Sierra NightSky

Sierra NightSky for the period starting December 16, 2016 by Jim Kaler.

The next skylights will appear December 30, 2016. Best wishes to all for the Holiday season.

We end 2016 with the Moon running through its late waning phases, beginning with the waning gibbous, which ends at third quarter on Tuesday, December 20, nine hours before the Sun hits the Winter Solstice. It then glides through its waning crescent phase until it hits new Moon on Thursday the 29th. Your last view of the waning crescent will be the morning of Tuesday the 27th with Saturn to the northwest of it. The Moon will appear to the west of Jupiter the morning of Tuesday the 22nd, to the other side the following night.

The big event is the passage of the Sun over the Winter Solstice at 4:44 AM CST (5:44 EST, 2:44 PST) on Wednesday the 21st, giving us the shortest day and longest night in the northern hemisphere. The Sun will rise as far to the southeast and set as far to the southwest as possible for any given latitude. Because of the tilt of the Earth’s axis and the eccentricity of its orbit, we’ve already passed the time of earliest sunset, which took place on December 7, and you’ll quickly see the evenings getting lighter.

Venus glows brilliantly in the southwest, You can’t miss it. Mercury, though, goes through inferior conjunction with the Sun on Wednesday the 28th and is quite invisible. Mars still reliably sets in the southwest at 9:30 PM. Of lesser significance, Uranus ceases retrograde (westerly) motion on Thursday the 29th. The Moon goes through apogee, where is it farthest from Earth, on Christmas Eve.

It’s Jupiter, however, that dominates much of the night, rising at 1 AM as the year comes to an end, an hour later as we open our period. Saturn remains in morning twilight.

With some luck you might see a few meteors from the underappreciated Ursid meteor shower, which peaks the morning of Wednesday the 22nd and appears to come from Ursa Minor – the Little Dipper – which is always nearly due north.

The Great Square of Pegasus and its attendant autumn constellations move off to the west to be replaced by those of winter. With the Sun at the Winter Solstice, you’ll see the Summer Solstice, on the Gemini-Taurus border, high at midnight. Below will be mighty Orion with his three-star Belt and Canis Major, which holds Sirius, the brightest star of the sky.

STAR OF THE WEEK: PHI PEG (Phi Pegasi)

Several are the classic stellar tests for minimal as well as excellent vision. The best known is certainly Mizar and Alcor in the Big Dipper, the Arabs’ “Horse and Rider?” Most people can rather easily make them out. How many stars can you see in the Pleiades, the “Seven Sisters?” Most people see six (hence the “Lost Pleiad), but keen-eyed observers can make out seven, even eight or nine or more. A much tougher test is to try to split the “Double-Double,” Epsilon Lyrae, whose stars (each of which is double yet) are about three and a half minutes of arc apart. Among the lesser-practiced tests is to see how many stars are visible inside the nearly blank Great Square of Pegasus. There is no agreed-upon or absolute value of course, but good eyes should at least see the four that carry Bayer Greek letters, as these came out of Tycho’s visual observations.

Here we find fourth magnitude Upsilon Peg (at 4.40 the brightest), fifth magnitude (4.60) Tau Peg, fifth magnitude Psi (4.66) and fifth magnitude (5.08) Phi. To these we might add 71 Peg (mag 5.32) and 56 Peg(5.38).

Of great significance is HR 8799, which has an orbiting planets that can be seen with direct imaging, though at sixth magnitude (5.99) the star’s a tough naked-eye find about halfway between Beta and Alpha Peg. Near it is fifth magnitude (5.49) 51 Peg with the first-known exoplanet, but it’s just outside the square and doesn’t count.

As a separate but curious issue, Phi Peg is just east of the equinoctial colure, the great circle that connects the equinoxes and the celestial poles. Because of the 26,000-year precession of the Earth’s axis, the equinoxes and solstices are moving east against the stars. Phi Peg will cross over around the year 3030. (It’s never too soon to plan a party).

Phi Peg is actually a reddish class M (M2.5) giant. (The square seems to attract them: so are Psi and 71 Peg.) Very little is known about the star, which lies at a distance of 463 light years (give or take 18). A temperature of 3400 Kelvin adopted from the spectral class allows an estimate of the cool star’s copious infrared radiation, yielding a total luminosity of 1860 Suns, which in turn gives a radius of 125 times that of the Sun, 0.58 Astronomical Units, 1.5 times the radius of the planet Mercury.

Given all the uncertainties, the star probably carries a mass of around twice that of the Sun. Some 1.5 billion years old, the best guess is that it has run out of helium fuel and is brightening for the second time with a dead carbon core and will before long shed its outer layers, allowing the core to turn into a white dwarf. There seems to be no evidence for any sort of companion to watch the action, the star circling the Galaxy on its own.

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. Read Dust to Dust in Stellar Stories.
Check out his site for more.