Red dwarfs, the commonest stars

What is the commonest kind of star? Not one like the Sun, and not any star you have ever seen. It is the red dwarf: a small, cool, dim star, glowing a dull orange-red. About three-quarters of all the stars in our galaxy are thought to be red dwarfs. Of the 60 stars nearest to the Sun, 50 are. And not one red dwarf anywhere in the sky is bright enough to see without a telescope.

The nearest star of all is one of them. Proxima Centauri is 4.25 light years away, closer than anything else beyond the Sun, and it is about a hundred times too faint for the eye. It was not found until 1915. If it took the Sun’s place, it would give the Earth about 600 times less heat, and so little light that noon would be about as dim as late dusk.

A red dwarf is a real star. It makes its light the way the Sun does, by pressing hydrogen into helium at its centre. But it is small. The smallest have about a twelfth of the Sun’s mass and are hardly wider than Jupiter. Proxima is about a seventh as wide as the Sun. With so little weight pressing down, the centre is cooler and the hydrogen burns very slowly indeed.

That slowness is their great secret. The Sun will shine for about 10 billion years. A small red dwarf will shine for about 10 trillion, a thousand times longer. A red dwarf also stirs itself like a pan of soup, so it can use up nearly all of its hydrogen, where the Sun will use only the part at its centre. The universe is 13.8 billion years old. No red dwarf has ever died of old age, anywhere. There has not been time.

They are not quiet, though. Many red dwarfs throw off sudden flares, bursts of light and X-rays from tangled magnetism at the surface. In March 2016 Proxima brightened 68 times over in a flare that lasted minutes. For a moment the star could just have been seen without a telescope, by someone under a dark sky who happened to be looking.

Red dwarfs have planets, plenty of them, and mostly small rocky ones. Proxima has at least two. One, Proxima b, is at least as heavy as the Earth and sits where water could be liquid. To be warm enough it must huddle close: its year lasts 11 days. That close, a planet is probably held with one face always towards its star, one side in endless day and the other in endless night, and takes the full force of every flare. Whether anything could live there is one of the big open questions.