IM Pegasi

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IM Pegasi
Observation data
Epoch J2000.0      Equinox J2000.0
Constellation Pegasus
Right ascension 22h 53m 02.2659s
Declination +16° 50′ 28.298″
Apparent magnitude (V) 5.892 (var. 5.7–5.9)
Spectral type K2 III / G3 V
Variable type RS CVn
Radial velocity (Rv) -12.3 km/s
Proper motion (μ) RA: -20.97 mas/yr
Dec.: -27.59 mas/yr
Parallax (π) 10.33 ± 0.76 mas
Distance 320 ± 20 ly
(97 ± 7 pc)
Orbit[1] [2]
Primary K2III
Companion unseen G3V
Period (P) 24.64877±0.00003 d
Eccentricity (e) 0.00
Inclination (i) 65° ≤ i ≤ 80°
Semi-amplitude (K1)
34.29±0.04 km/s
Semi-amplitude (K2)
62.31±0.06 km/s
K2III primary
Mass 1.6–2.0 M
Radius 13.3±0.6 R
G3V secondary
Mass 0.9–1.1 M
Radius 1.00±0.07 R
Other designations
HD 216489, HIP 112997, HR 8703, SAO 108231
Database references

IM Pegasi is a variable binary star system approximately 329 light-years away in the constellation of Pegasus. With an apparent magnitude of 5.7, it is visible to the naked eye. Increased public awareness of it is due to its use as the guide star for the Gravity Probe B general relativity experiment.[3] It was chosen for this purpose because its microwave radio emissions are observable with a large radio telescope network on the ground in such a manner that its precise position can be related by interferometry to distant quasars.

The two components of the binary system includes a K-type giant star and a G-type main sequence star. The primary star is estimated to be 1.8 times as massive and 13 times the diameter of the Sun. The secondary star is estimated to be similar to the Sun in size and mass. They orbit their common barycenter in a period precisely estimated to be 24.64877 days.


  1. ^ "The long-period RS Canum Venaticorum binary IM Pegasi. I. Orbital and stellar parameters". Astronomy and Astrophysics, v.347, p.932-936 (1999). 
  2. ^ "A Sun in the Spectroscopic Binary IM Pegasi, the Guide Star for the Gravity Probe B Mission". The Astrophysical Journal, 634:L173-L176, 2005 December 1. 
  3. ^ "Gravity Probe B confirms Einstein effects". BBC Online. 4 May 2011. Retrieved 5 May 2011. 

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