Jim Loudon Observatory - Proper Motion Observations



PROPERLY MOVED
by
Richard E. Hill

One of the greater frustrations for the double star observer is the wonderfully clear nights after the passage of a cold front or after a storm passes, when transperency is spectacular, the Milky Way glistens like beads of dew and the stars are a twinkling firmament. But then, that's the problem...the twinkling! This was a persistent problem in Michigan where I grew up in the 1950s and 1960s, especially in winter. But even in the desert southwest of the U.S. such nights can be experienced in abundance, particularly if you live near mountains where the cold air can slide off the mountains after sunset and wash over the foothills destroying seeing for hours or a whole night. Any attempt to use a telescope or micrometer with sufficient magnification for double star measures will only be met with discouragement on such a night. Yet I always felt that there must be some astrometric project connected to double star observing that can be gainfully employed. I faced this problem in 1984 when I had my newly constructed micrometer in hand and met with my first spate of five arc second nights.

Most of us observe double stars not only for their beauty but for the fun of seeing change in the sky. These innumerable, fantastically distant points of light were thought for centuries to be "fixed", unmovable. Today, most of the public and many amateur astronomers still think of them as such and are astounded to learn that orbital motion can be seen in as little as a year in stars that are of naked eye brightness! But armed with the same techniques and tools that serve to measure orbital motion the amateur astronomer can in only a year's time detect motion in stars with proper motions (p.m.) of a few arc seconds or more.

Barnard's Star and 61 Cyg are the first high proper motion (HPM) stars most amateurs learn about but there are many more bright stars that have fairly high (< 2.5"/yr) proper motions. In Sept. 1976 Willem J. Luyten at Univ. of Minnesota, published the LHS Catalogue (Revised and republished in 1979). It is a comprehensive list of stars with proper motions in excess of 0.5"/yr. Of the 73 stars listed in this catalogue about half are measurable an 8-inch telescope with p.m.s of 2.5" or more. Included in the later edition are finder charts for the fainter stars. While these would not be measurable in small apertures (being faint) they would be observable and with careful sketches noting the cardinal points carefully, motion could be seen in a few years. Still, there are plenty of the bright ones to work on that, if followed diligently, will provide much to do on less than ideal nights.

Armed with this catalog, and a decent set of star charts I set out to measure some of these stars. As you might expect, I started with the two aforementioned stars, Barnard's Star and 61 Cyg. Identification of the former was difficult and most important. I used the charts in Burnaham's Celestial Handbook. Since I used a telescope with 3 reflections it was necessary to copy the charts on plain paper (either by hand or copy machine), flip them over and shine a flashlight through them from behind. This gave me a very usable reverted view. The star field around both these stars is excellent for the project with many stars available to measure.

My first measure of Barnard's Star was in August 1985. Measures were taken just as with a wide double star system but in this case with the selected field stars, paying particular attention to stars in the future path of the HPM system. It was followed with further measures taken in 1988, 1991, 1992, 1998 and then again this year (2000). These measures now cover 165.5" of travel for an annual measured p.m. of 10.96" which is about 6% off the cannonical 10.31". If the 2000 measurement is omitted (and it was done with an inferior internal lighting system in the micrometer making measuring difficult) then the distance traveled from the 1985 measurement to the 1998 is 135.0" in 13.1 years for an annual p.m. of 10.31! Needless to say, this made me VERY happy to have the numbers come out so nicely.

With 61 Cyg. you have a bonus, a wide double as well as a HPM system. Measures were made of this system in the same years as Barnard's Star. The reduction of them from 1985-2000 yeilds a measured p.m. of 5.033" which compares to a fairly well with the published 5.204" in the LHS Catalogue. The A-B measures for those years showed a slowly increasing P.A. from 146.8 (1985) to 148 (1998) and a separation that remained around 30". These measures are in fair agreement with the predictions listed by Wolfgang Wepner in 291 Doppelstern Ephemeriden fur die Jahre 1975-2000.

Other stars have been measured in similar manner including: Groombridge 34 (LHS 3/4, p.m.=2.9"), Mu Cas (LHS 8, p.m.=3.8"), LaLande 21185 (LHS 37, p.m.=4.8"), LaLande 21258 (LHS 38, p.m.=4.5"), Groombridge 1830 (LHS 44, p.m.=7.0"), and Arcturus (LHS 48, p.m.=2.3"). Many of these are multiple star system. In one case, Omicron Eridani, there are three members all within reach of a moderate aperture. Many are red dwarf systems that actually appear reddish or orange in the telescope.

There are other stars and star systems that can be enjoyed by the amateur, more than enough to fill up those nights when won't quite cooperate to reveal the secrets of the many binary systems that populate our galaxy.