Once in a Lifetime
(the ride of Porrima)
If you are a double star enthusiats, it's not often that you have the chance to view a "once-in-a-lifetime" event. Doubles are for the most part fairly static objects. I know, they move; but it's easier to see grass grow than to notice any direct change in most binary systems during an ordinary lifetime. There is one glowing exception, at least for this "lifetime", and that is Gamma Virginis, known affectionately by the name Porrima. It was about 170 years ago that it's unusual orbit was first detected and raised some excitement.
In his book "A Handbook of Double Stars" Crossley devotes 5 pages to this double and its periastron (closest separation) in 1836. He quotes what Sir John Herschel said with regards to the orbital elements that he calculated: "If they be correct, the latter end of the year 1833, or the beginning of the year 1834, will witness one of the most striking phenomena which siderial astronomy has yet afforded; the perihelion passage of one star around another, with the immense angular velocity of berween 60 and 70 degrees per annum, that is to say, of a degree every five days. As the two stars will then, however, be within little more than half a second of each other, and as they are both large and nearly equal, none but the very finest telescopes will have any chance of showing this magnificent phenomenon."
2005 will see the two stars of Porrima return to the same region they inhabited in 1834 and best estimates put the time of this "go around" in late May. The separation will then be around .3 arcseconds, well beyond the ability of any of my telescopes to slpit it. I figure that it will take a scope with a 17" objective or larger to achieve this during its perihelion passage. The last time I was able to cleanly split this double was in the Spring of 2001. I used a 4" refractor with a 6mm ortho and a 2X barlow for 333X. The separation was at the "limit" of this instrument, being about 1.3 arcseconds and it took brief moments of excellent seeing to glimpse a thin line of black sky between the two components. This is not a "beautiful" double star, but a pair of ordinary yellow suns of equal magnitude. However, they will hold a special place among double star enthusiasts living during this "once-in-a-lifetime" event.