OMI HER (Omicron Herculis). As in real estate, for this star it's
mostly "location, location, location." In southeastern Hercules, but closer to classical Lyra, Omicron Herculis lies within a set of
stars that shows that Bayer used position as well as brightness when
he assigned Greek letters on his great
atlas, the Uranometria of 1603. Sadly it once had
a proper name, Masym (the wrist), that was later
transferred to Lambda Her, leaving Omicron
bereft of the honor. Rather bright (ok, fourth
magnitude, but 3.83), it's mixed in with a string that includes
Lambda and Nu
Herculis, both of magnitude 4.41, along with brighter (3.42) Mu Her. More importantly, with Nu, Xi, and
99 Herculis, Omicron forms a rough box that contains the Apex of the Sun's Way, the point
among the local bright stars towards which the Sun is moving at a
stately pace of 20 kilometers per second. Near the border with Lyra, the Apex depends some on the stars
selected. This particular version is for "standard solar motion."
If we pick the nearest stars to be our set, the Apex for "basic solar
motion" lies 4.5 degrees to the south-southwest in the direction of
89 Herculis. Nothing, it seems, is ever
simple.
That's two "locations." The third might be the distance of 338 light
years (give or take 6) for this class B (B9.5, right on the edge of
class A) dwarf, or in some
circles, giant (the difference not great). The measured
temperatures range from 8600 to 10,590 Kelvin, not so good. From
the average of 9660, we can calculate the amount of ultraviolet radiation, which
with the rest of the electromagnetic spectrum yields a total
luminosity of 296 times that of the Sun and
a radius of 6.2 times solar. The star stands out mostly for its
equatorial rotation speed of at least 171 kilometers per second,
which gives a rotation period of under 1.8 days (as opposed to the
25-day rotation period of the Sun). The high rotation stirs up the
stellar gases, preventing any sort of diffusion of the elements that
leads to odd chemical abundances at the stellar surface. It also
makes the star somewhat oblate, which could account for the range
in measured temperatures, as oblate stars are hotter at the poles
and cooler at their equators (a phenomenon oddly called "gravity
darkening"). Going along with the rapid rotation is a circumstellar
disk that radiates emission lines, making Omi Her into a "Be star"
whose icons are Gamma Cassiopeiae, Zeta Tauri, and Delta Scorpii. Omi Her hardly matches
their activity levels, however, as Gamma Cas has been seen to vary
by a couple magnitudes, Delta Sco by half a magnitude, whereas Omi
Her changes by perhaps a tenth of a magnitude. Theory yields a mass
of 3.5 times that of the Sun and shows Omi Herculis actually to be
a subgiant that has just
given up hydrogen fusion, the star thus falling right between its
two spectral classifications. With a current age of about 240
million years, it will someday eject its outer layers, leaving a white dwarf of 0.70 or so
solar masses behind. Is there a companion there to witness the
action? An old suggestion gives one at a separation of 0.1 seconds
of arc, but it has not been confirmed and most likely does not exist.
Written byJim Kaler 9/12/14. Return to STARS.