Stars · the brightest

Sirius: the brightest star in the night sky

Sirius A, a large blue-white star, drawn in Exo-View

Sirius, the Dog Star, is the brightest star in the night sky and one of our nearest neighbours, 8.6 light-years away. It is a white star twice the Sun's mass, with a companion that was found because of the wobble it causes: Sirius B, a white dwarf the size of the Earth. Exo-View flies you there.

Fly to Sirius See the Milky Way

Distance: 8.6 light-years, the fifth-nearest star system
Kind: White star, spectral type A1
Size: 1.7 times the Sun's width, with twice its mass
Light: About 25 times the Sun's
Surface temperature: About 9,900 K, against the Sun's 5,800 K
Companion: Sirius B, a white dwarf with about the Sun's mass in the size of the Earth, circling every 50 years
Seen in: The constellation Canis Major, the Great Dog

Why it looks so bright

Sirius is not an unusually powerful star: it gives out about 25 times the Sun's light. It outshines every other star in our sky because it is so near: of the stars close enough to compare, only a few are nearer, and most of those are too faint to see. It is best seen on winter evenings in the northern hemisphere, low in the south-east, and its steady white-blue flickers in colour through our unsteady air.

The dog days

To the ancient Egyptians Sirius was Sopdet, and when it first showed in the dawn sky just before sunrise, in midsummer, the Nile was about to flood, so its return began their year. The Greeks and Romans called the hot weeks when it rises with the Sun the dog days, a name that is still used.

A companion the size of the Earth

In 1844 Friedrich Bessel saw that Sirius wobbles as if it were circling something unseen; in 1862 the telescope-maker Alvan Clark spotted the companion, Sirius B. It is a white dwarf, the bare core of a star that ran out of fuel, with about the Sun's mass packed into a ball about as wide as the Earth and a surface hotter than Sirius A's. It is 10,000 times fainter than the main star and hard to see next to its glare. It goes round Sirius A every 50 years.

In Exo-View

The key below flies from the Sun to Sirius. Its companion circles it, drawn on their real orbit and to their real sizes, so you can see how small a white dwarf is beside a star.