A STAR ON THE MOVE ---------------- John Pazmino NYSkies Astronomy Inc www.nyskies.org email@example.com 2000 April 1 initial 2005 April 1 current Introduction ---------- I wrote this article before the days of NYSkies. It was published in the April 2000 Eyepiece, newsletter of Amateur Astronomers Association Inc, under editor Jack Dittrick. I reissue it here as a companion to my update piece for gamma Virginis, 'They're closing in!'. Some minor editing converted the typeset version to ASCII text. I also set off sections for easier reading.
Double stars ---------- Double stars are one of the features of the heavens home astronomers enjoy in small telescopes. These are pairs of stars, usually a gravitational unit, which to the eye or low power appear as one star. Under enough magnification they 'split' into their components. Once considered a curiosity, it turns out that some 1/3 of the stars in the sky are doubles. Because double stars have long periods, it is generally allowed that once observed, they are fixed features in the sky, like, say, a globular cluster. For the most part this is true when you think about it. If the stars are far enough apart to separate as distinct points in small scopes, they must be spatially far from each other, implying long periods. However, there are a surprising number of double stars with short enough periods, decades to a couple centuries, that within a lifetime their components sensibly alter position and distance from each other.
Here's Porrima! ------------- One such star is gamma Virginis (GA-ma VIRR-jih-niss), or Porrima (PO-rih-ma), now in our spring skies [in April 2000]. It's a longtime favorite of sky watchers and is one of the features exhibited at AAA starviewing sessions. gamma Virginis is usually an easy pair to resolve, even in very small scopes with modest power. Now that's about to change!
Some history ---------- First, a bit of history. Castelli discovered the first double star of all, Mizar, in 1617. By learning of Mizar from Castelli, Galileo later in 1617 made the first telescopic effort to capture parallax. He believed that the brighter member of Mizar was the nearer of the two and would shift in position relative to the dimmer star in step with the Earth's motion around the Sun. Of course he failed to see any parallax. OK, Galileo missed out on the discovery of the first double star. He did capture the second one, in 1617, the three brighter members of the Trapezium of theta Orionis. Thereafter, doubles stars were found by accident, a couple per decade. gamma Virginis itself was spotted first in 1689 by Richaud, a priest stationed in India, but the first astronomer to study it was Bradley at Greenwich Observatory in 1718. There being no reason to give close attention to doubles and there being no competent instruments to measure them with, such stars were left for the starbrowser to enjoy. Never the less, in 1767 Michell argued by probability's sake that the pairs being found thruout the sky may be real orbital systems, but he had no positive evidence.
William Herschel -------------- William Herschel, in his quest for parallax, discovered that many of the stars he studied exhibited true orbital revolution. He proceded exactly as Galileo did but with proper instruments and for a twenty year span. It is reasonably certain Herschel never knew of Galileo's attempts of over 150 years earlier. He devised the separation and position angle scheme for describing the aspect of a double star. His first measure of gamma Virginis was in 1781.
First orbit model --------------- William's son John did the horrendous calculation that led to a definitive description for the spatial orbit for gamma Virginis. This was in 1833 and was the second orbit for a binary star. xi Ursae Majoris was the first in 1830 by Savary. The calcs predicted an amazing future event. gamma Virginis would in 1834, according to Herschel, swing so close together that it would be outside the range of scopes to resolve! And in fact only the Dorpat Observatory just barely made out the two stars at their closest separation. This actually occurred in spring of 1836. The event was a crowning validation of Newton gravity theory, then only applicable within the solar system. Now it was truly a universal law. Since then, gamma Virginis has been under continuous observation, including from US Naval Observatory.
Occultations ---------- The orbit was refined as more measurements were logged in. One major boon for the star was its occultation on 1967 January 3 by the Moon. Timings of the two stars as they were hit by the Moon produced good clean precise data. gamma Virginis sits close enough to the ecliptic for frequent occultations. It, at 3rd magnitude, is the brightest star in the long gap between Regulus and Spica. The next occultation of gamma visible from the City [as at April 2000] is on 2003 August 2 in evening twilight. To track occultations of Porrima, you may need its designation in the Zodiacal Catalog, ZC 1821, by which occultable stars are generally referenced. Despite its repeated coverings by the Moon, seldom has duplicity of gamma Virginis been noted. In fact, the first definitive record of a double star's occultation was that of gamma Virginis on 1720 April 20-21. This was duly recorded by Jaques Cassini at Paris Observatory.
