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Star Watch

To questions that dogged astronomers, a Sirius solution

By Alan M. MacRobert
Globe Correspondent / February 6, 2010

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Look straight east after dinnertime this week, rather high, and there looking back at you will be the steady fire-eye of Mars.

Just a week ago, Mars was at its brightest and closest to Earth for the year, and it’s still blazing in full glory. It far outshines Castor and Pollux above it, Regulus way below it, and Procyon, the Little Dog Star, well off to its right.

You have to look farther afield, far to the lower right of Procyon, to pick up the one star in the sky that’s a proper match for Mars. That would be Sirius, the Big Dog Star (outside the right of the frame here). Sirius sparkles as icy white as Mars is fiery yellow-orange. Their difference in color is unmistakable.

And thereby hangs a tale.

Sirius is white because it’s white-hot, much hotter than our sun, and that’s the way it has been throughout human history. Like the sun, Sirius is a settled, stable, hydrogen-fusing star in the midst of its long prime of life. A million years is nothing to a star like this.

But for a while, historians of ancient Greece and Rome thought they had discovered otherwise.

Around 135 AD, the astronomer Claudius Ptolemy in Alexandria listed Sirius as one of six bright stars that have a “sub-red’’ or fiery character. The other five really do look fire-colored, and this led to centuries of speculation that Sirius has changed color within historical times. Several other ancient writers called it reddish, too, though others described it as white or blue-white, the way it looks now. Could the star have been a red giant just a couple of thousand years ago? Was Sirius’s faint companion star, now a white dwarf, the red giant? Did a passing interstellar dust cloud temporarily redden our view of Sirius, like smog reddening the setting sun?

Astronomers worked through all the possibilities they could think of. None turned out to be remotely plausible. Such drastic events happening so recently to such a nearby star would have left obvious traces, blown-off shells of nebular gas, for instance. Nevertheless, historians insisted that the words of the ancients trumped astrophysics. Why else would anyone call the star reddish or fiery in character, unless it looked that way?

The likely solution was published in 1992 by a Harvard classics student named Roger Ceragioli. For the Greeks and Romans, the most important thing about Sirius was the date when it first became visible rising in the dawn sky. This happened in mid- to late July, the hottest season. Accordingly, the ancients called these days the “dog days’’ after Sirius, the Dog Star. The name Sirius was said to come from the same Greek root as searing or scorching, in other words, firelike. Astrological associations of Sirius with fire, war, and red-raging madness abounded in ancient folklore.

So poets had every reason to use such colorful terms, regardless of how Sirius might actually look.

Moreover, Sirius does look reddish when first making its appearance over the horizon in the dawn, because of filtering by Earth’s atmosphere, just like the rising or setting sun.

Other attempts to find great astrophysical discoveries in bits of ancient folklore have also come to nothing, by and large. Ancient mythmakers and tale-tellers did not have the mind-sets of today’s astrophysicists and were not making scientific observations for their benefit.

Mars on the wane
Keep watch on Mars in the coming weeks and months, and you’ll see it fading. It’s dropping back into the distance as Earth pulls ahead of it in our faster orbit around the Sun. Mars will continue to dim for the rest of this winter, spring, and summer.

And there’s lots else going on in the eastern evening sky. Look above Sirius for the bright constellation Orion. Passing nearly overhead is the bright star Capella.

And if you have a pair of binoculars, look a little to the lower right of Mars, in the same field of view, for the dim Beehive Star Cluster, a tight swarm of dozens of faint suns.

Whole-sky maps
Easy-to-use maps of stars and constellations across the entire evening sky are available at the website SkyandTelescope.com/gettingstarted.

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