The University Record, February 11, 1997

Sirius sparkles in Michigan's southern skies in February

Sirius, the Dog Star, is not only the brightest star in Michigan's night sky, it is also the closest one, says U-M astronomer Richard Teske.

"Between 9 and 10 p.m. on February nights, Sirius can be found almost directly south of us about one-third of the way up from the southern horizon," Teske says. "Watch its colorful twinkling. The remarkable display is caused by warm currents rising from buildings and houses in the frigid winter air which momentarily deflect the star's light."

Sirius owes its prominence in our sky to its nearness to Earth, Teske explains. Sirius is only 8.6 light-years away from us. It is more than 20 times brighter and nearly twice the size of our sun, but most astronomers classify it as an ordinary, garden variety star. "Many of the stars we see at night are hundreds of times brighter than Sirius and dozens of times larger; but they appear fainter, because they are much farther away," he says.

Sirius grows steadily brighter as its distance from Earth diminishes; the star moves toward us while sliding off to one side. "It's as if we stood far back from the side of a road watching an automobile approach," Teske explains. "The star's pace is only slightly faster than the velocity of the Apollo astronauts as they rocketed from Earth to the moon. By the year 61,997, Sirius will be closest to Earth---just 7.8 light-years away---after that, it passes by and slowly recedes again."

Among astronomers, Sirius' greatest claim to fame is its strange companion---the cinder of a burned-out star. The two are locked in a mutual gravitational embrace that causes them to circle one another perpetually, completing one orbit every 50 years.

Too faint to be seen except with a large telescope, Sirius' companion is a type of star astronomers call a "white dwarf," because of its neutral color and size. "White dwarf stars are only about the size of Earth, yet each typically contains half as much matter as the sun," Teske says. "With so much material packed into such a small volume, their density is extraordinarily great. An old textbook of mine says that a teaspoon of white dwarf matter would weigh as much as seven circus elephants, but I suppose that depends on how one selects the elephants."

"Thousands of white dwarfs exist---some of them single stars, some that orbit around other stars as does Sirius' partner. They are the searingly hot cores of normal stars that long ago ceased to generate their own energy and so have died," Teske says. These star cores shed their outer layers and slowly cool off, radiating their remaining heat into space. Year by year, they glow more feebly, but 10 billion years or more will be needed before they wink out altogether---a time nearly equal to the present age of the universe, according to Teske. It is believed that Sirius' companion star became a white dwarf only 80 million years ago.

The extended periods of time required for these dense stars to cool to dark coals is one reason they have attracted astronomers' interest, Teske says. "As the corpses of stars that were born and died in the early history of the universe, the cinders represent a kind of fossil record of the first stars ever born."