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Sierra NightSky

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

Current Moon PhaseThe Moon's current phase, courtesy of USNO.

Sierra NightSky for the period starting July 25, 2014 by Jim Kaler

There sure is not much happening: summer doldrums perhaps. We lead off with the new Moon on Saturday, July 26, so our constant companion is not even visible as we open up Skylight's current week. But that has its distinct advantage, as we can then see the stars in all their glory in a dark sky, presuming we can get away from artificial lighting.

We do also get to see the Moon wax in its crescent phase during the latter part of our week, as first quarter is not reached until next week, on Sunday, August 3.

Your first look at the growing crescent will be in western twilight the evening of Monday the 28th. Be sure to admire the Earthlight, which illuminates the Moon's nighttime side. The only other thing the Moon has going for it is its apogee (farthest from Earth) on Sunday the 27th. The near coincidence with new Moon will weaken tidal amplitudes at the coasts.

Dark skies bring us to the evening's two bright planets, Mars and Saturn, the red planet to the right. Now to the east of Virgo's Spica, nearby Mars is rather quickly shortening the angular separation between it and much more distant Saturn, which in effect is just sitting there in Libra waiting. Mars will pass four degrees north of it on August 26.

As Earth pulls away from the red planet, Mars is slowly dimming and is now about the same brightness as Saturn, which is still pretty bright. But to see them, look early, as Mars sets shortly before midnight Daylight Time, Saturn following shortly after midnight. In the morning sky, Venus rises almost exactly at the first break of dawn, about 4 AM, as it ever so slowly slips away.

With the Moon gone, the starry sky comes alight. Perhaps you might spot some meteors from the Delta Aquarid shower, which peaks roughly midweek.

Then as the Big Dipper falls into the northwest, the Little Dipper goes over the top, above Polaris, while Draco curls between the two. To the south of, and parallel to, the Dipper's handle, find the two-star figure of Canes Venatici, the Hunting Dogs (of Charles II), and farther south the lovely apparition of Coma Berenices, a nearby cluster that makes "Berenice's Hair" (that of an ancient Queen). To the right of Polaris rules dim Cepheus, the King and most westerly of the constellations of the Andromeda myth. The rest of the gang, represented best by W-shaped Cassiopeia, will be along shortly, as will be the coolness of fall.

STAR OF THE WEEK: 17 COM (17 Comae Berenices)

Separated by 145 seconds of arc, fifth magnitude (5.29) 17 Com A (in Coma Berenices, Berenices Hair, south of the Big Dipper's handle) and seventh magnitude (6.65, almost sixth) 17 Com B are usually considered to be an easy "binocular" double. Merged together, they shine at magnitude 5.01. Flamsteed 17 sometimes refers to "A" alone, leaving "B" as HR 4751 from the Bright Star Catalogue ("A" being HR 4752). Both are given as class A (A0p) "peculiar" dwarfs, though 17 Com B is also listed as A2 to F2. The duplicity rests on their common motions both across and along the line of sight.

The Hipparcos parallaxes, however, call out a rather resounding "no," as the distance of 17-A is 238 light years, while that of 17-B is 223 light years. The likely respective errors of 4 and 6 light years cannot bring them any closer together than Alpha Centauri is to us, making it highly questionable that they constitute a binary system.

The common motion is more likely to be the result of their both being members of the Coma Berenices open cluster (which is the major part of the larger modern constellation), whose stars are moving through space together while slowly orbiting some vague common center of mass and also interacting with one another.

With a distance to the cluster of around 280-300 light years, 17 A and B are on the near side of it. We'll therefore consider them as separate stars that share a common name.

Even if the two were at the same distance, they would be separated by more than 10,000 Astronomical Units which, in the environment of a cluster, seems a bit much for the pair to be any kind of stable orbiting binary. One interesting possibility is that they WERE a pair that has been separated. About 400 million years old, the cluster as a whole is falling apart, its stars gradually escaping its weak gravitational grip.

It's no surprise that brighter 17 A is by far the more studied. The "p" suffix to the class makes it a "magnetic" double of the "Alpha-2 Canum Venaticorum" class (Cor Caroli, not far away), the star's field measured at around 1000 times that of Earth.

In the similar metallic stars, some chemical elements in their outermost layers sink under the force of gravity, while others are elevated by radiation. The result is a wacky, very non-solar chemical mix. The magnetic stars add tilted magnetic fields that concentrate some of the elements into "starspots" that, as the stars rotate, swing in and out of view.

As a result, 17-A varies between magnitudes 5.27 and 5.40 over a 5.08 day rotation period, with chromium, europium, and strontium strongly enhanced. With a poorly-known temperature of 10,200 Kelvin, the star shines with the light of 42 Suns, which in turn gives a radius of 2.1 times solar.

A projected equatorial rotation velocity of 18 kilometers per second gives a rotation period of less than 5.82 days, which with the true value shows the rotation axis to be inclined to the plane of the sky by 60 degrees. Application of theory yields a mass of 2.4 Suns and solidly confirms "dwarfhood."

The temperature of lesser 17 Comae B is around 7700 Kelvin, consistent with a class of A3, which we'll adopt: classes of these peculiar stars are compromised by the odd abundances (that of 17-A compromised too).

The star radiates at a rate of 8 Suns, its radius 1.6 times solar. A rotation speed of at least 26 km/s gives a rotation period of under 3.1 days, theory giving a mass of 1.7 Suns.

It too is a peculiar magnetic dwarf, with a field around 750 times the strength of Earth's.

While "A" and "B" are unlikely to be a binary, "B" itself is triple! It has a spectroscopic companion in a 68.3 day eccentric orbit, which if of low mass would orbit at an average distance of 0.4 AU. Then a 13th magnitude companion hovers at just under two seconds of arc that, if really bound to "B," from its brightness is red M2 dwarf at least 130 AU away and that takes more than 1000 years to go all the way around.

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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