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Is Sirius the most luminous star in the sky?

11mar26_430

Tonight for March 10, 2015

Look outside tonight, and can’t miss Sirius, the brightest star in the nighttime sky. Only the planets Venus and Jupiter outshine Sirius on March 2015 evenings.

You won’t mistake the planets for Sirius, if you look at a familiar star pattern near Sirius on the sky’s dome. No matter where you live on Earth, just follow the three medium-bright stars in Orion’s Belt to locate Sirius.

See the sky chart below:

If you're not sure the bright star you're seeing is Sirius, remember ... Orion's Belt always points to Sirius.

If you’re not sure the bright star you’re seeing is Sirius, remember … Orion’s Belt always points to Sirius.

The star Sirius.  Photo by EarthSky friend on G+ Martin Marthadinata.

The star Sirius. Photo by EarthSky friend on G+ Martin Marthadinata.

Mia asked EarthSky:

Isn’t there a brighter star than Sirius 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’s 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.

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.

To get a better idea of a star’s true luminosity, astronomers like to list stars according to “absolute magnitude.” Absolute magnitude measures the brightness of the stars as if they were all an equal 32.6 light-years distant.

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If you're an early riser, use the waning crescent moon to locate the planet Saturn and the star Antares over the next several mornings. The green line depicts the ecliptic.

If you’re an early riser, use the waning moon to locate the planet Saturn and the star Antares on the mornings of March 11-13, 2015. The green line depicts the ecliptic.

At 32.6 light-years away, our sun would barely be visible as a speck of light. In stark contrast, Aludra, Wezen, and Omicron 2 at 32.6 light-years away 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. So if all these stars were equally distant, these super-luminous stars in Canis Major would shine thousands of times more brilliantly than Sirius.

Once again, absolute visual magnitude measures the star’s brightness as it would appear to the eye at 32.6 light-years away. Apparent visual magnitude refers to a star’s brightness as seen by the eye from Earth.

Read more about stellar luminosity, the true brightnesses of stars

Sirius, from Matt Schulze in Santa Fe, New Mexico.

Sirius, from Matt Schulze in Santa Fe, New Mexico.

Bottom line: Sirius is our sky’s brightest star (although not as bright as the planets Jupiter and Venus), but not the most luminous star in the sky. In other words, it’s an ordinary star that only appears bright to us because it is relatively nearby.

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