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.