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.