Winter Circle

© 2000 by Robert C. Moler

No other seasonal sky has so many first magnitude stars as winter. First magnitude stars comprise all nighttime stars brighter than magnitude +1.5. The range actually runs from +1.49 to -1.42, nearly three magnitudes, but only includes 21 stars. One third of them, seven to be exact, are contained in The Winter Circle.

A note about the range: The Greek astronomer Hipparchus in the 2nd century BC ranked the visible stars in six magnitude steps. The brightest being of first magnitude, the dimmest to the unaided eye was sixth magnitude. Therefore magnitudes are a bit like golf scores. The lower the magnitude the brighter the star. Astronomers later created a mathematical framework for the magnitude scale in which a difference of 5 magnitude steps is a brightness difference of 100. Therefore each magnitude step is the 5th root of 100, or 2.512 approximately.

This is the last year for a while when the Winter Circle will remain pure, uncontaminated by bright planets. As can be seen, Saturn is visible to the right of the chart above. Next year both Jupiter and Saturn will be in Taurus, confusing the novice. They will move farther left each year.

At the top of the circle is Capella in Auriga the Charioteer. Capella is circumpolar in the Grand Traverse area; a constant reminder, even in summer that winter will soon arrive. Capella is 42 light years away, meaning the light we see from it tonight left there in 1958. Capella is actually two stars orbiting one another and too close to resolve with telescopes from earth. It is large yellow star, a bit cooler than the sun, but 90 times brighter than the sun, and a white star hotter and 70 times brighter than the sun. The stars orbit each other every 104 days.

Moving clockwise, the next star is Aldebaran, which appears at the upper left tip of a letter V group of stars that is the face of the bull. Aldebaran isn't actually part of the group, called the Hyades star cluster. The cluster is about 151 light years away, while Aldebaran is 65. The star has an orangish hue because its surface is cooler than the sun's. However Aldebaran is 40 times larger in diameter, and shines 125 times brighter than the sun. Aldebaran is the type of star that uses helium to generate its energy.

Continuing around we find Rigel whose longer Arabic name of which Rigel is the first part means Left Leg of the Giant. Rigel is a giant itself, actually a supergiant. Its surface temperature is more than twice as hot as the sun. It is 57 thousand times as bright as the sun and 50 times its diameter. It's actual distance is not well known, but is generally thought to be about 900 light years. Those with telescopes might be able to spot a close companion star to Rigel, just at the edge of the bright arc light image of Rigel itself.

Just past the bottom of the circle we find Sirius, the brightest night time star. The name Sirius means dazzling one, and it does twinkle merrily. It's also called the Dog Star because it lies in the heart of Orion's large hunting Dog, Canis Major. Sirius is so bright principally because it close to us. While 50 trillion miles doesn't seem close, that comes to only 8.6 light years, making Sirius only twice as far as the closest star to the sun. Sirius is pure white in color, compared to the sun's yellow. It was sacred to the ancient Egyptians, as Sothis, whose first appearance in the morning sky signaled the flooding of the Nile.

Climbing up and to the left, and at the same level as the constellation Orion's shoulders is the bright star Procyon in the constellation Canis Minor, or lesser dog. Procyon is an interesting name. It means "Before the dog", which itself means that Procyon, though east of Sirius rises before it. This is due to Procyon's more northerly position. This effect doesn't work south of the equator, however. Procyon is a star much like Sirius but 11.4 light years away.

Moving up to complete the outside of the circle is Pollux in Gemini, at the head of his namesake. His brother star Castor lies nearby, just missing the first magnitude list. Pollux the pugilist is a single star 34 light years away. I once took a photograph of Pollux during the daytime under special circumstances: it happened to be near the totally eclipsed sun.

In the center of the circle is the most famous of all these stars at least by name. The is Betelgeuse, popularly pronounced "Beetle Juice". The screenwriter of the movie of the same name was an amateur astronomer and liked the sound of the name. Betelgeuse is the bright red star is in the constellation Orion's shoulder. It's about 500 light years away. Despite its great distance, it is the star whose surface is easiest seen, after the sun of course. That's because it's so big, as large around as the orbit of Jupiter, it turns out. The Hubble telescope has sent back pictures of the star, and has found a bright spot, seen in ultraviolet light on it's surface. A star like Betelgeuse is so bloated that it can be described as a red hot vacuum, thus it's edge or limb is much darker than its center, rather than the sun's less pronounced limb darkening.

Enjoy these stars and their constellations as they slowly slide westward night by night. They will begin to disappear in evening twilight when spring arrives to present us with a much more sparse sky with but three first magnitude stars.


Questions? Comments? Send Email to me at bob@bjmoler.org

Updated: 01/31/00