IOTA HER (Iota Herculis). Iota Herculis, of no proper name, is the "next to the last stop" before Hercules runs into Draco (Tau Her just a bit more to the north), and is so close to Draco's head that it more appears to be a part of the Dragon rather than part of the Hero. As Hercules is traditionally placed upside down, Iota represents (as seen in the Uranometria) one of the figure's legs. Iota Her is a hot (17,800 Kelvin) class B (B3) subgiant, a star whose internal hydrogen fuel has just run out (or will shortly) and is preparing to become a true giant. At a distance of 495 light years, the star radiates a power 2500 times that of our Sun, its mass 6.5 times solar, its radius 5.3 solar. As massive stars evolve quickly, Iota is but 45 or so million years old. For a class B star, it is a very slow rotator, spinning at a mere 10 kilometers per second at its equator (5 times faster than the Sun), which really suggests that we are viewing the star nearly pole-on (such that it is really rotating much faster than we see). It is also slightly metal-poor, its heavy element content 60 percent that of the Sun. The star has two other significant characteristics. First, it is a good example of the little-discussed class "slowly pulsating blue variables," which seem to be something of a cooler version of the far more well-known "Beta Cephei stars," which chatter away with multiple sub-day pulsation periods. Iota Her has a principal period of 4.48 days, combined with three others, which produce variations far too small to be seen with the human eye. It is also a multiple star. Iota Her is foremost a "spectroscopic binary" with a 113.804 day period, implying a separation of about one Astronomical Unit (the distance between Earth and Sun). At a distance of roughly 30 AU lies another companion that should take somewhere around 60 years to make a circuit. Much farther out, nearly two minutes of arc away, is yet another, a dim star of magnitude 11.8 that lies over 18,000 AU away and that takes a good million years to make a circuit. At such a distance we might assume that this companion would be just a line of sight coincidence, but over the past century, it and Iota proper are seen to move through space together. At 495 light years, this distant neighbor would be a class K1 dwarf (appearing from Iota itself like a yellow-orange "Venus"), one that Galactic tides will surely set free.
Written by Jim Kaler. Return to STARS.