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.
Written by Jim Kaler. Return to STARS.