V391 Pegasi

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V391 Pegasi
Observation data
Epoch J2000.0      Equinox J2000.0
Constellation Pegasus
Right ascension  22h 04m 12.2s
Declination +26° 25′ 08″
Apparent magnitude (V) +14.57
Spectral type sdB
Variable type V361 Hydrae (or sdBVr)
Distance4570 ly
(1400 pc)
Absolute magnitude (MV)+3.84
Mass0.5±0.05 M
Radius0.23±0.03 R
Luminosity15.4 L
Surface gravity (log g)5.15 cgs
Temperature29300 ± 500 K
Age>10 × 109 years
Other designations
HS 2201+2610
Database references

V391 Pegasi, also catalogued as HS 2201+2610, is a blue-white subdwarf star approximately 4,570 light-years away in the constellation of Pegasus. The star is classified as an "extreme horizontal branch star". It is small, with only half the mass and a bit less than one-quarter the diameter of the Sun. It has luminosity 15.4 times the Sun.[citation needed] It could be quite old, perhaps in excess of 10 Gyr. It is a pulsating variable star of the V361 Hydrae type (or also called sdBVr type).[citation needed]


Subdwarf B stars such as V391 Pegasi are thought to be the result of the ejection of the hydrogen envelope of a red giant star at or just before the onset of helium fusion. The ejection left only a tiny amount of hydrogen on the surface - less than 1/1000 of the total stellar mass. The future for the star is to eventually cool down to make a low mass white dwarf. Most stars retain more of their hydrogen after the first red giant phase, and eventually become asymptotic giant branch stars. The reason that some stars, like V391 Pegasi, lose so much mass is not well known.[citation needed]

Planetary system[edit]

In 2007, a gas giant planet designated V391 Pegasi b was found in orbit by variable star timing method. This planet around an "extreme horizontal branch" star provided clues about what could happen to the planets in the Solar System when the Sun turns into a red giant within the next 5 billion years.[citation needed]

The V391 Pegasi planetary system
(in order from star)
Mass Semimajor axis
Orbital period
Eccentricity Inclination Radius
b >3.2±0.7 MJ 1.7±0.1 1170±44 0.00



  • R. Østensen; J.-E. Solheim; U. Heber; R. Silvotti; et al. (2001). "Detection of pulsations in three subdwarf B stars". Astronomy and Astrophysics. 368 (1): 175–182. Bibcode:2001A&A...368..175O. doi:10.1051/0004-6361:20000488. Retrieved 2007-09-16.
  • C. S. Jeffery (2005). "Pulsations in Subdwarf B Stars". Journal of Astrophysics and Astronomy. 26 (2–3): 261. Bibcode:2005JApA...26..261J. doi:10.1007/BF02702334.

External links[edit]

Coordinates: Sky map 22h 04m 12.2s, +26° 25′ 08″