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The Sky This Week, 2017 January 10 - 17

Colors in the starry sky.
Orion over Paris, Virginia, 2014 March 27
imaged with a Canon EOS Rebel T2i DSLR

The Moon dominates the overnight hours this week, beaming brightly down from nearly overhead as she passes through the Full Moon phase on the 12th at 6:34 am Eastern Standard Time. January’s Full Moon is popularly known as the Wolf Moon, a name derived from Native American lore, or the Moon After Yule. Look for the waning gibbous Moon rising within a degree of the bright star Regulus late on the evening of the 14th. Shortly after midnight on the 17th the Moon will occult the second-magnitude star Porrima at 12:08 am EST.

Despite the best efforts of the Moon to wash out the winter sky, our evening hours now play host to the year’s best collection of bright stars. Nine of the 25 brightest stars in the sky can be found within the confines of Orion and the surrounding constellations. These stars also offer some of the most noticeable color variations that you can see with the unaided eye. One of the most striking examples of brightness and color can be found in the first-magnitude star Betelgeuse, which marks one of the "shoulders" of the familiar constellation of Orion, the Hunter. The ruddy tint of Betelgeuse stands in stark contrast to the blue-white colors of Orion’s other bright luminary, Rigel, and the distinctive stars of the Hunter’s Belt. Betelgeuse is classified as a red supergiant star that is in the later stages of its evolution. It has exhausted the hydrogen in its core and is now fusing hydrogen into helium in an ever-expanding shell surrounding the core. This has caused the star’s outer layers to swell to vast proportions; its radius is over 800 times that of the Sun. Its girth is so vast that if it occupied the Sun’s place in our solar system the Earth would be inside its outer layers! The mass of Betelgeuse is over 10 times that of Old Sol, and this large mass means that the star is now at a very unstable phase of its evolution. Of all the bright stars in the sky Betelgeuse is the most likely candidate to explode in a supernova, but we have no way of predicting when this might occur. It is well over 600 light-years away, so it could have blown up 300 years ago and we’ll still have to wait another 300 years to find out! In classical mythology Orion was killed by the sting of a lowly scorpion, and we still see this in the constellations today. The brightest star in Scorpius, Antares, is a very similar star to Betelgeuse, but you’ll never see them in the sky together. Throughout the ages Betelgeuse has set when Antares rises as the Hunter and the Scorpion perpetually chase each other across the heavens.

Venus reaches her greatest elongation east of the Sun on the morning of the 12th. You should have no trouble finding Venus in the early evening sky as she moves eastward through the dim stars of Aquarius. For the next few weeks Venus will climb northward along the ecliptic before she begins her fall back toward the Sun in late February. Through the telescope she resembles the first quarter Moon, and over the next few weeks her phase will become more crescent-shaped as her apparent diameter grows ever larger.

Mars may also be found in Aquarius just to the northeast of Venus. Venus continues to close the gap with Mars, but the dazzling planet won’t be able to catch him before she starts her fall toward the horizon. Mars will continue his eastward slog and will remain visible in the evening sky until the late spring.

Jupiter now rises about half an hour after local midnight, which means that he now crosses the meridian at around 6:00 am EST. With sunrise still taking place at around 7:25 am this is a great opportunity to spend a little time looking at the giant planet before you start your days’ activities. Jupiter’s four large moons were first observed by the astronomer Galileo Galilei 407 years ago this week.

Saturn and Mercury can be spotted low in the southeast about half an hour before sunrise. Both planets are around zero magnitude and are located about 10 degrees above the horizon, separated by about six degrees as the week opens. By the week’s end the gap between them grows as Mercury moves east of Saturn and gradually brightens.