IOTA-2 SCO (Iota-2 Scorpii). In spite of the number of supergiants in the sky, they are truly rare stars. Not only are they the progeny of the exceedingly rare blue class O stars (the most massive of "normal" stars that fuse hydrogen in their cores), they do not live in the supergiant state (in which they have dead helium cores, or are helium-burners, or are in even more advanced states) as long as the short-lived O stars do. We see a number of them only because they are very bright. Most supergiants are red (class M) or blue (class B). To find a pair of supergiants that belong to neither of these classes within a quarter degree of each other suggests that they must somehow be physically related. Such would seem the case for Iota Scorpii, the star at the eastern bend of the tail of Scorpius, the Scorpion. It is actually two stars, Iota-1, a third magnitude class F bright supergiant, and Iota-2, a fifth magnitude (4.81) class A (A2) supergiant that is a lesser version of Deneb in Cygnus. If Iota-1 and Iota-2 are related, the two stars would have the same distances. Iota-1 is measured to be 1790 light years, away, Iota-2 3700 light years, so that would seem to settle the issue. Yet the distances are so great that the measurement errors are huge, enough to allow the distances actually to be the same. The matter is resolved by the line-of-sight radial velocities, Iota-1 approaching us at 28 kilometers per second, Iota-2 at 10. They cannot be related. The amount of ultraviolet light from Iota- 2's 8800 Kelvin surface is trivial. After we have factored in 0.6 magnitudes of interstellar dust absorption (which corresponds to a factor of 1.7 in brightness), the highly uncertain distance of Iota-2 suggests a star with a luminosity 21,000 times that of the Sun, which is actually close to what we would expect from a star of this class, showing the distance to be fairly good. Temperature and luminosity give a radius 60 times solar. From Iota-2's minimum rotation speed of 39 kilometers per second, the star takes over 80 days to make a full turn. Half a minute of arc away lies an 11th magnitude "companion" about which nothing is known. If the star is really related, it must be a class F dwarf not much bigger than the Sun. Given the thickness of stars in the local Milky Way, the pairing is probably just a coincidental alignment. Like all supergiants, Iota-2 is losing mass, though at a relatively low rare of about 2 billionths of a solar mass per year. Since the star's mass hovers around 12 times that of the Sun, and is perhaps a bit less, its fate is uncertain. It will either explode or, if the mass is indeed a bit on the low end of the range, will turn into a rare massive white dwarf, perhaps one made not of carbon and oxygen (as most white dwarfs are) but of neon, oxygen, and magnesium.
Written by Jim Kaler. Return to STARS.