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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
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
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.
References for Cloudy Evening
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