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.