STELLAR  MOTIONS

The stars in space are not fixed. They are constantly moving, however because they are so far away, their motions can only be detected by sensitive instruments. Only over centuries can the naked eye detect the relative changes of the positions of stars. The Sun, Moon and planets move quickly over the sky since they are so close to us. In 1916, Edward Emerson Barnard announced the discovery of a star that moves 10.3 arc seconds annually across the celestial sphere. Later parallax examinations of this object showed it to be the 2nd nearest star from the Sun (outside of the Alpha/Beta Centauri system in the southern hemisphere) at a distance of 1.8 parsecs (5.95 light years).  In 175 years, Barnard's Star moves an amount equal to the angular size of the Moon across the sky. The yearly motion of a star across the sky is called proper motion  and only 200 stars have proper motions that exceed 1.2 arc seconds per year.

Studies of proper motions are paramount importance for galactic kinematics, which is the study of the motions of star in our galaxy, The Milky Way. By analyzing the proper motions of the stars, astronomers can determine how our galaxy rotates, and watch star clusters move through space.  

The photos below illustrate the rapid motion of Barnard's Star taken with a Meade telescope at Ft. Davis, Texas. During the 11 year baseline, Barnard's Star has moved 113 arc seconds, which is more than twice the apparent size of Jupiter.

                               May 1993         Photos by Richard Nugent       May 2004

For the story of Barnard's Star and the premature claim of a planet circling it, go here.

                                        The Flying Star - 61 Cygni

The double star system, 61 Cygni is located in the constellation of Cygnus, the Swan. This pair of yellow 5th magnitude stars, easily visible in any small telescope, was the first pair of stars to have their distances measured. This important discovery was announced in 1836 by Frederick Bessel. For 20 centuries, from the time of the ancient Greeks, astronomers attempted to measure the distances to stars, resulting in one failure after another. This was due to the enormous distances of the stars and the crude instruments and techniques used. Bessel found 61 Cygni to be 10.4 light years away (he was only 10% off than the modern value). He used the technique known as trigonometric parallax, in which the star is observed in different times of the year as the Earth swings around in its orbit. As the Earth moves around the Sun, the nearby stars appear to wobble back and forth in a 6 month pattern, due to the change in the orientation of the observer on Earth.

61 Cygni also has a large proper motion, 5.2 arc seconds/year which can be seen in this pair of photos below:

                      October 1993          Photos by Richard Nugent                May 2004

61 Cygni is also a binary star system, in which the 2 stars orbit around each other. Thus they travel through space together as a system. The orbital period of 61 Cygni  is around 700 years.