Alpha Equulei is the brightest star in the constellation Equuleus, the Little Horse, at apparent magnitude +3.92. It also has the traditional name Kitalpha or Kitalphar, which is a contraction of the Arabic title "Qit'at al Faras", meaning "Part of the Horse".
Properties
Kitalpha is approximately 186 light years from Earth, and has a total luminosity of 75 suns. However, Alpha Equulei is actually a spectroscopic binary, consisting of two components so close together that their spectral class is mixed. The brighter of the two (by about 50%, 45 times the Sun's luminosity) is a class G0 III giant, the fainter a white class A5 V main-sequence star. The blend of starlight makes actual temperatures difficult to measure, but should respectively be around 5500 and 8500 Ks.
Kitalpha is one of the few close doubles whose orbits have been calculated from their spectrum (through use of Doppler shifts) and by direct observation (by interferometry). The stars are typically separated by 0.01", or only 0.66 AU, orbiting around each other in nearly circular paths every 99 days.
Evolution
When the system was born, approximately half a billion years ago, it consisted of a pair of white class A stars, one just a bit brighter than the other. Now, the giant is dying and most likely has a contracting helium core, while the main-sequence star is still fusing hydrogen. To be dying first, the giant must be the more massive, and from orbital calculations consistently weighs in at 2.1 solar masses, whereas its companion contains 1.9 solar masses. The fainter will follow its companion into gianthood before another half billion years have elapsed. The giant will eventually expand further, and may someday encroach upon its neighbor, transferring mass between the stars.
[Adapted from STARS by Jim Kaler, Professor Emeritus of Astronomy, University of Illinois]