What is the brightest star in the night sky?

Astronomers measure brightness in magnitudes, and the scale runs backwards: the lower the number, the brighter the star. Each step of one is about two and a half times brighter. Sirius has a magnitude of -1.46, which puts it below zero, brighter than any other star in our sky. The faintest stars you can see on a really dark night are about magnitude 6.5.

Only the Moon and some of the planets outshine it: Venus and Jupiter, and Mars and Mercury at their best. Venus can reach magnitude -4.92.

So when can you actually see it? From Europe, North America and the rest of the north, Sirius is an evening star from about December to March, low in the southern sky. From the far Arctic it never rises at all. From Australia, South Africa and South America it climbs high overhead on summer evenings. Sirius is bright enough to shine through the glare of a city, so you don't need a dark sky to find it.

The easiest way to find Sirius is to start with Orion. On a winter evening in the north, face south and look for three bright stars in a short, straight row. That is Orion's Belt. Follow the line of the belt down and to the left, for a little over two fists held at arm's length, and the first very bright star you reach is Sirius. In the Southern Hemisphere the picture is the other way up, and the belt points up towards it.

Sirius is part of the constellation Canis Major, the Great Dog, which is why it is called the Dog Star. It follows Orion, the hunter, across the sky like a dog at his heels.

Like every star, Sirius rises in the east and sets in the west, because Earth turns beneath the sky. From northern Europe it stays low all night, never far above the southern horizon. Look for it after the sky is fully dark, when it has had time to climb clear of houses and trees.

On a cold night you may see Sirius flicker red, green and blue, several times a second. The colours are not coming from the star. Sirius is a white star, hotter than the Sun.

The flashing happens in our own air. Earth's atmosphere bends starlight a little, and it bends each colour by a slightly different amount, like a prism. Moving pockets of warmer and cooler air shift those colours in and out of your line of sight, so you see one colour, then another.

The effect is strongest close to the horizon, where starlight passes through much more air before it reaches you. That is why it is so striking from Europe and much of North America, where Sirius never climbs high. From a southern summer sky, with Sirius overhead, it shines far more steadily.

Sirius is not alone. In 1844 the astronomer Friedrich Bessel found small, regular wobbles in its path across the sky, and realised that an unseen companion must be pulling on it. In 1862 Alvan Clark and his father saw the companion for the first time. It is called Sirius B, and sometimes the Pup.

The two stars circle each other once every 50.13 years. Sirius B is a white dwarf: the hot core left behind by a star that has run out of fuel. Long ago it was the heavier of the pair, so it aged faster, threw off its outer layers and shrank.

What is left is extraordinary. Sirius B holds about 1 times the mass of the Sun, yet it is only about 11,000 kilometres across. Earth is about 1.1 times as wide. Sirius B is so much fainter than Sirius that it is lost in its glare, and you need a large telescope to see it.

The ancient Greeks, Romans and Egyptians watched Sirius closely. In high summer it rose at the same time as the Sun, hidden in the dawn glare. They believed its heat added to the Sun's and made those weeks the hottest of the year. The Romans counted the dog days from 3 July to 11 August, the weeks around the time Sirius rose with the Sun.

The heat of summer has nothing to do with Sirius. It comes from the tilt of Earth's axis: in the northern summer, the North Pole leans towards the Sun. But the name has stayed, and so has the habit of looking for Sirius as the year turns.

Sirius may be a destination too, of a kind. Voyager 2, now in interstellar space, is heading in its direction. It will take hundreds of thousands of years to pass it, at a great distance.