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Tuesday, December 1, 2015

Sky Tonight—March 26, Is Sirius the most luminous star in the sky?

Courtesy of EarthSky
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Look southward at dusk and nightfall, and you can’t miss Sirius, the brightest star in the nighttime sky. Mia asks, “Isn’t there a brighter star in absolute magnitude which appears dimmer because of its distance?”

Yes, Mia, you are right. Sirius, in the constellation Canis Major (the Greater Dog), looks extraordinarily bright in Earth’s sky because it is only 8.6 light-years away. Many stars on the sky’s dome are intrinsically more luminous than Sirius, but appear fainter because they lie farther away. Apparent visual magnitude refers to a star’s brightness as seen by the eye from Earth, and absolute visual magnitude measures the star’s brightness as it would appear to the eye at 32.6 light-years away.

At least three stars in Canis Major are thought to be thousands of times more luminous than Sirius: Aludra, Wezen, and Omicron 2. Although the distances to these faraway stars are not known with precision, Aludra and Omicron 2 reside an estimated 3,000 light-years distant, and Wezen at about 2,000 light-years.

How far is a light-year?

At 32.6 light-years away, our sun would barely be visible as a speck of light, but Aludra, Wezen, and Omicron 2 would outshine Sirius (at its distance of 8.6 light-years) by some one to two hundred times. At 32.6 light-years, Sirius would be about the same brightness as the Gemini star Castor. If all these stars were equally distant, these super-luminous stars in Canis Major would shine thousands of times more brilliantly than Sirius.

The Stellar Magnitude System by Alan M. MacRobert

by Bruce McClure

Astronomy Picture of the Day from NASA/JPL

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