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The Sky This Week, 2019 December 10 - 17

Brightening the long winter night.
Orion rising over the Catalina Mountains, imaged from Catalina, Arizona, 2019 January 14.
Orion rising over the Catalina Mountains
imaged from Catalina, Arizona, 2019 January 14,
made with a Canon EOS Rebel SL2 DSLR.

The evening hours are dominated by the bright light of the Moon for most of the week as she rides high along the ecliptic through the heart of the Great Winter Circle.  The Full Moon Before Yule occurs on the 12th at 12:12 am Eastern Standard Time, then wanes to last Quarter on the 18th.  On the evening of the 10th she glides just north of the Hyades star cluster near the bright star Aldebaran, the “eye” of Taurus, the Bull.  By the end of the week Luna turns southward in her course, passing just over three degrees north of the star Regulus in Leo, the Lion.

You probably won’t notice it, but starting on the 13th the time of sunset will gradually start to shift a bit later.  Our earliest sunsets now occur at 4:46 pm here in Washington, and by the time that Christmas rolls around Old Sol will set six minutes later than he does now.  That said, we are still losing daylight in the mornings, as our latest sunrise won’t occur until early next year.  However you look at it, though, we are now experiencing the longest nights of the year.

Fortunately, when the Sun is shy and scudding over the southern horizon, the Moon reaches her highest declinations for her full phase for the year, nestled among a bevy of the sky’s brightest stars.  Once the Moon leaves the scene, those bright winter stars linger on, adds at least a little extra light to the longest nights.  Dominating this group of stars is the distinctive figure of Orion, the Hunter.  Of all the constellations of the season, Orion stands out despite the best efforts of the Moon to wash out his distinctive features.  Although they are not the constellation’s brightest stars, the three stars that mark Orion’s “Belt” are almost universally synonymous with the boreal winter sky.  They were one of the first star patterns that I noticed at a very early age, and Orion was the first constellation I explored with my new Christmas-present telescope.  

A casual look at Orion with the naked eye shows that most of the Hunter’s stars are tinted blue, and this betrays their common origin in a vast star-forming region that’s centered on the Great Orion Nebula in the asterism known as “The Sword”.  This small clump of stars hangs below the left side of the bright Belt stars, and if you look at the group with binoculars you will immediately see that the middle star in the group has a fuzzy appearance.  A small telescope will show a tight knot of four stars surrounded by wisps of glowing gas even under urban skies and bright moonlight.  Move to a dark sky site on a moonless night and the extent of the nebulosity grows, filling the eyepiece with tendrils of soft diffuse light.  The stars that have formed in this cloud now make up many of the brighter stars in Orion.  They are very far away and thus very luminous.  The bright blue stars that dominate the constellation are well over 1000 light-years away, and to appear that bright over such a vast distance they shine with the luminosity of tens of thousands of Suns!  

The one exception is perhaps the most distinctive star in the constellation, Betelgeuse, which marks Orion’s right shoulder.  You will immediately notice its reddish tint, far different from its bright companions.  Betelgeuse is located at about half the distance to the rest of the blue stars in Orion, and while the blue stars are comparatively young in their evolutionary age, Betelgeuse is nearing the end of its stellar life.  It is a “red supergiant” star that has exhausted the hydrogen in its core, and it now fuses hydrogen into helium in an ever-expanding shell around that core.  This causes the star’s girth to expand dramatically, and if there were a planetary system analogous to our solar system around the star all of the planets out to the orbit of Mars would be swallowed by its outer layers!   Betelgeuse is also an irregular variable star whose light changes by over a full magnitude over the course of several years.  At times it is brighter that its rival Rigel, the star that marks the Hunter’s left knee.  This year, however, it is decidedly fainter than Rigel and may be on the way to its faintest minimum since 1941.

Venus may now be easily seen in the evening twilight sky.  As the sky darkens on the evenings of the 10th and 11th you will see the planet Saturn just to the north of the dazzling planet.  Venus will quickly abandon her more distant companion leaving Saturn to set at the end of evening twilight.  Venus will continue to climb in the evening sky and will be our evening companion until late in the coming summer.

Ruddy Mars can be spotted before sunrise low in the southeastern sky.  On the morning of the 12th he will pass by the wide double star Zubenelgenubi in the constellation of Libra, the Scales.  In binoculars Mars and the star will almost appear to touch, and they will be close enough to each other to appear in the same field of view of a telescope.  This will be the closest appulse of a planet and star for the year, so try to catch the sight if you can.