SULAFAT (Gamma Lyrae). An exquisite quintet of fainter stars lies
next to brilliant Vega, and with this great
luminary make Lyra, the celestial Lyre.
Anchoring the harp's southern end are a close pair of stars, Sheliak and Sulafat. Both names refer to
the harp itself, "Sulafat" meaning "tortoise" in Arabic and
referring not to the animal but to the shell that makes the lyre's
musical body. Toward the fainter end of third magnitude (3.24) and
second brightest in the constellation (a bit brighter than
Sheliak), the star still received the Gamma designation from Bayer,
he apparently lettering Lyra's stars in something of a spiral
pattern. Sulafat lies at the cool end of class B and has a color
very similar to Vega. Unlike Vega, however, it is a giant star.
From its rather large distance of 630 light years (and factoring in
a bit of ultraviolet radiation and absorption by interstellar
dust), we find a total luminosity 2100 times that of the Sun, which really places the star in the
region of "bright giants," its luminosity and temperature of 10,000
Kelvin combining to give a diameter of 15 solar. Otherwise,
Sulafat is quite ordinary, which in itself makes it interesting.
It is one of the few evolved stars that quite unambiguously tells
us just what it is doing. Its temperature and luminosity, combined
with the theory of stellar ageing, yield a mass just a bit over
five times that of the Sun. The star, its death cycle underway,
ceased fusing the hydrogen in its core about 150,000 years ago, the
quiet helium core now surrounded by a hydrogen fusing shell. In
another 200,000 years Sulafat will have chilled to become an orange
giant and will begin to brighten as the core helium begins to fuse
to carbon and the star (eventually losing its outer layers)
prepares to become a heavy carbon-oxygen white dwarf, a dense
shrunken cinder about the size of Earth. Sulafat is perhaps best
known as a comparison for other celestial sights. It makes a
perfect reference with which to see the variations of neighboring
Sheliak. Then it and Sheliak almost exactly bracket one of the
sky's most wondrous sights, the famed "Ring Nebula in Lyra." Even
a small telescope pointed between the stars reveals a celestial
smoke ring, a classic "planetary nebula" (so called because of its
disk-like appearance). The Ring is the ejected outer envelope of
a star that is now effectively a white dwarf. Sulafat will produce
one quite like it some 25 million years down the road.