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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 November 6, 2015 by Jim Kaler

The next skylights will appear November 20, 2015.

The Moon begins our fortnight in its waning crescent phase. Having passed just south of Jupiter the morning of Friday, November 6, it then makes a marvelous triangle with Venus and much fainter Mars the morning of Saturday the 7th, the crescent just a degree south of the brilliant planet and below the red one. The morning of Sunday the 8th, you might spot the Moon up and to the left of the star Spica. Thinning completely out, the Moon passes new around noon on Wednesday the 11th, so your last glimpse of it will be the morning of Tuesday the 10th in growing eastern dawn.

We get to see the Moon again as a waxing crescent in fading western twilight the evening of Thursday the 12th when it invisibly passes a few degrees north of Saturn. The crescent then fattens until first quarter is reached the night of Thursday the 18th, after which the Moon enters the waxing gibbous phase. The Moon passes its apogee (when it is farthest from Earth and angularly smallest, though you can't really tell the difference) on Saturday the 7th.

The two bright planets, Venus and Jupiter, continue to grace the morning sky. In the middle of our term, Jupiter (now in southern Leo well to the southeast of Regulus and south-southwest of Denebola) rises around 1:30 AM as it heads toward the evening sky in mid-December. Venus follows, up by about 3 AM. While brighter Venus rises ever-earlier in the morning sky, Jupiter rises ever later, and thus the two are slowly separating. In between is Mars, which, while bright (just shy of first magnitude), is outmatched by the other two. Saturn is effectively gone, lost to evening twilight. Mercury is in superior conjunction with the Sun (on the other side of it) on Tuesday the 17th, while Neptune, in Aquarius, ceases retrograde motion as our period ends.

The famed Leonid meteor shower peaks the night of Tuesday the 17th (really the following morning), but don't expect much this year, as the dense cloud of debris from Comet Tempel-Tuttle is long gone. In a dark sky one might make out 15 or so meteors per hour radiating from the constellation Leo.

Eyes are now on the elegant Great Square of Pegasus, the Flying Horse, as it soars high to the south in mid-evening. Lesser known is Equuleus, the Little Horse, that gallops faintly to the southwest of its much larger mate. Directly east of the Square find the flat triangle that makes most of Aries, the Ram, which in ancient times held the Vernal Equinox, that point now in Pisces as a result of the 26,000-year wobble of the Earth's axis. From off the northeast corner of the Square comes the string of stars that best identifies Andromeda. Then later in the evening look to the east to see the stars of winter, from Auriga to Orion, begin to replace those of autumn.


About 10 degrees northwest of the Circlet of Pisces is a pair of stars a degree apart, fifth magnitude Rho (4.90) then Sigma (5.16) Pegasi (in Pegasus, the Flying Horse of the Andromeda myth), that point also northwesterly toward fourth magnitude Xi Peg. Rho is a class A1 dwarf 312 light years away.

Much closer, at a distance of 89.0 light years (with an uncertainty of just 0.7), Sigma is a class F (F7) subgiant more like the Sun. It also carries a special treasure, a faint companion called Sigma B, rendering bright Sigma itself "Sigma A," which is usually known by its Flamsteed number, 49 Pegasi.

Sigma Peg proper has been subjected to a two dozen temperature measures that average to a pretty precise 6245 Kelvin. With little but visual radiation to deal with, the distance gives a luminosity of 5.15 times that of the Sun, which in turn yields a radius 1.94 times solar. Cooler than the "rotation break" around F2 (above which stars rotate freely), Sigma is a leisurely rotator, with a projected equatorial speed of 6 kilometers per second (triple the solar value), allowing a rotation period of up to 17.4 days.

Application of theory gives a mass of 1.28 Suns and shows that the star, if not a subgiant (a star whose core hydrogen fuel has run out) is pretty close to being one after a lifetime of just over four billion years. The age explains the slow rotation, as magnetic effects act as a brake, the star consistently no longer showing much, if any, magnetic activity.

At a separation of 250 seconds of arc lies fourteenth magnitude Sigma B, a class M4 red dwarf. You'd pay little attention to it (red dwarfs are everywhere) except that it's keeping pace with Sigma A and seems quite to belong to the brighter star. (Be careful of the name. Possible faint companions of 10th and 16th magnitude respectively 16 and 15 seconds away, which may be just in the line of sight, have been called "B" and "C," rendering the red dwarf "Sigma D." We'll stick with "B.")

Such companionship is not uncommon; after all, the nearest bright star, Alpha Centauri, is followed by the red dwarf Proxima Cen (which is slightly closer us). However Sigma Peg B has another surprise in store.

Close examination reveals that it too is double, made of two red dwarfs just a couple tenths of a second of arc apart (reminiscent of Castor C). Separated by at least 8 Astronomical Units, the two take more than 30 years to orbit each other.

Given a physical distance of 6800 AU, the orbital period of the twin red dwarfs around Sigma proper must be at least 415,000 years. It's remarkable that with an age some four billion years that the companionship has survived. If indeed it has, as the two may actually be drifting apart.

Of equal interest is that Sigma is speeding along at 68 kilometers per second relative to the Sun, roughly 4.5 times average, suggesting that it's a visitor to the local community from a different part of the Galaxy. Consistently, Sigma's iron-to-hydrogen ratio is low, 60 percent solar, and the red dwarfs are as bit faint for the class, shifting them a somewhat toward the realm of subdwarfs, though any such conclusions are at best made on minimal data.

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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