of the 88 constellations recognised today. Wit ...

. It has the

variable star designation In astronomy, a variable star designation is a unique identifier given to variable stars. It uses a variation on the Bayer designation format, with an identifying label (as described below) preceding the Latin genitive of the name of the conste ...

GZ Pegasi, while ''57 Pegasi'' is the

Flamsteed designation A Flamsteed designation is a combination of a number and constellation name that uniquely identifies most naked eye stars in the modern constellations visible from southern England. They are named for John Flamsteed who first used them while co ...

. The system is faintly visible to the naked eye as a point of light with an

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 ...

that fluctuates around 5.105. It is located at a distance of approximately 780 light years from 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 ...

based on parallax, and is drifting further away with a radial velocity of +14 km/s. The variability of this star was discovered by J. Stebbins and C. M. Huffer in 1930. It was classified as a long-period variable of type Lb in 1974. However, based on a constant period and the shape of the

light curve In astronomy, a light curve is a graph of light intensity of a celestial object or region as a function of time, typically with the magnitude of light received on the y axis and with time on the x axis. The light is usually in a particular frequ ...

, it was later reclassified as a

semiregular variable In astronomy, a semiregular variable star, a type of variable star, is a giant or supergiant of intermediate and late (cooler) spectral type showing considerable periodicity in its light changes, accompanied or sometimes interrupted by various irre ...

of type SRa in 1978. It varies in brightness from magnitude 4.95 down to 5.23 with a period of 92.66 days. 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 ...

of 57 Peg displays blended features that indicate this is a binary system consisting of an aging red giant star on the asymptotic giant branch with a stellar classification of M4IIIa, and a fainter but hotter A-type main-sequence companion of class A3V. This spectroscopic binary system has poorly constrained orbital elements with an estimated

orbital period The orbital period (also revolution period) is the amount of time a given astronomical object takes to complete one orbit around another object. In astronomy, it usually applies to planets or asteroids orbiting the Sun, moons orbiting planets ...

of 100–500 years. The red giant primary has been designated as an

S-type star An S-type star (or just S star) is a cool giant with approximately equal quantities of carbon and oxygen in its atmosphere. The class was originally defined in 1922 by Paul Merrill for stars with unusual absorption lines and molecular bands no ...

that shows enriched levels of s-process elements in its

. However, it does not display significant lines of radioactive

technetium-99 Technetium-99 (99Tc) is an isotope of technetium which decays with a half-life of 211,000 years to stable ruthenium-99, emitting beta particles, but no gamma rays. It is the most significant long-lived fission product of uranium fission, produci ...

in its spectrum, indicating that the s-process elements must have been acquired from a mass transfer event from a formerly asymptotic giant branch companion that had passed through multiple

dredge-up A dredge-up is any one of several stages in the evolution of some stars. By definition, during a ''dredge-up'', a convection zone extends all the way from the star's surface down to the layers of material that have undergone fusion. Consequently, t ...

events. This would suggest it has a

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 ...

as a tertiary companion, but this is incompatible with the data findings. Instead, it might have been misclassified as an S-type star. There is a magnitude 10.06 visual companion at an angular separation of along a

position angle In astronomy, position angle (usually abbreviated PA) is the convention for measuring angles on the sky. The International Astronomical Union defines it as the angle measured relative to the north celestial pole (NCP), turning positive into the ...

of 198° from the primary, as of 2015. Designated component B, this star was first reported by F. G. W. Struve in 1827.

References

{{DEFAULTSORT:55 Pegasi M-type giants A-type main-sequence stars Asymptotic-giant-branch stars Semiregular variable stars S-type stars Spectroscopic binaries Pegasus (constellation) BD+07 4981 Pegasi, 57 218634 114347 8815 Pegasi, GZ