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The Winter Circle
updated: Jan 18, 2014, 11:00 AM

By Chuck McPartlin

The night skies of winter in the northern hemisphere contain the densest grouping of bright stars visible during the entire year. Luckily for us in Santa Barbara, our winter weather is pretty conducive to going out and enjoying views of the stars.

We can see these bright stars and constellations prominently displayed at around 10 PM in mid-January, and easy to find because they are arrayed around a readily recognizable constellation - Orion the Hunter, with his distinctive belt of three stars. Face generally south, toward our ocean horizon, and look up.

The constellations and brilliant stars surrounding Orion form a large asterism known as the Winter Circle, centered on the bright orange-red star in the shoulder of Orion. This is Betelgeuse, the Armpit of the Giant. Betelgeuse is a red supergiant star about 700 times the diameter of the Sun, at an estimated distance of 570 light years.

On the other side of Orion's belt from Betelgeuse is a blue supergiant star known as Rigel, the Foot. Rigel is 74 times the diameter of the Sun, and about 860 light years away. It's intrinsically about four times brighter than Betelgeuse, and much hotter. The color, or spectral type of a star tells you its surface temperature. Cooler stars are redder, and hotter stars are bluer. Yellow stars like the Sun are somewhere in the middle. The surface temperature of Betelgeuse is about 3,700 Kelvin (6,200° F). The Sun's is about 5,800 K (10,000° F). Rigel is at 11,500 K (20,200° F).

If you follow the line of the belt stars up and right, relative to the figure of Orion, they point the way to another bright orange star. This is Aldebaran, an eye of Taurus, the Bull. Its name comes from the Follower of the Pleiades, the beautiful star cluster in the shoulder of the Bull. Aldebaran is a red giant star about 43 times the diameter of the Sun, and about 67 light years away. Its surface temperature is 4,000 K.

Counterclockwise and up from Aldebaran is a bright yellow star - Capella. This is the brightest star in the constellation Auriga, the Charioteer, and in the classical Greek depiction is the head of a goat being held by the chariot driver. Capella is a yellow giant star about 43 light years distant, 14 times the diameter of the Sun. Its surface temperature is 4,900 K.

Continuing counterclockwise, we come to Castor and Pollux, the head stars and names of Gemini, the Twins. Castor is a multiple star, with 6 components. Its main star is white with just over 3 times the Sun's diameter, and 51 light years away. Its surface is at 9,500 K. Pollux is an orange giant, 10 times the size of the Sun, and around 34 light years away. Its surface is at 4,800 K. An exoplanet with just over twice the mass of Jupiter has been detected in orbit around Pollux.

Next in our circle, we come to Procyon, the lucida of Canis Minor, the Little Dog. Orion is accompanied by two hunting dogs, and Procyon and the dimmer star Gomeisa make up the little dog, who's nothing but a hot dog. The name Procyon comes from "before the dog", because at northern latitudes it rises earlier than the next star in the circle. Procyon is a yellow-white main-sequence star about 12 light years away, and twice the Sun's diameter. Procyon's surface is at 6,500 K.

Following the belt of Orion down and left brings you to Sirius, the brightest star in the sky (except our Sun). This is the head of Canis Major, the Big Dog, and is often called the Dog Star. It is not intrinsically very bright, but it is the closest star to our Sun that can be seen easily without optical aid from the latitude of Santa Barbara, at a distance of just under 9 light years. It is a bit less than twice the diameter of the Sun, with a surface temperature of 9,900 K.

The ancient Egyptians would wait for Sirius to rise just before the Sun - a signal that the Nile River was about to flood. And when Sirius is in the daytime sky with the Sun, in August, it was thought that it added to the heat of the Dog Days of summer.

Sirius is a blue-white main-sequence star. Its name derives from "sizzling". Since we always see it fairly low to the south, its light passes through lots of our atmosphere, and any turbulence or temperature variations cause it to twinkle fiercely, refracting it into glints with all the colors of the rainbow. Astronomical Unit members often get questions from the public in the winter about the mysterious colorful flashing light out over the ocean.

That bright object in the knees of Gemini that outshines even Sirius? That's the planet Jupiter, currently adding to the luster of our Winter Circle. Jupiter is a bit closer to us than the stars in the circle, only about 35 light minutes away. It shines mainly by reflecting the light of the Sun.

Take a look at it in binoculars, and see whether you can spot its four bright Galilean Moons. Through a telescope, you can also see two dark cloud bands in Jupiter's atmosphere that make it look like a Double Cheeseburger in space.

Bon appétit!

References for Cloudy Evening

Bright Stars

 

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