Observation
This star system was catalogued in the 1862 Bonner Durchmusterung ("Bonn astrometric Survey") as BD +18°4794B. It later appeared in Pickering's 1908 Harvard Revised Photometry Catalogue as HR 8210. The designation "IK Pegasi" follows the expanded form of the variable star nomenclature introduced by Friedrich W. Argelander.
Examination of the spectrographic features of this star showed the characteristic absorption line shift of a binary star system. This shift is created when their orbit carries the member stars toward and then away from the observer, producing a doppler shift in the wavelength of the line features. The measurement of this shift allows astronomers to determine the relative orbital velocity of at least one of the stars even though they are unable to resolve the individual components.
In 1927, the Canadian astronomer William E. Harper used this technique to measure the period of this single-line spectroscopic binary and determined it to be 21.724 days. He also initially estimated the orbital eccentricity as 0.027. (Later estimates gave an eccentricity of essentially zero, which is the value for a circular orbit.) The velocity amplitude was measured as 41.5 km/s, which is the maximum velocity of the primary component along the line of sight to the Solar System.
The distance to the IK Pegasi system can be measured directly by observing the tiny parallax shifts of this system (against the more distant stellar background) as the Earth orbits around the Sun. This shift was measured to high precision by the Hipparcos spacecraft, yielding a distance estimate of 150 light years (with an accuracy of ±5 light years). The same spacecraft also measured the proper motion of this system. This is the small angular motion of IK Pegasi across the sky because of its motion through space.
The combination of the distance and proper motion of this system can be used to compute the transverse velocity of IK Pegasi as 16.9 km/s. The third component, the heliocentric radial velocity, can be measured by the average red-shift (or blue-shift) of the stellar spectrum. The General Catalogue of Stellar Radial Velocities lists a radial velocity of -11.4 km/s for this system. The combination of these two motions gives a space velocity of 20.4 km/s relative to the Sun.
An attempt was made to photograph the individual components of this binary using the Hubble Space Telescope, but the stars proved too close to resolve. Recent measurements with the Extreme Ultraviolet Explorer space telescope gave a more accurate orbital period of 21.72168 ± 0.00009 days. The inclination of this system's orbital plane is believed to be nearly edge-on (90°) as seen from the Earth. If so it may be possible to observe an eclipse.
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