89 HER (89 Herculis). To the eye, Flamsteed 89 of Hercules presents nothing remarkable. Closer examination with the spectrograph, however, shows it to be a class F (F2) "yellow supergiant" (never mind that it is really white -- the "yellow" is a jargon term for its mid-range temperature of around 6500-7000 Kelvin). A close superficial match is Iota-1 Scorpii. However, 89 Herculis is in the wrong place! Supergiants are the immediate progeny of massive class O and hot B dwarfs, and stick to the plane of the Galaxy, the Milky Way, where massive stars are created by thick clouds of interstellar matter. "89," however, is rather well off the Milky Way. Something is amiss. Indeed, 89 Herculis is the only naked eye example of an extremely rare type of star, one highly evolved and in the last stages of its death. Slightly variable (by just under a tenth of a magnitude over a period of 65 days), 89 Her is a member of the somewhat similar class of "UU Herculis stars." Study is limited by the lack of a good distance. Direct parallax gives about 3000 light years, but the measurement errors are quite high and the distance not reliable. Given that distance and factoring in a bit of absorption of starlight by interstellar dust, 89 Her shines with the light of 7000-9000 Suns. The star is also a strong source of infrared radiation that comes from heated dust grains that lie in a huge surrounding gaseous shell (made of hydrogen but rich in oxygen), whose inner edge lies 100 Astronomical Units (2.5 times Pluto's distance from the Sun) from the star itself, and whose outer edge extends 100 times farther. The star, creator of the shell through a powerful wind, has a radius about 60 times solar. Emission lines in the spectrum tell of continuing mass loss. "89" has been through the major phases of internal core fusion, first the fusion of hydrogen to helium, then helium to carbon and oxygen (much as Mira, Omicron Ceti, is doing now). In the jargon of astrophysics, Mira -- a long-period variable -- is an "asymptotic giant branch" (AGB) star, one that is brightening as a giant for the second time with a dead carbon core (the first brightening is with a dead helium core). At the end, such stars slough off their outer envelopes in powerful winds. "89" thus represents the next stage for Mira, and as such is a remarkable "post-AGB star" that is in the process of baring its ancient nuclear-burning heart. As the interior star -- which is only superficially a "supergiant" -- heats with the removal of its outer layers, it will illuminate the fleeing shell of its own making and create a beautiful "planetary nebula," an illuminated expanding shell of dusty gas of the kind depicted so well by Hubble images. The star inside will thereafter shrink to become a feeble white dwarf. You can see this rare transition star with no telescope at all, and while it at first appears like any other, the mind can conjure its real and amazing nature.
Written by Jim Kaler. Return to STARS.