Double Stars To Follow

PART III: GAMMA VIRGINIS AND XI URSA MAJORIS

Martin Gaskell

This is the third of a short series on the best double stars to follow. This part covers two systems well placed for spring observing. Gamma Virginis is good for small scopes, and Xi Ursa Majoris will require an aperture of 10" or more.

GAMMA VIRGINIS

This series is about double stars "that are actually doing something" and Gamma Virginis, is certainly a star starting to do something. The two components of Gamma Virginis are rushing towards periastron (closest approach) in only ten years time and this provides a rare opportunity to see rapid orbital motion in even the smallest of telescopes.

Gamma Virginis is the third magnitude star about 15 degrees to the "upper right" of Spica in the direction of Leo. For the deep sky folks, if you can find the Virgo cluster of galaxies, Gamma is the third magnitude star directly south of the cluster! The system is quite nearby, only 32 light years away. The two stars are almost identical F-type main sequence stars, a little hotter than our own sun and each about 3.5 times brighter. The identical magnitudes makes resolution of the system easier and this will be a plus when they start to get very close in a few years time. They orbit each other in a very eccentric, comet-like orbit that takes them as far apart as 70 astronomical units (about twice the distance from the earth to Neptune or Pluto) and as close as 3 au (closer than we get to Jupiter).

Gamma Virginis was one of the first star ever discovered to be double. A missionary in India, named Richaud, discovered the duplicity in 1689. It was rediscovered by Bradley, Pound and Cassini in 1718-1720. The first useful measurement (of the position angle only) was made by Sir William Herschel in the fall of 1781. In the 1820's and early 1830's its motion was so rapid and obvious that in 1833 his son, Sir John Herschel, calculated its orbit. This was only the second time that the orbit of a double star had been calculated. John Herschel predicted periastron in the spring of 1836. At apastron (maximum separation) the stars are 6 arc seconds apart (this last happened in the 1920's) but the orbit is very eccentric so that at closest approach the stars are only half an arc second apart. In the 1830's this generated considerable excitement. Here was a star which, in only a decade, went from being a fairly easily resolvable close pair to being unresolvable in every telescope in the world except for the great Dorpat refractor used by the famous Otto Struve! The verification of John Herschel's reduction helped fuel the double star fever of the latter 19th century.

It is now 1995 and you get a chance to relive the excitement of the 1830's! According to the most recent (1990) orbital calculation of Dr. W. D. Heintz, Gamma Virginis has a 168.68 year period and the next periastron will occur less than ten years away, in late February 2005. Between now and then Gamma Virginis is closing rapidly. The very eccentric orbit makes Gamma Virginis different from most other double stars. For the majority of double stars the main change is in the position angle. For Gamma Virginis, now only will the position angle (PA) be changing rapidly (about 3 degrees per year right now; increasing to 70 degrees per year in 2004/2005) but the separation can be seen to be changing rapidly at a rate of about 0.14 arc seconds per year (easily detectable with a cardboard micrometer) and this rate will increase slightly towards periastron. In the table below I give the predicted separation and position angles for the next few years. The cumulative change in separation between now and 2005 is so large that it will be obvious in a small telescope without making any quantitative measurements. Just note the position angle in your note book relative to the east-west drift in the telescope, estimate the separation relative to the resolution of your telescope and write down your impressions in your observing book so that you can refer back to them in future years. When 2005 comes, unless you have a very big telescope (more than 10" aperture), you won't be able to resolve it at all! Then even a very big telescope will only resolve it in extremely good seeing.

As with the stars I mentioned in the first two articles, there is some uncertainty in the orbit of Gamma Virginis. This is because it is only now completing its first well-studied orbit. If you use the older ephemeris given in Norton's 2000.0 (a period of 171 years) you will get positions which are off by quite a lot -- over 2 degrees in PA and over a quarter of an arc second in separation. The differences between the old and the new Heintz ephemeris have been easily detectable with my cardboard micrometer. How good is the Heintz ephemeris itself? I gave Gamma Virginis particularly intensive study last spring (1994) with our 6" Newtonian ("Tel'Poke") and came up with a PA in agreement with the 1990 ephemeris but separation about 0.05" too large. This is about the same as my calibration uncertainties, but 5 years of observations by more experienced amateur observers with larger telescopes gave a mean separation 0.08" larger than the ephemeris and speckle observations with the 26" at the US Naval Observatory in 1994 gave a separation 0.04" wider than the ephemeris so it looks like the Heintz ephemeris is under predicting the separation slightly. Any discrepancies will be larger as periastron approaches so I'm going to be keeping a close eye on Gamma Virginis with my cardboard micrometer. From these numbers you can see, that at least for a bright pair like Gamma Virginis, the $0.25 cardboard micrometer on Tel'Poke compares quite favorably with a $100,000 piece of equipment on a 26" refractor at the US Naval Observatory! Come the spring of 2000 however I am going to need to have built a slightly larger telescope!

Gamma Virginis

XI URSA MAJORIS

For those who already have a larger telescope I must briefly mention Xi Ursa Majoris, the most southerly obvious naked eye star of Ursa Major (it's just above the east end of Leo). This system is only 26 light years from the sun. The stars orbit each other with a period of 60 years so they've been seen to go through three complete orbits since they were discovered by William Herschel in 1780. Xi Ursa Majoris is famous for being the first star to have its orbit determined, by M. Savary in 1828, 5 years before John Herschel calculated the orbit of Gamma Virginis. The stars went through a closest approach in the sky of 0.85" in 1992 and will be at periastron this (1995) year (the difference is due to perspective caused by the tilt of the orbit as seen from the Earth). Between now and the year 2000 the system will widen from 1.10" to 1.77". At present it can just be resolved by a 6" but an 8" or 10" would be better. It is certainly possible to measure it with a cardboard micrometer on a 10" -- I measured it in 1992 when the separation was 0.89". If you can't measure it, this is a system to just watch as the stars swing round each other. The change in PA is rapid -- well over a degree per month. Since I measured it in 1992 the PA has changed from 13 degrees to 312 degrees this (1995) spring, a change of 61 degrees! This is the sort of change you can see in the sketches in your observing notebook.

Here is a star which you might be able to follow through an entire orbit if you live long enough! Below is a brief ephemeris due to W. D. Heintz. It was calculated 30 years ago, so I would not be surprised if there are deviations from it at a level which could be detected by a cardboard micrometer and a 10" - 12" telescope.

Xi Ursa Majoris