61 CYGNI

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

Written by Giles Sparrow

S tep 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 crossshaped 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.

This view was shattered in the early 17th century. Early telescope observations by Galileo and others revealed moons orbiting Jupiter – conclusive proof that not everything in the universe circled the Sun – while Johannes Kepler made the theoretical breakthrough that orbits were usually ellipses rather than perfect circles. Together these discoveries backed up the idea of a Sun-centred or heliocentric universe, which had been on the table since Polish priest Nicolaus Copernicus published his controversial book On the Revolutions of the Heavenly Spheres from his deathbed in 1543.

One of several quite reasonable objections to the idea of Earth moving around the Sun was why our changing point of view didn’t affect the positions of the stars through the year. Surely if our point of view looking towards distant stars was changing by tens of millions of miles over the course of a year, the stars should appear to move back and forth due to the same effect that makes objects near at hand shift their apparent direction when seen from two slightly different viewpoints, called parallax. Most astronomers reconciled the evidence by concluding that the stars must lie at truly vast distances from Earth – so far away that parallax was undetectable to the simple telescopes of the time.