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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 July 17, 2015 by Jim Kaler

The next skylights will appear July 31, 2015.

With the Moon just past new (on Wednesday, July 15), we first see the waxing crescent climb the western evening sky heading toward first quarter the night of Thursday the 23rd roughly about the time of moonset in North America. It then enters the waxing gibbous phase, reaching full just after our fortnight ends, on Friday the 31st, again about the time of moonset. For reasons that apparently involve the Maine Farmer's Almanac, Sky and Telescope, and various interpretive errors, this second full Moon in a month is called a blue moon, the event happening once a year or so. While it highlights the length of the lunar phase cycle, the blue moon has no physical meaning. The Moon goes through apogee, where it is farthest from Earth, on Tuesday the 21st.

The early evening of Saturday the 18th will feature a spectacular lineup, with the thin crescent Moon tucked just under Venus, Jupiter to the right, and the star Regulus hovering above them all. Even the next night will be great, with the Moon now up and to the left of the remaining trio. Look next the night of Saturday the 25th to see the Moon just to the right of Saturn, the Moon passing north of the planet after midnight. By the night of Saturday the 26th, the Moon will have flipped to the other side of Saturn.

Jupiter and Venus stay close to each other during the second half of the month. The two came into conjunction on July 1, and will do so again on Friday the 31st with brighter Venus now 6 degrees to the south. By then they will be setting in bright twilight and difficult to see. So enjoy them early. Afterward look for Saturn northwest of Antares, the planet transiting the meridian shortly after sunset and setting not long after local midnight. Mercury, out of sight, goes through superior conjunction with (on the other side of) the Sun on Thursday the 23rd, while Uranus resumes normal easterly motion against the stars on Sunday the 26th.

The Delta Aquarid meteor shower, which is active in late July and early August, peaks around the 28th and 29th. Its 20 meteor per hour rate however will be ruined by the bright Moon.

As the sky darkens, look for bright Arcturus west of the meridian, with the rest of kite-shaped Bootes to north it. Following to the east is the graceful curve of stars that forms Corona Borealis, the Northern Crown, then Hercules, bright Vega (which marks Lyra, the Harp or Lyre), and finally Cygnus, the Swan, which holds Deneb and flies along the bright Milky Way.

STAR OF THE WEEK: ZETA CRB (Zeta Coronae Borealis)

Actually "stars," as it's double. Or is it triple, or maybe quadruple, even quintuple.

To partially quote Laurel and Hardy, "Well here's another nice mess... "

Zeta Coronae Borealis, rather obviously in Corona Borealis, the Northern Crown, is a barely-fifth magnitude (4.58) classic double made of a mid-fifth magnitude (4.96) class B7 dwarf called Zeta-2 CrB coupled to a sixth magnitude (5.91) class B9 dwarf tagged Zeta-1 (because it's the more westerly of the two) that lies 6.1 seconds of are away.

They've changed their distance from each other by only 0.9 seconds of arc since 1779, and pretty clearly belong together.

From Smythe and Chambers in the nineteenth century: "A fine double star, in the middle of the space over the wreath, and 10 degrees N. a little easterly, from Alpha. A 5, bluish white; B 6, smalt blue. This is a beautiful object."

Juxtaposition effects do odd things to apparent star colors, since Zeta-2 should be the more blueish one, while Zeta-1 is white. ("Smalt blue" refers to deep blue cobalt glass.) At a distance of 473 light years (give or take 36) for the pair, Zeta-1 is by far the simpler. Assuming a temperature of 11,000 Kelvin from the class (needed to allow for ultraviolet radiation), it radiates at a rate of 112 solar luminosities, which gives the star a radius of three times solar and, with a minimum equatorial rotation velocity of 20 kilometers per second, a rotation period of under 7.5 days.

Now abandon hope etc. Almost everything about Zeta-2 is contradictory.

Since 1925, it has been known to be a spectroscopic double (with two sets of spectra shifting back and forth in wavelength) that seems to be made of similar B7 (or perhaps B6) stars in a tight 12.824-day orbit. But in 1997, analysis of new data found it to be triple, made of an inner pair (Zeta-2 A and B)in a 1.72357-day orbit around which goes another star (Zeta-1 C) with a period of 251 days, the old 12.8-day orbit dropped as erroneous.

But then a further study in 2005 reconstituted it, so now we have a quadruple star with Zeta-1 D (?) going around the 1.7-day AB pair every 12.5842 days, then with Zeta-1 C orbiting the inner triple in 251 days.

Confused yet?

Further analysis leads to even more frustration. Assume that the B7 (B6?) pair dominates and we split the resulting luminosity in two. Temperature estimates start at 16,580 Kelvin, much too high for the B6 class, which should be around 13,000 K. Best we can guess is that the bright pair shines at around 95 Suns each with masses of maybe 3.2 times solar. But don't trust it as theory can't properly fit the parameters.

Given what we have, the AB pair is but 0.05 AU, just 11 solar radii, apart (plenty enough for clearance). Raising the temperature increases both the luminosity (because of more UV light) and mass. But we don't know the contribution of Zeta-2 C or, if really present, Zeta-2 D. Assuming C and D have inconsequential masses, they orbit the AB pair at mean separations of 0.20 AU and 1.4 AU. Further speculation is futile. Looking further afield Zeta 1, separated by at least 930 Astronomical Units from Zeta-2, must take at least 920 years to make a circuit.

The whole affair shows that while the characters of some stars are known with exquisite accuracy, when we get into others, especially close multiples, all bets are off and confusion reigns.

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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