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. Return to STARS.