ALPHA SCT (Alpha Scuti). Eighty-eight formal constellations grace the sky (along with many more informal ones plus figures that have long-since fallen into disuse). Over half come down from ancient times. The rest are of "modern" origin, that is, created between about 1600 and 1800 as astronomers furiously competed with "filling in the blanks" between the ancient constellations. Lying in the Milky Way between ancient Aquila and Sagittarius is the modern constellation Scutum, the Shield, which honors the Polish king John Sobieski for his defense of Vienna in 1683. Though the stars are faint, the northern part of the Shield stands out mightily as a bright patch of the Milky Way. None of its stars carry proper names, even the brightest of them, which is known simply as Alpha Scuti, abbreviated as an unpronounceable "Alpha Sct." Alpha Scuti, which shines at only fourth magnitude (3.85), is yet one more orange class K giant, though one with a bit of a difference, at class K3 slightly cooler than most, its temperature measured at 4300 Kelvin. From its distance of 175 light years it radiates 132 solar luminosities from a surface swollen to 21 times that of the Sun, about a quarter the size of the orbit of Mercury. Decidedly single, Alpha Scuti is reported to be slightly variable, its brightness varying by about 10 percent. No one seems to know the period of oscillation, however (if indeed it has one), or what class of variable it might be. The most interesting aspect of the star is its ambiguous evolutionary status. As solar-type stars age after they cease hydrogen fusion in their cores, they (1) brighten as they become giants with dead helium cores; (2) shrink a bit as they begin to fuse helium to carbon; (3) fuse carbon; (4) brighten even more with dead carbon cores; (5) lose their outer envelopes and become dim white dwarfs (like Sirius B and Procyon B). Alpha Scuti, a 1.7 solar mass star that is at least 2 billion years old, could be in state 1, 2 or 4, which somewhat overlap. That the star is variable suggests that it might indeed be in state 4, and beginning to brighten for the last time.

Update: The new Hipparcos reduction gives a larger distance of 199 light years (give or take 2), which raises the luminosity to 171 Suns and the mass to around twice solar.
Written by Jim Kaler 9/07/01; updated 4/29/11. Return to STARS.