Friday, December 19 2014

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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 7, 2014 by Jim Kaler

The next Skylights will appear Friday, December 19.

It's yet another two-weeker and it's jammed full of good stuff. We begin just shy of full Moon, which is passed the morning of Saturday, December 6, near Moonset in North America. Look near dawn for it glowing above the northwestern horizon. We then swing through the waning gibbous phase, third quarter on Saturday the 14th near sunrise, and finally through a good portion of the waning crescent.

The evening of Friday the 5th, the Moon plows through the Hyades star cluster in Taurus, which because of the lunar brightness will be hard to see, though you may spot Aldebaran (which is not actually a member of the cluster). The Pleiades will be northwest of them. Then look the night of Thursday the 11th to see the Moon passing five degrees south of Jupiter, while the following night it will make a fine triangle with Jupiter and Regulus in Leo. At the end we see the crescent bearing down on Saturn the morning of Friday the 19th. The Moon passes apogee, where it is farthest from Earth, in the middle of our fortnight, on Friday the 12th.

Jupiter remains ascendant, rising before 10 PM as we begin our period, shortly before 9 by the end of it. It crosses the celestial meridian to the south just before dawn. The giant planet ceases its direct motion against the stars of Leo to the west of Regulus on Tuesday the 9th, whence it begins its westerly retrograde trek.

Mars, climbing through Capricornus, continues its 8 PM setting time as it slowly falls behind the Earth. Returning to the scene, Saturn clears the start of twilight shortly after we began this tale northwest of the three-star head of Scorpius.

Not that anybody will notice, but Mercury passes superior conjunction with the Sun (on the other side of it), on Monday the 8th. You won't see it again until 2015.

Venus, however, is slowly becoming visible within evening twilight. And because of Earth's axial tilt and orbital eccentricity, the earliest sunset as seen from mid-northern-latitudes arrives on Monday the 7th. By the time of the start of winter (and shortest day) on December 21, the evening sky is notably lighter.

One of the better and more interesting meteor showers, the Geminids, peaks the morning of Saturday the 14th. Capable of more than a meteor a minute, the shower will be marred by a quarter Moon. The Geminids are the debris of Comet 3200 Phaeton. Once thought to be an asteroid, Phaeton orbits the Sun in a short period of just 1.43 years and comes well inside the orbit of the Earth (but does not intersect us).

Right in the middle of things, in early evening find the Great Square of Pegasus high to the south, Andromeda streaming off its upper left corner. Then around midnight climbs one of the great glories of the sky, Orion, the Hunter, with his silvery three-star belt and the magnificent red supergiant Betelgeuse at the upper left. Wait a bit then to see the rising of brilliant Sirius, the brightest star of the sky.


Fourth magnitude (4.13) Kappa Pegasi, of no proper name, 112 light years away (give or take 3), is the second-most western star with a Greek letter in Pegasus (the Flying Horse, of course). It's just barely beaten out for the western honor (such as it is) by second magnitude Enif (Epsilon Peg). No matter, position is irrelevant to the Kappa's nature as a nifty very close triple that requires interferometry to resolve and study. The AB pair are but a couple tenths of arc apart, if that. Defying convention, Kappa Peg B is the brighter of the two rather than the fainter, which has led to considerable confusion, allowing A and B to get mixed up.

Both are given as class F subgiants (combined, F5). The two orbit every 11.567 years at a mean distance between them of 8.04 Astronomical Units (not far from Saturn's distance from the Sun), a high eccentricity taking them between 10.6 and 5.5 AU apart. Application of Kepler's laws yields a total mass to the system of 3.88 Suns. A closer look shows Kappa Peg B also to be double, its components only a few thousandths of a second of arc apart. (Kappa-A was at one time also thought to be binary, but it apparently isn't.) The lesser companion (Kappa Bb) orbits brighter Kappa Ba with an amazingly short period of just 5.971 days!, the two in near-circular orbit (as would be expected) just 0.086 AU apart, a quarter Mercury's distance from the Sun. These parameters then give a total mass to Kappa B of 2.39 Suns.

Kappa Peg Kappa Peg B and A orbit each other every 11.6 years averaging about 8 Astronomical Units apart. Here we see the apparent orbit of Kappa A around Kappa B, which is not at the focus of the apparent ellipse because of the orbit's tilt and orientation relative to the plane of the sky. Note the small scale on the axes, the two stars very close to each other, at least as seen from Earth. (W. I. Hartkopf and B. D. Mason, Sixth Catalog of Orbits of Visual Binary Stars, US Naval Observatory Double Star Catalog, 2006.)

Kappa Peg B and A have more recently been catalogued at magnitudes 4.94 and 5.04, which combine to 4.24, a tenth of a magnitude fainter than found earlier, the difference of little consequence. Temperatures and individual classes are also a bit insecure. Kappa-B may be as hot as F2, but temperature measures suggest more like F5. No matter, B and A have respective luminosities of 1.5 to 1.6 times that of the Sun (depending on whether they are real subgiants or old dwarfs) with radii of 2.4 solar.

Subtracting the mass of Ba alone from the total given by the orbit gives a mass to the companion (Bb) of 0.8 to 0.9 Suns, making it out to be a K0 or G8 dwarf. To be proper, we ought to subtract its brightness from the magnitude of B given above, but it really makes no significant difference. The evolutionary masses of the whole system then add up to around 3.9 to 4.0 Suns, which is the value given by Kepler's Laws, suggesting that the orbits are pretty good (not to mention theory, though there is some circular reasoning involved).

The real interest is in what is going to happen in the future. Near or at their main sequence hydrogen-fusing lifetimes of some 2.5 billion years, both Kappa B and A will turn into giants with helium cores and then into more advanced giants with carbon cores. While Kappa A will not cause a problem with B, the consequences for the little one that orbits Kappa B will be severe, as the two will probably merge. Mass loss combined with binary action may someday also produce a highly structured planetary nebula before one or both Kappa B and A turn into white dwarfs.

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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