2005 periastron ------------- In 1990 Heintz at Sproul Observatory worked up a new orbit, the one currently in force [in April 2000], with a period of 168.68 years and a closest separation in, uh, late February of 2005. This seems like a long way off [from 2000] but the two stars are right now rapidly closing in. This is a star on the move! The stars may pass beyond separability in small home telescopes in 2001, when they close within 1 arcsecond apart. This scene will enchant those blessed with large scopes and truly still air.
Caveat spectator -------------- There is an older orbit, by Strand from 1935, still in circulation. It gives the closest approach for 2008, three years beyond 2005 due to its 171.37 year period. Measures logged in during the 1990s seem to put Porrima a bit farther apart than the Heintz orbit predicts, by about 0.04 arcsecond. So this is a star that calls for close attention, even after over three centuries of study. By the geometry of gamma's orbit the closest spatial approach, the periastron, happens to be within weeks of the closest angular approach, such that some authors use the terms interchangeably. Think of a comet's path on the sky. The angularly closest approach to the Sun is generally very different from the closest spatial approach at perihelion.
Diffraction micrometer -------------------- To follow gamma (and any other binary star) you need a means of actually assessing the orientation and distance of the pair. After looking around among double star observers, Dr Martin Gaskell, University of Nebraska, recalled a technique I actually played with some thirty years ago. He pointed me back to the diffraction micrometer, made from a hunk of cardboard. Yes, cardboard. You can make it in an afternoon and be measuring binaries that very night. I explain the gadget in a companion article 'The diffraction micrometer' hung on the [NYSkies files section as '\pazmino\diffmic.txt.] It gets a bit technical but, hey, for the price of a large box of candy (to get the thin, stiff, smooth cardboard) there has to be some trade off.
Orbit diagram ----------- To guide your viewing, [there are orbit charts on various websites for observing double stars] spotting the stars at various years. See how elongated, like a comet, the orbit is. In the tradition of binary star work, the brighter star is held fixed and all the motion is thrown onto the fainter. What's plotted is the relative orbit as if you were standing on the primary looking at the companion.
Facts and figures --------------- In the litterature you may find gamma listed under its binary star designation, Struve 1670 or Aitken 8630. In older works a Struve star was written with a capital sigma before the number. Newer books either write out 'Struve' or use 'STF' before the number. An Aitken star was and is always 'ADS' before the number. On the sky, gamma is 40% of the way from Spica to Denebola. It's of 3rd magnitude, sort of yellow-white in binoculars. Position is, for 2000, RA = 12h 41.7m, dec = -01d 27m. In the scope the two stars are quite equal, 4th magnitude, but one is a trifle brighter and that's the primary. In 2000 the pair stands 1.4 arcsec apart and is aligned along 80-260 degrees, with 0 at north and 90 at east on the sky. In spatial measure their projected distance apart is 16.5 AU.
Observing tips ------------ You need inspect gamma Virginis only a couple times in each viewing season. Once every two months is far enough sessions. In 2004, when the stars really get spinning around, you'll watch it every couple weeks. Your records will be kept in a durable manner to last out the many years you'll be watching. Record the observing site, instrument details, full date, year & decimal, separation, position angle, comments on the night. The position angle is recorded to the tenth of a degree; the separation, the hundredth of an arcsecond. You can not consciously discern a 1/100 arcsec feature by eye in the scope, yet with the diffraction micrometer you really can capture a valid measure with that precision. The year & decimal merely express the day count as a decimal. We do not use Julian day numbers in binary star work. When we started observing binaries, long before variable stars became important, we used the decimal year system. We simply stayed with it ever since. Thus, an observation made on 2000 April 16 is (2000) + (107) / (366) = 2000.292. Yes, 2000 is a leapyear. Note that the hour within the day is not necessary. Even gamma Virginis doesn't spin that rapidly. You can plot your sightings directly on [copies of[ the orbit map with a protractor and the chart's distance scale.
The 1836 spectacle ---------------- Now I send you outside to gamma Virginis with the original account of that first observed closest approach in 1836
'At length, about the beginning of June, 1836, a letter arrived from Sir John Herschel, addressed to Mr. Baily, in which he detailed his observations on the single state of this star, at the ville of Feldhausen, Cape of Good Hope, in his 20-foot reflector. Under the date of February 27th, that unwearied astronomer says: gamma Virginis, at this time, is to all appearances a single star. I have tormented it under favorable circumstances, with the highest powers I can apply to my telescope, consistently with seeing a well- defined disc, til my patience has been exhausted, and that on several occasions, whenever the definition of the stars generally, in that quarter of the heavens, would allow of observing with any chance of success, but I have not been able to procure any decisive symptom of its consisting of two individuals.'