Look southward at dusk and nightfall, and you can’t miss Sirius, the brightest star in the nighttime sky. Only the planets Venus and Jupiter are brighter, but they’re in the western sky, due to be near the waxing crescent moon beginning around March 23. Venus and Jupiter are temporary visitors to our sky at this time of year. Sirius is a permanent fixture of March skies, where it always outshines other stars.
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.
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 distance 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.
Bottom line: Sirius is our sky’s brightest star (although not as bright as the planets Venus and Jupiter, now in the west after sunset), 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.
The Stellar Magnitude System by Alan M. MacRobert