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Star Watch

Look south to see winter's brightest constellations

Email|Print| Text size + By Alan M. MacRobert
Globe Correspondent / February 2, 2008

The brightest constellations of winter shine high in the south these evenings, and among them glitters icy-white Sirius, the brightest star in the night sky. It is the Dog Star, following faithfully behind Orion the Hunter, as Orion stalks westward.

Even if you live under city light pollution where stars are practically invisible, these will shine through. Just avoid nearby lights that will blind your night vision completely. The fainter parts of the constellations drawn here might be invisible, but you should have no trouble with Orion's main stars - including the three-star row of Orion's Belt - and Sirius.

You can also spot Procyon to their left. Its name means before the dog, and when it rises above the eastern horizon, Sirius is not far behind.

From a dark-sky site, the whole panorama is much more striking. Orion's upraised club and outstretched shield become dimly visible. So does Sirius's entire constellation: Canis Major, the Big Dog, whose stars can be connected to form a very realistic, proud-looking canine, as shown here.

Procyon is part of the constellation Canis Minor, the Little Dog, Orion's other hunting partner. This one requires more imagination. It has only one star other than Procyon that is readily visible.

Sirius to Cambridgeport

Sirius will forever be linked to a neighborhood in Cambridge by Brookline Street and Memorial Drive. The 19th-century workshop of Alvan Clark & Sons, the foremost telescope makers of their time, was located here. They specialized in refractors, the traditional telescope design, with a big lens in front instead of a big mirror inside.

On the night of Jan. 31, 1862, Clark and one of his sons were putting the final touches on the largest telescope lens ever made up to then, 18 1/2 inches in diameter. They set it up to test it on Sirius. Clark Jr. started examining the star and exclaimed, "Father, Sirius has a companion."

The Clarks did not realize the full significance of their find, but within a day word got to George P. Bond, director of Harvard Observatory a few miles up the road, who did. Astronomers in Europe, he knew, had deduced that Sirius must have an invisible companion orbiting it, judging by a very slight, 50-year wobble that had been measured in Sirius's position on the sky. Bond managed to confirm the sighting using Harvard's 15-inch telescope on Observatory Hill.

Word reached Europe by sailing ship in early March. There the world's leading astronomers - who had already searched unsuccessfully for the "dark companion of Sirius" - were embarrassed at being bested by upstart Americans who had hit on it by accident. The episode greatly boosted the Clark firm's reputation for making the world's best telescopes.

But that was only the start. The Dog Star's companion became known as the Pup. The gravitational effect that each star had on the other, as they revolved in orbit, told astronomers that the companion had to be nearly as massive as Sirius, that is, about as heavy as the sun. Yet its faintness showed that it had just more than a hundredth of the sun's light output. That it was white hot - even hotter than the sun - meant that it had to be tiny to appear so dim. By 1915 astronomers determined that the pup had to be only about the size of Earth, despite having about 300,000 times Earth's mass.

This meant it was made of something no one had ever dreamed of, a material so dense that a teaspoon of it would weigh several tons.

The identification of such an alien form of matter was one of the great scientific breakthroughs of the day and spurred advances in atomic theory to account for it. We now know that Sirius B (the Pup's formal name) is the nearest white dwarf star, and that these superdense objects are the common end stage of normal stars that have lived out their lives and used up their nuclear fuel. The sun, by the best modern calculations, will end up as a white dwarf 7.8 billion years from now.

And as luck would have it, Procyon too turned out to have a white-dwarf companion. This one is even dimmer and orbits closer to its primary star, so it was harder to find. It, too, was first identified by a slight, decades-long wobble in the primary star's position.

So both the big and little dog stars following Orion have similar, very remarkable little pups. It is one of the many fascinating coincidences that keep cropping up in skywatching lore.

Alan M. MacRobert is a senior editor of Sky & Telescope magazine (SkyandTelescope.com) in Cambridge. His Star Watch column appears the first Saturday of every month.

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