At night you cannot miss the spectacle of looking for and seeing Sirius, the brightest star in the night sky. Is it the brightest star? Yes and no. For astronomers, the word luminous refers to the intrinsic brightness of a star. To clarify, Sirius is the brightest star in our sky (although not as bright as the planets Jupiter and Venus, of course), but it is not the brightest star in the sky. In other words, it is an ordinary star that only appears very bright to us because it is relatively close to us: 8,611 light years away.
Sirius is in the southern hemisphere constellation of Canis Maior, Canis Major, and looks extraordinarily bright in Earth’s sky. No matter where you live on Earth, follow the three medium-bright stars in Orion’s Belt to locate Sirius.
Sirius is more than 20 times brighter than our Sun, and more than twice as massive.
Colors?
Many people comment that they see Sirius with flashes of various colors. It happens when you see Sirius low in the sky. The colors are just the ordinary rainbow colors in the white light of the stars. All starlight is made up of this mixture of colors. However, we notice the bright colors of Sirius more easily because it is much brighter than most stars. The extra thickness of Earth’s atmosphere near the horizon acts like a lens or prism, breaking the starlight into the colors of the rainbow. When you see Sirius low in the night sky, you are looking through more atmosphere than when the star is high.
If you look, you will notice that Sirius is less bright and less colorful (more strictly white) when he appears higher in the sky.
Brightest stars
Isn’t there a star brighter than Sirius in absolute magnitude?
Of course. Many stars in the dome of the sky are inherently brighter than Sirius but appear dimmer because they are farther away.
At least three stars in the constellation Canis Major are believed to be thousands of times brighter than Sirius: Aludra, Wezen, and Omicron 2. Although the distances to these distant stars are not precisely known, Aludra and Omicron 2 reside in an estimated 3,000 light-years distant, and Wezen about 2,000 light-years away. That is in contrast to Sirius’ distance of 8.6 light years.
To get a better idea of the true luminosity of a star, astronomers like to list stars according to their absolute magnitude. The absolute magnitude measures the brightness of the stars as if they were, for example, an equal distance of 32.6 light years.
At 32.6 light years away, our Sun would barely be visible as a point of light. However, Aludra, Wezen, and Omicron 2, 32.6 light-years away, would outshine Sirius 100 to 200 times. At 32.6 light years, Sirius would be roughly the same brightness as the star Gemini Castor (at its known distance of 52 light years). So if all of these stars were equally distant, these super-luminous stars in Canis Major (Aludra, Wezen, and Omicron 2) would be seen to shine thousands of times brighter than Sirius. But, as we said at the beginning, since Sirius is closer, he wins the game.
Remember: One light year equals about 9.55 trillion kilometers.
