61 CYGNI THE STAR THAT CHANGED EVERYTHING

In the newly released History of the Universe in 21 Stars (and 3 Imposters), the star 61 Cygni has been revealed to be the smoking gun in our understanding of the cosmos

- Giles Sparrow

Step outside and take a look at the early evening sky at this time of year and the constellation of Cygnus, the Swan, is hard to miss. This prominent cross-shaped constellation hangs high overhead, looking like a swan with outstretched wings and a long neck stretching along the Milky Way towards the southern horizon. The bright-blue star Deneb marks the Swan’s tail high in the sky – the upper-left corner of the famous northern ‘Summer Triangle’ – while binoculars show that Albireo, on the tip of its beak, is a beautiful double star.

Cygnus is packed with beautiful and fascinating objects, so it’s easy to overlook 61 Cygni, an apparently insignificant star tucked behind the Swan’s right or eastern wing. But it’s worth a look, not only because binoculars will show it as an attractive double star – a pair of yellow-orange stars separated by roughly the same diameter of Saturn in Earth’s skies – but also because of its pivotal role in the history of astronomy. Out of all the stars in the sky, 61 Cygni was the first one to have its distance precisely measured, proving conclusively that the stars are blazing objects like our own Sun seen over vast distances.

Ancient and medieval astronomers mostly believed in an Earth-centred model of the universe – a system with Earth sitting at the centre of the cosmos, orbited by the Moon, Sun and planets, with the stars as either tiny lights affixed to an outer, all-encompassing sphere, or holes in that sphere permitting light to shine through from beyond. In general the universe was assumed to be compact, with the stars not much further beyond Saturn, then the most distant and slowest moving planet.