Algenib (Gamma Pegasi)
Algenib (Gamma Pegasi) is a star in the constellation Pegasus, magnitude 2.8.
A blue-white heavyweight at a corner of the Great Square, swelling and shrinking every 3.6 hours.
Every 3.6 hours Algenib swells and shrinks. The change in its light is tiny — about a tenth of a magnitude — but it marks the star as a Beta Cephei variable star, one of a family of hot, massive stars that throb in and out. Slower rhythms ride on top of the fast one, so astronomers call it a hybrid pulsator.
Behind the pulse is a blue-white heavyweight of nearly nine solar masses, five and a half times the Sun’s width and about 6,000 times as luminous, with a surface hotter than 22,000 kelvins. Its spectral type, B2 IV, marks a subgiant: running low on hydrogen in its core and starting to leave the main sequence. In 2025 interferometers found a close companion, about which almost nothing is yet known.
Its name was once shared with Alpha Persei — a muddle the IAU settled in 2016 by naming Perseus’s star Mirfak. In Hindu astronomy, Algenib and Alpheratz together form the 26th nakshatra, or lunar mansion, Uttara Bhādrapadā.
Algenib (Gamma Pegasi) is 390 light-years away: the light we see from it set off around 1635, 25 years after Galileo first turned a telescope to the sky.
Did you know?
- Caught by its spectrum. In 1911 Keivin Burns noticed Algenib’s spectral lines shifting back and forth. In 1953 D. Harold McNamara showed why: the whole star is pulsating.
- Slow spinner — or pole-on. Hot stars usually spin fast, but Algenib’s spectrum shows almost no rotation. Either it turns unusually slowly, or we are looking down on one of its poles.
- First Star of the Wall. Chinese astronomers grouped Algenib with Alpheratz in the asterism Bìxiù, the Wall. Algenib is its first star.
The south-east corner of the Great Square of Pegasus, a blue-white star of 3rd magnitude. Compare it with Alpheratz, the next corner to the north: Algenib is clearly the fainter. Its pulsations are far too small to see by eye.
When and where to see it
It is best placed in the evening sky around October, when it crosses the meridian at about 10 pm local time (around midnight in September). It can be seen from everywhere north of 75°S and never sets north of 75°N.
At its highest it stands 55° up from 50°N, 70° up from 35°N, 75° up from the equator, 40° up from 35°S.
Facts
- Type
- Star
- Constellation
- Pegasus
- Designations
- Gamma Pegasi, γ Peg, 88 Peg, HR 39, HD 886, HIP 1067
- Magnitude
- 2.8
- Position
- RA 0h 13.2m, Dec +15° 11′ (J2000)
- Spectral type
- B2IV
- Colour
- blue-white (B−V -0.23)
- Distance
- 390 light-years
- Discovered
- Known since antiquity
Open AstroPilot to see when Algenib (Gamma Pegasi) is highest tonight from your location, how easy it is with your telescope or binoculars, and an eyepiece view.