The Stars of Winter
Observer's Notebook for February, 2002
Bright Stars and Planets in the Coldest and Darkest of Seasons
by John Rummel
Winter months and bone chilling temperatures bring sky gazers some of the most brilliant constellations of the year. Take a look at the sky this month, for example. By about 7:30 pm mid month, Orion has reached his highest point above the southern horizon. His distinctive three-star belt is familiar to almost everyone, regardless of interest or background in astronomy. Orion's left shoulder, Betelgeuse, and his right knee, Rigel, are both on the top 10 list of brightest stars (10th and 7th place, respectively). Two other stars just missed being in the top 25, Belletrix, his right shoulder, and Alnilam, the middle star of the belt.
Starry Night Pro depiction of the winter constellations. Planet positions drawn for February 15, 2002.
Below and to the left of Orion is the brightest star in the nighttime sky, Sirius. Often called the dog-star, it anchors the beautiful constellation Canis Major, Orion's principle hunting dog. Some stars are bright simply because they're close. Others are bright because they're gigantic, super hot/bright stars in their own right. Sirius falls into the first category. At just 8.6 light years, it ranks as one of the closest stars to our solar system. Rigel, by comparison, is some 770 light years away, but is over 1600 times brighter than Sirius. If Rigel was as close to us as Sirius, it would shine far brighter then even Venus at its brightest. In fact, Rigel is one of the most luminous stars known.
Speaking of extremes, consider Betelgeuse again, the beautiful orange left shoulder of Orion. At over 400 light years, Betelgeuse is bright because it is a red supergiant star. Betelgeuse actually pulsates in size. Over a period of five to six years, its size ranges from 600 to 900 times the diameter of our own sun. At its largest, it balloons up to nearly the diameter of Mars' orbit! It pulsates in brightness with this same period as well, but its behavior is very irregular. Betelgeuse comes by this odd behavior honestly though. It's an aging star, nearing the end of its life. Having exhausted most of its hydrogen fuel by fusing it into heavier elements, it has become very unstable, and is a leading candidate among stars in our area of the galaxy to go supernova someday soon. Don't worry though, in the life-span of stars, "soon" could be thousands of years away.
Clustered around Orion are several other notable constellations and their own brilliant stars. Clockwise from the left is Sirius (see above), and then Procyon in Canis Minor, the little dog. Next up in the sky is Gemini with the twin stars Castor and Pollux. Though not as bright as Sirius and Rigel, the twins shine bright enough to make the top 25 list, shining at 23rd and 17th brightest respectively.
Joining the twins this year is the bright planet Jupiter. Shining much brighter than Sirius, Jupiter will remain the brightest object (other than the moon) in the night sky until Venus rejoins the evening skyshow in the spring.
Continuing up above Orion is the bright white star Capella in the constellation Auriga. At number 6 on the list of bright stars, Capella narrowly edges out Rigel, but like Sirius, that's because it's close (just 42 light years away).
Below and to the right of Orion and Auriga is Taurus the Bull. Taurus' centerpiece star is the fiery orange Aldebaran, number 14 on the all-time brightest list. Aldebaran is also a red giant, though it's not even in the same ballpark as the supergiant Betelgeuse. Aldebaran is also bright because it is close, just over 65 light years away.
Taurus' appearance is also skewed this year because of a visitor - the giant planet Saturn. In fact, if you imagine the V-shape of the Hyades cluster as being the face of the bull, Saturn completes the picture by giving the Taurus his second eye (Aldebaran is the first).
Rounding out our survey of the stars of winter is the small beautiful cluster M45, known popularly as the Pleiades. Find it above Aldebaran and Saturn, nearly at the top of the sky this month. Most people can only make out six or seven stars in the cluster, though some sharp-eyed individuals claim to be able to spot 10 or 12. The cluster actually contains hundreds of stars and makes a great target for binocular observing. The stars in this cluster are mostly between 350 and 450 light years away, and are all hot young blue stars. Most average about 1000 times the brightness of our sun. Each of the stars visible to the unaided eye in this cluster are larger and brighter than Sirius, but again, their distance causes them to fade to the limits of visibility.
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