Gamma Pegasi

Gamma Pegasi

Template:Location mark Location of γ Pegasi (circled)
Observation data Epoch J2000      Equinox J2000
Constellation	Pegasus
Right ascension	Template:RA
Declination	Template:DEC
Apparent magnitude (V)	Template:Val
Characteristics
Spectral type	B2 IV
U−B Template:Engvar	−0.85
B−V Template:Engvar	−0.23
Variable type	β Cep
Astrometry
Radial velocity (Rv)	+4.1 km/s
Proper motion (μ)	RA: +1.98 mas/yr Dec.: –9.28 mas/yr
Parallax (π)	6.9474±0.4293 mas
Distance	Template:Rnd ± Template:Rnd ly (Template:Rnd ± Template:Rnd pc)
Absolute magnitude (MV)	Template:Val
Details
A
Mass	Template:Val Template:Solar mass
Radius	Template:Val Template:Solar radius
Luminosity	Template:Val Template:Solar luminosity
Surface gravity (log g)	Template:Val cgs
Temperature	Template:Val K
Metallicity [Fe/H]	−0.34 dex
Rotational velocity (v sin i)	8 km/s
Age	Template:Val Myr
Metallicity [Fe/H]	{{{metal_fe2}}} dex
Other designations
Template:Odlist
Database references
SIMBAD	data

Gamma Pegasi is a binary star in the constellation of Pegasus, located at the southeast corner of the asterism known as the Great Square. It has the formal name Algenib Template:IPAc-en; the Bayer designation Gamma Pegasi is Latinized from γ Pegasi and abbreviated Gamma Peg or γ Peg. The average apparent visual magnitude of +2.84 makes this the fourth-brightest star in the constellation. The distance to this system has been measured using the parallax technique, yielding a value of roughly Template:Convert.

Nomenclature

Gamma Pegasi is the star's Bayer designation. Although it also had the traditional name Algenib, this name was also used for Alpha Persei. In 2016, the International Astronomical Union organized a Working Group on Star Names (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 Algenib for this star (Alpha Persei was given the name Mirfak).

The asterism of γ Pegasi and α Andromedae, in Hindu astronomy, is called Uttara Bhādrapadā (उत्तरभाद्रपदा) or Uttṛṭṭāti. It is the 26th nakshatra. In Chinese, (), meaning Wall (asterism) refers to an asterism consisting of γ Pegasi and α Andromedae . Consequently, the Chinese name for γ Pegasi itself is (, Template:Langx.)

Properties

File:GammaPegLightCurve.png

A light curve for Gamma Pegasi, plotted from TESS data

In 1911, American astronomer Keivin Burns discovered that the radial velocity of Gamma Pegasi varied slightly. This was confirmed in 1953 by American astronomer D. Harold McNamara, who identified it as a Beta Cephei variable. (At the time he actually identified it as a Beta Canis Majoris star, which was subsequently designated a Beta Cephei variable.) It pulsates radially with a period of 0.15175 days (3.642 hours), but also shows the behavior of a slowly pulsating B star (SPB) with three additional pulsational frequencies. Hence it is considered a hybrid pulsator. Its magnitude varies between +2.78 and +2.89 over the course of each pulsation cycle. In 2025, interferometric observations discovered a companion around the star.

The primary is a large star with 8.8 times the mass of the Sun and 5.5 times the Sun's radius. The stellar classification of B2 IV suggests this is a subgiant star that is exhausting the hydrogen at its core and is in the process of evolving away from the main sequence. With a projected rotational velocity of Template:Val, it is either rotating very slowly or it is being viewed from nearly pole-on. Gamma Pegasi A has a total luminosity of 6,000 times that of the Sun, which is being radiated from its outer atmosphere at an effective temperature of more than 22,000 K. At this temperature, the star glows with a blue-white hue. Algenib has a weak magnetic field (from -10 G to 30 G, an upper bound on a dipolar magnetic field strength of about Template:Val).

The secondary has an angular separation of Template:Val from its primary, little is known about it.