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Department of Physics | University of Durham | |
Level One |
The mass and structure of our Galaxy can be investigated by the measurement of the motions of stars in the local solar neighbourhood, i.e. those within a few hundred parsecs of the Sun.
The motion of any star is determined by the radial velocity and the proper motion measurements.
The radial velocity (VR) is the line-of-sight velocity component of a star. It can be measured directly from the Doppler shift of the star's spectral lines. Typical values for nearby stars are a few tens of kms-1.
The proper motion (mu) is the tranverse component of the star's motion on the plane of the sky and is measured in units of arcsecond per year. (1 degree = 60 arcminutes = 3600 arcseconds).
The discovery of proper motions was made by Edmund Halley in 1718. He noticed that the positions of three bright stars (Sirius, Arcturus and Aldebaran) were over half a degree different from those recorded by Hipparchus more than 1800 years earlier. Most stars are so far away that their proper motions are not measurable. A star with a large proper motion indicates that it is nearby. In fact, the first successful measurement of annual parallax was made by Bessel in 1840 who rightly deduced that the star 61 Cygni was nearby because it had a large proper motion. For more background information see Searching Out the Nearest Stars and Our Nearest Celestial Neighbors.
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Ursa Major (i.e. the Great Bear, the Plough) is one of the best known northern constellations. Five of the bright stars in Ursa Major lie at a distance of 25 parsecs and form a sparse physical group. This group is streaming past the Sun with a proper motion of 0.1 arcsecond per year. Over thousands of years the pattern of the constellation changes. The diagram shows how Ursa Major's shape changes because of the proper motion of the stars. The top panel is how Usra Major looked 100,000 years ago, the middle panel shows the current pattern and the lower panel shows how Usra Major will look in 100,000 years. The two stars not in the Ursa Major moving cluster are the end stars, i.e. Dubhe (Alpha UMa) and Alkaid (Eta UMa). |
In practice a proper motion for a star is determined by measuring it's angular displacement relative to distant stars which do not have any measurable proper motions. Hence the distant stars are used effectively as reference points that fix the co-ordinate system. Of course, a priori, it is not known which stars in a field are in the far background. Fortunately it is a very reasonable initial assumption that most faint stars in any field will be sufficiently far away to have little or no proper motion. Usually a series of images taken several years apart are used in order to reliably track the motion of the nearby star.
This experiment illustrates the measurement of stellar proper motions. It is based on the measurement of the proper motion of Barnard's star. This is a nearby red dwarf which has the largest known proper motion.
This page is maintained by jrl. Last updated: 2000-Nov-08