BETA SCT (Beta Scuti). It's no wonder that Johannes Hevel
(Hevelius), when inventing his mostly northern modern
constellations in the late 1600s, placed a shield (Scutum, in honor of John Sobieski, King of Poland from
1674 to 1696) between Aquila and Sagittarius, where the Milky Way brightens in a patch reminiscent
of that piece of defensive armor. While the stars of Scutum
themselves hardly form anything that looks like a shield, they are
easily found as they bracket the Milky Way's lovely small star
cloud, with Beta Scuti on the northern side, and Alpha and the famed variable star Delta
Scuti on the southern. Of Hevelius's seven constellations that
were finally adopted in the twentieth century, Scutum was the only
one not accepted by John Flamsteed, who considered it part of Aquila. As a result, Scutum has no Flamsteed numbers of its own,
only those of Aquila. Beta Sct is thus also 6 Aquilae, and Alpha
and Delta are respectively 1 and 2 Aql, names that are never used
(imagine the confusion). Since the constellation was not invented
until well after Bayer's time,
the Greek letters were supplied later.
Peculiar history aside, Beta Scuti is a fourth magnitude (4.22)
yellow-white class G (G5) bright giant at a distance of 690 light
years. It's far enough away within the
Milky Way to suffer just over half a magnitude of
absorption by interstellar dust, which if absent would boost the
star to magnitude 3.71. Distance and a temperature of 4700 Kelvin
(to allow for infrared radiation) yield a luminosity 1800 times
that of the Sun. But hold for a moment: the
star is a spectroscopic double
with a period of 2.28 years. The companion has been listed as a
class A (A0) hydrogen-fusing dwarf, which suggests a luminosity of
around 40 Suns that (ignoring the differences in infrared emission)
should be subtracted from the total, making the giant shine at 1760
Suns. Luminosity and temperature then yield a radius of 64 times
that of the Sun (0.30 Astronomical Units, three-fourths the size of
Mercury's orbit), and a healthy mass 5.5 times solar. A slow
projected equatorial rotation speed of 8 km/sec gives it a rotation
period that could be as long as 400 days. Having begun life as a
hot class B3 star some 75 million years ago, it is now quietly
fusing its internal helium into carbon and oxygen. Metal
abundances are about 60 percent of solar, typical of such local
class B stars. It also seems to have a slight carbon-nitrogen
enhancement. If we assume that the class A companion carries 2.5
solar masses, then the average separation between the two is 3.5
AU. From the companion (which will take another 500 million years
before it too begins to become a giant), Beta Sct proper would
appear some 10 degrees across.
Update: The new Hipparcos reduction places Beta Sct much farther
away, at a distance of 916 light years (with an uncertainty of
about 80 l-y). The luminosity of the primary (Beta Sct A) is thus
raised to 3100 Suns, which almost takes it into the supergiant
realm. The radius is then moved upward to 84 times that of the
Sun, rendering it a dozen degrees across as seen from the
companion. From the temperature and the new luminosity, if the
star is still evolving with a dead helium core, the mass must be
around 7 Suns, but in the more likely case that helium has already
fired to fuse to carbon, it is reduced to a still-mighty 6 Suns,
the age some 55 million years.
Written by Jim Kaler 10/26/07; updated 4/29/11.
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