Double Vision


Charlie Fredrickson - March 15, 1998


This is an exciting time of year for double star lovers! There are a number of binaries seen now that are actually "doing something", and some of great historical interest. One of those doubles with a quickly changing appearance is Gamma Virginis, also known as PORRIMA. The two equally bright white components of this binary are rushing towards periastron (closest approach) in its 168 year orbit in under seven years time. Right now the stars are separated by just under 2 arc-seconds, and as such is a great test for a three-inch aperture instrument, and should be fairly easy for a four inch scope. Within the next few years, however, the distance shrinks quickly, so that by the year 2005, most telescopes on earth will be unable to separate the pair!! The orbit of the binary is shown in the accompanying diagram. Don't think the optics of your instrument are on the fritz, if you cannot resolve it at this time, and you see an old listing of its separation as 4.5 arc-seconds!! Gamma Virginis is a fairly close system to us, being about 32 light years distant. The general visual impression in a telescope is sometimes described as "a pair of headlights"!

The fact that the components of Gamma Virginis is of equal magnitude actually helps to resolve the pair, as opposed to another binary nearby, Epsilon Bootes, also known as IZAR. This components of this beautiful pair, discovered by the Double Star legend F.G.W. Struve, are pale golden yellow and blue-green in color. The primary at magnitude 2.4 is much brighter than the secondary at magnitude 5, which makes the pair difficult in three inch scopes, despite it's separation of 2.8 arc-seconds, and not exactly easy in even larger instruments. This is one that is much easier to resolve when seeing conditions are good. The positions of the two stars have not changed appreciably in the years that it has been observed, which indicated that the period must be many thousands of years in duration. It is a much more distant object than Gamma Virginis, being at least 300 light years away.

I should also mention Xi Ursa Majoris. Xi is the southern-most naked eye star of Ursa Major, being just above the east end of Leo. This pair orbits each other in about 60 years, so it has gone thru three complete orbits since its discovery by William Herschel in 1780. It was also the first to have its orbit determined, by M. Savary, in 1828, five years before John Herschel computed the orbit of Gamma Virginis. It is currently at a separation of 1.6 arc-seconds, so it is presently a good test for about a four-inch scope. Just a few years ago, in 1993, it went thru closest approach, and was under 0.9 arc-seconds separation! Xi Ursa Majoris is another fairly close neighbor, being only 26 light years distant.