Sirius, the brightest star in the night sky

Sirius is the brightest star in the night sky and one of the closest: its light takes about 8.6 years to reach us. It belongs to Canis Major, the Great Dog, and NASA's headlines still call it the Dog Star.

Where is it from where I am?

How to spot it: Orion's three belt stars point at it. Source (opens in a new tab)

Why it is so bright

Sirius is bright in our sky mostly because it is near. NASA puts it at 8.6 light-years (opens in a new tab), the fifth-closest star system known, and gives the main star about twice the Sun's mass. In the Hipparcos satellite's measurements it is magnitude −1.44, first on ESA's list of the brightest stars (opens in a new tab); on the magnitude scale, lower numbers are brighter.

The companion you cannot see

Sirius has a partner. Sirius B is a white dwarf, the leftover core of a star that has already run through its life. According to NASA (opens in a new tab) it is about 7,500 miles across, a little smaller than Earth, yet it holds 98 percent of the Sun's mass, and it is some 10,000 times fainter than Sirius A. That glare is why nobody saw it until 1862. The two circle each other every 50 years.

How to find it

Find Orion first. His belt is three stars in a short, straight row, and that row points at Sirius: NASA's tip (opens in a new tab) for northern skywatchers is to follow the belt to the left and slightly downward. From the Southern Hemisphere the whole scene is turned round, but the belt still points the same way along the sky.

Low in the sky, Sirius often seems to flash different colours. That is our own air: near the horizon (opens in a new tab), turbulence in the atmosphere makes its light shimmer and flicker.

Where on Earth, and when

It rises from anywhere south of about 73°N.

South of about 73°S, it never sets.

It climbs at least 20° high, the finder's threshold for a suggestion, from latitudes between 87°S and 53°N.

It is highest in the evening sky, at about 9 p.m. local time, in mid-February, from either hemisphere, wherever it rises. With daylight saving time, that is nearer 10 p.m. on the clock.

Worked out by this site from the Bright Star Catalogue, for a flat, clear horizon. Hills, buildings, haze and city light all cut into this; the air also lifts objects near the horizon by about half a degree. For your own place and night, use the sky finder.