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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 August 28, 2015 by Jim Kaler

The next skylights will appear September 11, 2015.

We open our two-week period with the Moon just shy of full, that phase passed during the day on Saturday, August 29. The Moon will thus rise in the end stage of its waxing gibbous phase the night of Friday the 28th and be barely in the waning gibbous phase as it rises just past sunset the evening of Saturday the 29th. The gibbous Moon then continues to wane until third quarter is reached the morning of Saturday, September 5, after which our companion slims as a waning crescent until our fortnight runs out. The waning gibbous passes close to Uranus on Tuesday the 1st (not visible in North America) and then will appear very near Aldebaran in Taurus the night of Friday the 4th, those in the northeastern US actually seeing the Moon occult the star. The real winner will be the morning of Thursday the 10th, when the Moon will make a classic pairing with Venus, the planet just up and to the right of the slim crescent. At the same time much dimmer Mars will appear to the left of the crescent, the three almost in a row with the Moon in the middle. Even the previous morning, Venus and the Moon will make a pretty sight with the crescent well above the bright planet. On Sunday the 30th, celebrating the coming September, the Moon passes perigee, where it is closest to Earth.

As apparent from the above, Venus is starting to make a big splash in the morning sky. Rising just before dawn, the brilliant planet, the "morning star," rapidly climbs the morning sky. Mars is there too, down and to the left of the Venus, but rises later and is hard to spot, while Jupiter rises within morning twilight and for now remains hard to see. Back in the evening, Mercury passes its greatest eastern elongation relative to the Sun on Friday the 4th, but sets within bright twilight. Saturn is better. To the right of Antares, in the middle of our period the ringed planet does not set until around 10:30 PM Daylight time, but you still need to look early, as at best it appears not far above the southwestern horizon. Finally, as August moves into September, Neptune passes opposition to the Sun.

Even with a bright Moon we can admire the Summer Triangle of Vega in Lyra (fourth brightest star of the sky, number two in the northern celestial hemisphere) at the northwestern apex, Deneb in Cygnus (the Tail of the Swan but at the top of the Northern Cross) at the northeastern apex, and Altair in Aquila at the southern point. At the other end of the Northern Cross, marking the Swan's head, is Albireo, one of the prettiest double stars of the sky, the pair shining blue against orange. South of Albireo is the modern figure of Vulpecula, the Fox, and farther down, just north of Aquila shoots the obvious ancient pattern that makes Sagitta, the Arrow, Through Cygnus and Aquila flows the Milky Way, which brightens through Sagittarius and then for mid-northerners drops out of sight below the horizon.


Toward Cygnus's western border with Draco, a bit over 10 degrees northwest of Deneb, lies (from Smythe rev. Chambers 1881) "A fine double star...A 5 1/2, bright white; B 8 lilac," the combined magnitude now settled at magnitude 4.92 (mid-fifth). The fainter of a close pair always seems to receive the nicer color, which we now know is just a proximity effect. In reality the stars are both white (but still a "pretty object"). First measured by William Herschel at a separation of 4.0 seconds of arc, the duo consists of a barely eighth magnitude (7.52) star going around a mid-fifth (4.96) magnitude class A (A4) dwarf.

But that's not all! Closer interferometric scrutiny reveals Psi Cygni A (the brighter of the pair) to be broken into another double, rendering the star a nifty triple. Psi Cyg Aa, at magnitude 5.6, is coupled with Psi Ab (magnitude 6.1), the two just a tenth of a second of arc apart. From its brightness, Ab is probably an A7 dwarf, whilst the class of Psi B is listed all over the place, from A7 to F4; as a compromise, we'll adopt F0 (dwarf).

Over the past two-plus centuries, the separation between Psi A and B has closed to 2.8 seconds of arc, which is not enough coverage for an orbit to be constructed. The inner pair, however, go around each other much faster. An orbit fitted to the positional data gives a period of 54.1 years and, at a distance from us of 281 light years (give or take 9), an average separation of 12.2 Astronomical Units.

But something is amiss.

At nearly 10 AU it takes Saturn some 30 years to orbit the Sun. The far more massive stars should take much less than 30 years to orbit, not more.

Application of Kepler's laws to the orbital parameters yields a combined stellar mass for Aa and Ab of 0.6 solar masses, which is patently way out of line for class A or F stars. Given the small portion of the orbit observed, however, such a result is not at all surprising. A longer baseline, ideally the observation of a complete circuit, will settle the matter.

So what do we have here? Aa is given a temperature of 8050 Kelvin, low for an A4 star, while from their classes we estimate 7800 for Ab and 7400 for Psi B. With little infrared or ultraviolet to deal with, the luminosities come in at 33 Suns for Psi Aa, 21 for Ab, and just six for Psi B, while radii are calculated to be 3.0, 2.5, and 1.5 times solar.

Theory then gives respective masses of 2.2, 2.0, and 1.5 Suns. With the inner pair summing to 4.2 Suns, a period of 21 years, rather than 54, would be more like it.

We'll just have to wait and see.

A high projected equatorial rotation velocity of 256 kilometers per second applied to Aa gives a rotation period of under 0.6 days, but it's hard to know how contamination from Ab factors into it. At a separation from Psi A of at least 650 AU, B must take at least 700 years to orbit. From B, the inner pair might look like brilliant points of light separated by perhaps a degree, each radiating the light of a couple dozen full Moons.

And yet more: fourteenth magnitude Psi C is 24 seconds away, eleventh magnitude Psi D nearly three minutes distant.

These, however, are moving too fast to relative to Psi A to belong to the system, and alas are mere line of sight coincidences. We'll have to stick with a trio. (Thanks to Bill Hartkopf for instructive commentary.)

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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