Sirius B

Sirius is the brightest star in the night sky. In 1844 the German astronomer Friedrich Bessel noticed that it does not move across the sky in a straight line but wobbles, as if something unseen were tugging on it. He concluded that Sirius has a hidden companion. On 31 January 1862 the American telescope maker Alvan Graham Clark, testing a new 18.5-inch lens, saw it: a faint point of light right beside Sirius. It became known as Sirius B.

Small and heavy

Sirius B turned out to be very strange. Its orbit showed that it has about the same mass as the Sun. But it is so faint that it had to be tiny. Modern measurements put its radius at about 5 600 kilometres, a little smaller than Earth. The whole mass of a Sun is packed into a ball you could fit inside our planet.

Sirius B is a white dwarf: the exposed core of a star that has used up its fuel. It was once a star of about five solar masses, much heavier and brighter than Sirius A is now. It burned through its fuel in about a hundred million years, puffed off its outer layers, and left behind its dense core. Its surface is still about 25 000 K (24 730 °C), far hotter than the Sun’s, and it will take billions of years to cool.

A test of Einstein

Gravity at the surface of a white dwarf is so strong that light climbing away from it loses energy and is shifted towards the red, as Einstein’s general relativity predicts. In 1925 Walter Adams reported measuring this shift in Sirius B’s light. His result was later shown to be wrong, contaminated by light from Sirius A. In 2005 Martin Barstow and colleagues used the Hubble Space Telescope to measure the shift cleanly and found it matches a mass of about 1.02 Suns.

Why it is hard to see

Sirius B orbits Sirius A every 50 years, between 8 and 32 times the Earth–Sun distance away. Seen from Earth it is never more than a few arcseconds from Sirius A, which outshines it about ten thousand times. With a large telescope and steady air it can be glimpsed, but most people who look at Sirius never see its dead companion.