Observer's Notebook for February, 2000

The Brightest Star

by John Rummel

Warm summer nights and astronomy are perfect partners. What can be better than lying on a blanket on a balmy night casually studying the milky way as it arches overhead?

Is it any wonder that most casual watchers of the sky are largely unfamiliar with the winter stars? Cold winter nights can be discouraging, but sky-watchers who venture out will not be disappointed. For instance, no summer night can match a winter sky for the sheer number of bright stars. In fact, of the 20 brightest stars in the sky, northern winter nights hold 7 of them, all in approximately the same part of the sky. The familiar figure of Orion has numbers 7 and 10 on the list of bright stars, Rigel and Betelgeuse. Neighboring Gemini has Pollux, number 17 on the list. Close by are Aldebaran in Taurus (number 14), Procyon in Canis Minor (number 8), and Capella in Auriga (number 6).

Top dog on the list though, coming in at number one is Sirius. The brightest star in the sky (other than our own sun, of course) graces our winter skies all night long. Sirius is master of an otherwise undistinguished constellation, Canis Major, the big dog. For thousands of years, possibly longer, Sirius has been known as the "dog star."

Finding Sirius is easy. Find the unmistakable belt-stars of Orion, and follow them down and to the left. They point roughly at Sirius. Confirm its identification by its brightness, and you know you've found the dog star.

Nearly 10 times as bright as Betelgeuse or Rigel in neighboring Orion, Sirius is one of our sun's closest neighbors in the galaxy. At just under 9 light years distant, Sirius is a bright, hot star - significantly larger and brighter than our own sun. Inhabitants of a planet circling Sirius would see our sun as a 1st magnitude star somewhere between Vega and Altair, permanently spoiling the appearance of the "summer triangle" for Sirians.

To present day observers, Sirius appears white with a possible hint of blue. But ancient writers, including the great Ptolemy, spoke of Sirius as being "ruddy" or reddish. It seems very unlikely that a star could have changed colors so quickly (2 thousand years is barely an eye-blink in the life of a star). This discrepancy is one of the enduring mysteries to historians of astronomy.

For those of us in the northern hemisphere, Sirius is the closest star easily visible. Of the close naked-eye stars, only Alpha Centauri A and B are closer, but these are visible only from southern latitudes. In the northern sky, Sirius is practically a back-fence neighbor. A few other stars are closer, but all are too faint to be seen without telescopes and finder charts. Of the other brighter stars in Canis Major, Sirius is alone in being so close. The other stars brighter than 4th magnitude range from about 60 to over 3000 light years away.

Like about half the stars in the sky, Sirius is part of a binary system. It has a very faint companion, known as the pup, or more properly, Sirius B, which can only be glimpsed with professional equipment, or under extremely favorable circumstances. Sirius B's existence was inferred for years because astronomers monitoring Sirius had noticed a slight wobble in its position over several decades. In 1862, renowned telescope maker Alvan Clark was testing a newly ground 18 inch lens when he noticed the unmistakable presence of the faint companion, later identified spectroscopically as the first definitive white dwarf star.

Bundle up in your warmest clothes, take a thermos of hot soup and the finder chart located on the next page, and go out for an evening of winter star gazing. Reacquaint yourself with the winter constellations, and find a few new ones.

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