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.