IOTA-1 SCO (Iota-1 Scorpii). Supergiants are rare, and to find two of them
within a quarter-degree of each other is rarer still. To find two
forms of supergiants that for the class are rare in themselves
draws the eye, or at least should. Nevertheless, these two
magnificent stars, which have no proper names and are known as
Iota-1 (the western and brighter) and
Iota-2 Scorpii, are quite
neglected, perhaps by contrast to Scorpius's brighter magnificent stars, which include
one of the most prominent of all supergiants, Antares. Iota-1 is
a mid-third magnitude (3.02) yellow-white class F (F2) supergiant
that visually dominates its fourth magnitude (4.81) neighbor, the
class A (A2) supergiant Iota-2. The alignment is only accidental,
however, Iota-2 roughly double Iota-1's distance. Iota-1, the
focus of this story, is so far away that its distance is known only
to an accuracy of about 40 percent. From the uncertain parallax
and distance of 1800 light years, plus a correction for significant
dimming (a factor of two) by the Milky Way's interstellar dust, we
derive a luminosity 29,000 times that of the Sun, the temperature of 6700 Kelvin showing
that most of the light is emitted in the visual spectrum where we
can see it. Luminosity and temperature tell of a 12 solar mass
star with a radius 125 times that of the Sun, or about 60% the size
of Earth's orbit. Rotating at speed of at least 36 kilometers per
second at the equator, it takes at least half a year to make a full turn.
Even this great calculated luminosity is too low for a "class Ia"
supergiant like Iota-1. If it is really like its "Ia" siblings,
the star is really four times as luminous and must be twice as far
away as the parallax indicates, which bumps the mass to 20 solar.
Most supergiants are red, or at least hot and blue. Even at 20
solar masses, "yellow" supergiants such as this one are quite rare,
and are in a state of evolutionary transition in which (with dead
helium cores) they are in the process of becoming red supergiants.
(That there seem to be a good number of such supergiants in the sky
is an artifact caused by their great luminosities.) Now losing
mass at a rate of about a tenth of a millionth of a solar mass per
year, over a million times greater than the flow rate of the solar
wind, the star is a good candidate for exploding as a supernova.
A dim tenth magnitude companion lies 37 seconds of arc away. If
the pairing is real, and not line-of-sight, the little one must be
a class F dwarf a bit more massive than the Sun that orbits at
least 20,000 Astronomical Units from the big one and takes at least
800,000 years to make a full circuit. Thanks to Jeff Bryan, who
suggested this star.