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Sierra NightSky

"Let your soul stand cool and composed before a million universes" Walt Whitman

Sierra NightSky for the period starting Setember 12, 2014 by Jim Kaler

The Moon begins our week in its late waning gibbous phase, passing third quarter the night of Monday, September 15, shortly before Moonrise in North America. It then fades as a waning crescent heading towards new Moon the middle of next week, on Wednesday the 24th. The morning of Friday the 19th the crescent will make a fine sight near Jupiter, shining up and to the right of the giant planet as they rise in the east.

The two pairs of planets that have been gracing the evening and morning skies are separating. Mars, not quite keeping up with Earth as they orbit, moves toward its namesake Antares in Scorpius, while Saturn dives downward toward the early evening horizon, setting around 9:30 PM Daylight Time, less than an hour after the end of twilight.

Mars follows shortly thereafter. The morning scene is more extreme as Venus falls out of sight, rising well after dawn's onset. Jupiter on the other hand rises brilliantly in full darkness around 3:30 AM in Cancer to the southeast of the Beehive cluster. By the end of October, it will be rising before local midnight (1 AM Daylight Time).

In mid-evening the Summer Triangle of Vega (in Lyra),Deneb (Cygnus), and Altair (Aquila) straddles the celestial meridian, Hercules to the west, the Great Square of Pegasus, a sign of fall, well to the east.

Far to the south lies Sagittarius, its Little Milk Dipper stuck into the brightest part of the Milky Way. With the Moon pretty much out of the Way, the milky stream is at the height of its evening glory, its dark clouds signaling regions of star formation.

STAR OF THE WEEK: 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.

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 328 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.

Do you have a favorite star or one you would like to see highlighted on the Star of the Week? Send a suggestion to Jim Kaler

Sierra NightSky thanks to Jim Kaler.
Check out his site for more




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