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| Brightest Stars on Aug 27, 2013

61 Cygni, the Flying Star

61 Cygni is not bright. But it moves exceptionally rapidly against the background of more distant stars. Its motion reveals its nearness to Earth.

This star, 61 Cygni is not among the brightest stars. In fact, it takes some effort just to find this star, because it is not much brighter than the faintest stars visible to the unaided human eye. However, it is among the most important of stars visible without optical aid. It has one of the largest proper motions – that is, sideways motions along our line of sight – of any star in our sky. Its large proper motion has given 61 Cygni the nickname Flying Star.

In contrast to the “fixed” stars, why does 61 Cygni move so rapidly along our line of sight? The reason is 61 Cygni’s nearness to Earth. It is one of the closest stars to Earth. While not actually the closest star to the sun (that honor goes toAlpha Centauri), 61 Cygni is just 11.4 light years distant, making it the fourth-closest star visible to the unaided eye, after Alpha Centauri, Sirius, and Epsilon Eridani.

61 Cygni’s motion across our sky cannot be easily detected with the eye alone over the span of a human lifetime, by the way. Astronomers discovered its large proper motion via careful observation.

Size comparison of the Sun (left), 61 Cygni A (lower) and 61 Cygni B (upper right).  Image via RJHall and Wikimedia Commons.

Size comparison of the Sun (left), 61 Cygni A (lower) and 61 Cygni B (upper right). Image via RJHall and Wikimedia Commons.

This star is classified as a K2V, which means that it is an orange (K2) “main sequence” (V) star. In fact, 61 Cygni is a binary star, although the double nature cannot be seen with just the eye. There is a pair of K-type dwarf stars in the single point of light we see as 61 Cygni. They orbit each other in a period of about 659 years.

From hottest to coolest, the spectral sequence is OBAFGKM, with the sun being a yellow G type star, compared to the two K-type components of 61 Cygni. Even taken together, the two stars of 61 Cygni cannot match our local star in total energy output.

How to see 61 Cygni 61 Cygni is roughly halfway between easily seen Deneb, in Cygnus the Swan, and the end of the Swan’s east wing, the star Zeta Cygni (otherwise known as Geinah). Several other similarly dim stars are located nearby, and an accurate star chart is need to properly identify 61 Cygni. As with Cygnus itself, 61 Cygni is best observed in the fall and winter.

History and Mythology While 61 Cygni boasts no mythology, being barely visible to the eye, the ancients apparently leaving no written reference to it at all, its role in the history of astronomy is assured. As early as the late 1700s, astronomers recognized that its apparent motion among the stars is far greater than the average. Although this motion would take centuries to notice with the unaided eye, telescopic observations revealed a motion so startling that 61 Cygni became known as the “Flying Star.” This relatively fast motion indicates that the star is astronomically nearby, prompting German astronomer F. W. Bessel to use 61 Cygni to be the first star to have its distance measured by observation. As a result it is sometimes called “Bessel’s Star.”

61 Cygni’s position is RA: 21h 06m 51s, dec: +38° 44′ 29″