This is the second of what I hope will be a short series on the best double stars to follow. By this I mean the ones that are "actually doing something." In the first article (page 4 of the July issue of "The Prairie Astronomer") I described how I learned that you don't need a filar micrometer and a big refractor on a massive German equatorial mounting to follow double stars. My mention that you could do it with a piece of cardboard on a 6" Newtonian generated quite a bit of interest among members, and David Knisley has asked me to do a program on it at the April meeting (Tuesday April 25). I'll give a full description of my methods there with a detailed description of my results, an audience demonstration, and the first annual (?!) return visit of "Tel'Poke" - the Gaskell family's ultra-low-cost, home-made, 6" equatorial Newtonian with a high precision clock drive (this year's upgrades to Tel'Poke are almost a program in themselves!).
This month I want to talk about Castor (= Alpha Geminorum), perhaps the best-known double in the Winter/Spring sky, and according to some people the best double in the northern sky. The high declination of Castor makes it measurable all spring (it can still be done in late May.) Next month (if I get time) I want to write about Gamma Virginis, a star undergoing very exciting changes. Since Castor is a star you'll probably look at often and show to friends I'll describe the system and its history a bit.
Castor occupies a famous place in the history of astronomy -- it was the first system beyond the solar system in which gravity was shown to be operating. We might take this for granted today, but in 1803 when Sir William Herschel announced that Castor A and Castor B were orbiting each other, this generated an excitement which was to last for decades.
Castor was probably first resolved way back in 1678 by G. D. Cassini but the duplicity was not rediscovered until 1718 by James Bradley (the Astronomer Royal in England) and James Pound. Bradley and Pound made a fairly good estimates of of the position angle (PA = angle of the secondary relative to the secondary measured counter-clockwise from north) but unfortunately did not measure the separation (double star micrometer work didn't really begin in earnest for another 100 years). This is a pity as a good measurement by them (accurate to 0.25", say) would have helped eliminate much of the uncertainty in the orbit. The English minister, Rev. John Michell proposed (on statistical grounds which would seem overwhelming convincing today) that Castor, and close binaries like it, were true gravitationally bound systems, but the theory was not accepted until Sir William Herschel's famous announcement in 1803 that Castor A and Castor B were orbiting each other.
To complete the story of the Castor system, a third star, Castor C, a 9.1 magnitude star 72.5" away in PA 164, visible in any small telescope, has also been shown to be a distant member of the system. It is probably orbiting A and B with a period of 10,000 years. Each of A, B and C has been shown to be a spectroscopic binary (periods of 9.2, 2.9 and 0.8 days respectively) so there are six stars total in the system. Castor C is interesting because the orbit of the components is edge on causing eclipses of amplitude about 0.5 magnitudes (easily detectable visually). Castor C is given the variable star designation YY Gem.
The most recently available photographic parallax measurements give the distance to the system as 45 light years with an uncertainty of about 5 light years. The separation between the A components is 1/25th of an AU. The B components are about 90 AU away from the A components on average, or three times further away than Pluto is from the sun. The B components have a separation of 1/30th of an AU. The C components are more than 1000 AU away (300 times further apart than the sun and Pluto are) but are only 1/60th of an AU apart. Robert Burnham Jr. in his famous "Celestial Handbook" gives an interesting scale model of the Castor system. If A1 and A2 are 2.3 inches apart then B1 and B2 are 340 feet away and have a separation of 1.7 inches. C1 and C2 are 4500 feet away and have a separation of 0.9 inches. It would be fascinating to be on a planet in such a system! If the 45 light year distance is correct, the A and B stars are less massive than stellar evolution theory would predict. The A stars, for example, each seem to be similar to Sirius in spectral type and luminosity, but their masses come out to be about the same as the mass of the sun (Sirius is about twice as massive as the sun).
When Pound and Bradley discovered the duplicity of Castor, the companion was almost due north of the primary. When the famous Wilhelm Struve began micrometric measurements in 1826 the companion was almost due west of the primary. In my lifetime it has gone from being due south of the primary in 1954/55 to due east in 1982. Thus the system has been seen to rotate three-quarters of the way round.
You might think that there is nothing more to be learned about such a famous and well-studied system as Castor, especially not from visual measurements, but this is not true. The orbit of A and B is uncertain by an amount which is detectable using my cardboard micrometer. The main reason is that A and B have not yet been observed to go through a full orbit. As we saw in the previous article on 70 Ophiuchi, even when a system has been observed to go through several orbits there can be still be significant discrepancies when one tries to predict future behavior. We will see this to be the case again with Gamma Virginis. For Castor even the orbital period of A around B is quite uncertain. It has been calculated many times. In 1933 it was calculated to be 340 years, in 1940 to be 380 years, in 1956 to be 511 years, in 1958 to be 420 years and most recently (in 1988) to be 467 years. This is a 50% uncertainty! The semi-major axis of the orbit (the average AB distance) is also uncertain with values ranging from 6.3" to 7.4". This would be much less uncertain if the early 18th century observers had measured the apparent separation. It is agreed that the orbit of A and B is tipped towards us, making an angle of about 65 degrees with the plane of the sky (90 degrees would be seeing the orbit edge-on). A and B appeared to be at their closest in 1969/70, but because of the tilt of the orbit we are not sure when the real closest approach of the stars ("periastron") took place. Orbital calculations place it between 1950 and the late 1960's.
What do these uncertainties mean for the amateur observer in the mid- to late-1990's? In the table below I have calculated the predicted positions from three ephemerides due to P. Muller in 1956, W. Rabe in 1958 and W. D. Heintz in 1988 (the later is the one used in Norton's 2000.0). It will be easy to distinguish between the predictions using a cardboard micrometer. Castor is one of the best stars in the sky to measure in this way. It is so bright and so far apart that you can do it under seeing conditions which would make measuring other stars impossible. If you have a Celestron Microguide eyepiece with a degree scale this should also let you distinguish between the PAs. You should be able to see a change between now and 1996 if you are careful. If you don't go in for quantitative measurements you will have to wait longer, but the change in PA will be obvious even from eyeball estimates over a 5 to 10 year period. This should also be true for the separation as well. Have a look at Castor this month, and even if you can't make an accurate measurement, try to estimate the PA as accurately as you can. Write it down in your observing book for your future enjoyment! I note with pleasure that the estimated PA I scribbled down in the margin of my "Norton's" in 1972 is now off by a full 100 degrees!
PREDICTED SEPARATIONS AND POSITION ANGLES OF CASTOR AB
Date Muller 1956 Rabe 1958 Heintz 1958
PA Sep. PA Sep. PA Sep.
Mar 1, 1995 = 1995.08 72.1 3.54" 66.6 3.44" 70.5 3.45"
Jun 1, 1995 = 1995.42 71.8 3.57" 66.2 3.46" 70.1 3.48"
Oct 1, 1995 = 1995.75 71.4 3.60" 65.8 3.49" 69.7 3.51"
Mar 1, 1996 = 1996.08 71.1 3.63" 65.4 3.52" 69.4 3.54"