KITALPHA (Alpha Equulei). Most ancient constellations dominate their parts of the sky. A few, however, are remarkably obscure and hard to find, among them Equuleus, the Little Horse, its brightest star (Kitalpha) only mid-fourth magnitude (3.92). Though Kitalpha is indeed the Alpha star, the "alpha" part of the proper name has nothing to do with the Greek letter. The name actually comes from a short Arabic phrase that means "part of a horse," as only the head and shoulders of little Equuleus are represented in the sky. From the star's distance of 186 light years, we calculate a luminosity 75 times that of the Sun. However, the star deceives. Kitalpha is not one star, but two, so close together that the class is mixed, G + A, two spectra present at the same time. The brighter of the two (by about 50%, 45 times brighter than the Sun) is a class G (G0) giant, the fainter a white class A (A5) dwarf. The giant is in the process of dying and most likely (like the dimmer component of Capella) has a contracting helium core, while the dwarf is a common unevolved hydrogen-fuser. The blend of starlight makes actual temperatures difficult to measure, but should respectively be around 5500 and 8500 Kelvin. What makes the star especially interesting is that it is one of the few close doubles whose orbits have been calculated from the spectrum (through use of Doppler shifts, which relate subtle changes in spectral wavelengths to velocities) and by direct observation. The latter was accomplished with the US Navy's Mark III Interferometer, which makes use of the ability of light waves to interfere with each other and that allows extraordinary resolution of detail. Kitalpha-A, the giant, is typically separated from Kitalpha-B (the ordinary dwarf) by a mere 0.01 seconds of arc, the angle made by a US penny (2 centimeters across) at a distance of 400 kilometers! Orbiting in nearly circular paths around each other every 99 days, the two are separated by 99 million kilometers, only 0.66 Astronomical Units, 90 percent Venus's distance from the Sun. To be dying first, the giant must be the more massive, and from orbital calculations consistently weighs in at 2.1 solar masses, whereas the dwarf contains 1.9 solar masses. Approximately half a billion years ago, when the system was born, it consisted of a pair of white class A stars, one just a bit brighter than the other. The fainter will follow its companion to gianthood before another half a billion years has elapsed. The giant will eventually expand further, and may someday encroach upon its neighbor, leading to mass transfer between the stars and some interesting celestial fireworks.
Written by Jim Kaler. Return to STARS.