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The Sky This Week, 2020 January 7-14

A fading giant in Orion
Orion over the Northern Neck, New Year's Eve 2019-2020
Orion over the Northern Neck, New Year's Eve 2019-2020
Imaged with a Canon EOS Rebel SL2 DSLR from Mollusk, Virginia, 2020 January 1.
Note the brightness of Betelgeuse compared to that of Rigel. Normally the
two stars are comparable in brightness.

The new year finds a waxing Moon high in the sky, brightly lighting the stark winter terrain below.  Full Moon occurs on the 10th at 2:21 pm Eastern Standard Time.  January’s Full Moon is popularly known as the Wolf Moon or Moon After Yule.  Skywatchers in Europe and Asia have the opportunity to see a penumbral lunar eclipse at this time.  That said, the effect on Luna’s disc is very subtle.  Observers may notice a slight dimming of the Moon’s southern limb, but none of her disc will touch the darker umbral shadow of the earth.  Don’t plan to take a last-minute trip abroad to catch it!  Look for the Wolf Moon between the stars Castor and Pollux, the Gemini Twins, and Procyon, brightest star in the constellation Canis Minor, the Little Dog.  By the end of the week she treks across the stars of Leo, the Lion.

With the winter solstice a couple of weeks behind us, the days are now gradually getting longer.  This is especially noticeable at the time of sunset.  On the evening of the 11th Old Sol dips below the horizon at 5:06 pm EST, some 20 minutes later than his earliest setting time back on December 7th.  By the end of the month sunset will approach 5:30 pm.  Spring is on the way!

The bright Moon once again washes out much of the night sky, but winter’s bright constellations are pretty hard to hide in her glow.  The most prominent of the season’s constellations is Orion, the Hunter, whose distinctive outline is familiar to almost all of the Earth’s inhabitants.  This year is shaping up to be an especially good year to check out Orion on a nightly basis, since one of his most prominent stars has been behaving strangely.  Normally the star that marks the Hunter’s left “shoulder”, the red-tinted Betelgeuse, shines with about the same apparent brightness as the blue star Rigel, one of Orion’s “knees”.  However, a casual glance at the constellation will show that Betelgeuse is barely brighter than any of the Hunter’s three “Belt” stars.  Betelgeuse has long been known to vary in brightness, but it hasn’t been this faint for well over a century.  Betelgeuse is, however, a very unusual star.  It is classified as a “red super-giant”, a massive star that has evolved through most of its life cycle.  Its interior is layered like an onion, with different layers fusing heavier elements in shells surrounding an iron-rich core.  As these shells expand they cause the outer convective layers of the star to swell to immense proportions; some estimates indicate that its diameter is larger than the orbit of Mars around our Sun.  These tenuous outer layers are relatively cool, giving Betelgeuse its characteristic red color and allowing the formation of molecular and “dust” layers.  Changes in the opacity of these outer layers may be the mechanism that causes in the star’s apparent brightness to fluctuate.  On the cosmic scale Betelgeuse is in a very unstable state, and it is one of the leading candidates for becoming a supernova in the near (i.e. sometime in the next 100,000 years!) future.  We don’t know when the current dimming trend will stop, but in all likelihood Betelgeuse will probably brighten back to its “normal” luster over the next few years.  For now, you can do a little “citizen science” by comparing the brightness of Betelgeuse to the other prominent stars in Orion and charting your results.

Venus is now the only bright planet gracing the evening sky.  You will find her high in the southwest shortly after sunset, blazing away among the dim stars of the autumnal constellations.  She sets at around 8:00 pm EST. 

You’ll have to wait until the pre-dawn hours to spot the next visible planet.  Mars rises shortly after 4:00 am along with the first of summer’s stars.  This week the red planet is moving through the stars of Scorpius, and by the week’s end he will be within a few degrees of the star Antares, another red supergiant star whose name means “rival of Mars”.  Antares and Betelgeuse are similar in their sizes, distances, and evolution, but you will never see them in the sky together.  In one of the myths surrounding Orion, he was killed by the sting of a scorpion, so the two are placed on opposite sides of the sky, never to encounter one another again.

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