Birth of Friedrich Bessel
On July 22nd 1784 the German astronomer Friedrich Wilhelm Bessel was born. He was the first person to measure the distance to a star. He worked out the distance to the star 61 Cygni in 1838.
This star is not bright enough to have been given a Greek letter. In a constellation the brightest stars are labelled using the Greek alphabet with alpha being the brightest followed by beta, gamma and so on through too omega. However this will only give us 24 letters.
John Flamsteed made a thorough catalogue of the naked stars in the sky, that is to say stars that can be seen without needing a telescope. This was published in 1725 after his death. All these bright stars including those with a Greek letter would have a number preceded by the letters FL. The FL letters standing for Flamsteed. Today astronomers don’t normally add the FL but just use the number of the star. Hence FL 61 Cygni is usually referred to as 61 Cygni.
61 Cygni is sometimes called the ‘Flying Star’ because it moves quickly across the sky is only 11 light years away. Its distance was measured by using what is known as the parallax method.
The parallax angle p is illustrated in the following figure.
If this angle, which corresponds to a small shift in apparent position of the star when observed from different points of the Earth’s orbit around the Sun, can be measured accurately, the distance can then be determined from simple trigonometry.
Although not a bright star 61 Cygni has its place in astronomical history.
Bessel was director of the Konigsberg observatory in Germany when he made this discovery he held this position from 1810 until his death in 1846.
He worked out the position of about 75,000 stars and predicted the positions of the then unknown companion stars of Sirius and Procyon.