THEY'RE CLOSING IN!! ------------------ John Pazmino NYSkies Astronomy Inc email@example.com www.nyskies.org 2005 March 28
Introduction ---------- In April 2000 I wrote a historical piece about the star Porrima, or gamma Virginis. I related its discovery as a double star and the studies of its orbit over the centuries. I reissued it thru NYSkies in April 2005 as a companion article. Please read that first, 'A star on the move', as a prerequisite for this update here. Porrima is now in the evening sky attaining a maximum altitude of about 49 degrees in the course of the night. It'll be with us in the evening sky thru late summer, when it recedes into twilight. Some of us watched gamma Virginis when it was in the morning sky since late fall of 2004, but the bulk of NYSkiers waited until March for the more convenient hours of viewing.
Amazing star ---------- The 'A star on the move' piece has the detailed description of gamma Virginis. Here I give a brief summary. It was first noticed as a double star in 1689 but real study wasn't started until 1718. By 1800 Porrima was recognized as a true binary system, not just a chance lineup of unrelated stars. An orbit was first worked up in 1833, revealing that the two stars in Porrima would round periastron in 1834. Periastron is analogous to perigee or perihelion; it's the 'near' end of the orbit's major axis. This event was closely observed. The stars swang so close, angularly, that no telescopes of the era could resolve them. The stars appeared to merge into a single dot! By reverse calculation in the 20th century, periastron actually occurred in spring of 1836.
Facts and figures --------------- Some catalog names for Porrima are
Aitken ADS 8630 Bayer gamma Bonner D'g BD-00:2601 Boss GC 17170 Bright Star HR 4825, 4826 Flamsteed 29 Draper HD 110379, 110380 Hipparchos HIP 61491 Hubble GSC 4949-1113 Smithsonian SAO 138917 Struve STF 1870 P & P M PPM 400177 3rd Fundamtl Kat FK3 477 Tycho TYC 4949-1120-2 Washington WDS 12417-0127 Zodiacal ZC 1821
Catalogs may give data for only one component, a blend of the two, or each separately. The celestial parameters are, for epoch 2000:
Right ascension 12h 41.6s Declination -01d 26.9m Ecliptic long 190.13 Ecliptic lat +02.79 Galactic long 297.77 Galactic lat +61.33
The two stars are nearly equal in size, brilliance, diameter, and other properties as follows
diameter 2 Sun mass 2.1 Sun output 4.6 Sun surf temp 7,100K spectrum F0 lum class V (main seq) tot app magn +2.7 ind app magn +3.4, +3.5 tot abs magn +2.4 ind abs magn +3.1, 3.2 Sun magn +5.2 dist 38.6ly, 11.8pc
Normal binary ----------- Thruout most of the 20th century, when most of us began our astronomy lifes, gamma Virginis was well separated, several arcseconds apart, in even quite small telescopes. It was a favorite target for spring and summer public starviewings. Porrima was for the early 2000s a favorite star for occultations by the Moon. It sits near the ecliptic about 60% from Denebola to Spica. Each component suffers its own occultation, allowing for tight bounds on their orientation and separation. The first such occasion so exploited was in 1720 for the first good measurements of the pair. We just ended a season of Porrima occultations but most of the events were not visible from New York City. The next season begins in 2016 with the next City event in 2017:
UT date event ----------- ---------------- 2003 Aug 03 previous occultation over NYC 2005 Jan 03 last occultation of old season 2016 Feb 25 first occultation of new season 2017 Jan 18 next occultation over NYC
Orbit --- Orbits were calculated in 1908 and 1935. The latter, by Strand, is still in wide circulation in reference books from the mid to late 20th century. New orbits were issued by Heintz in 1990 and Soderhjeim in 1999. The latter gives orbit elements as follows
Period = 168.9y SMAxis = 3.68" Inclination = 148 AscNode = 37 PeriT = 1836.4 Excentricity = 0.89 ArgPeri = 257
From the Soderhjeim orbit comes this ephemeris of gamma:
Year dec dist ang dec dist ang dec dist ang dec dist ang ---- --- ---- --- --- ---- --- --- ---- --- --- ---- --- 2003 .00 0.87 236 .25 0.82 232 .50 0.76 228 .75 0.69 222 2004 .00 0.63 216 .25 0.56 208 .50 0.50 198 .75 0.43 185 2005 .00 0.38 168 .25 0.35 147 .50 0.35 124 .75 0.38 103 2006 .00 0.44 86 .25 0.51 73 .50 0.58 64 .75 0.65 56 2007 .00 0.73 50 .25 0.80 46 .50 0.87 42 .75 0.93 38 2008 .00 1.00 35 .25 1.06 33 .50 1.12 30 .75 1.18 28 2009 .00 1.24 26 .25 1.29 24 .50 1.35 23 .75 1.40 21 2010 .00 1.46 20 .25 1.51 19 .50 1.56 18 .75 1.61 16
The year is given in decimals. .00 is January 1st; .25, about April 1st; .50, about July 1st; .75, about Oct 1st. 'Dist' is the angular distance in arcseconds between the pair. 'Ang' is the direction from the main, primary, star and the comes. North is 0; east, 90; south, 180; west, 270.
Orbital parameters ---------------- Altho the names of the elements look familiar, the definitions are quite different from the usual ones in solar system orbits. You have to be versed in double star work to make proper use fo these figures. The semimajor axis is given in arcseconds, not linear measure. This is simply due to the initially uncertain or unknown remoteness of the star. The linear separation is obtained once the distance to the star is known. It wasn't until the mid 1990s, quite ten years ago!, that clean distance measures were in hand from the HIPPARCOS spaceprobe. Porrima stands 38.6 lightyears, or 11.8 parsecs, away. By the definition of a parsec, the linear equivalent of an angular extent on the sky is
(lin sep in AU) = (ang sep in arcsec) * (dist in pc)
This gives the projected value, as if the two stars were placed flat against the celestial sphere. It ignores the inclination of the orbit. So, for Porrima at the start of 2005, we have
(lin sep) = (0.38 arcsec) * (11.8pc) = 4.13 AU
The ascending node, argument of periastron, and inclination are defined on the plane of the celestial sphere. There is no common base plane for all binaries, such as the equatorial plane of the Milky Way. Thus, you can not directly compare orbits among double stars like you can with, say, planets or comets in the solar system.
Orbits and orbits --------------- None of the orbits prove out! We can excuse the earlier computations, but why are there problems with a star so well and continuously monitored for over 250 years? The answer is partly in the the observations of the 1830s. Porrima's orbit is strongly elongated, like a long period comet. It spends most of its period near apoastron, where the pair moves slowly for decades. Near periastron, things get weird. Despite the valiant efforts of the 1830s, the instruments and skills were too crude. There are no firm measurements in the critical years surrounding the periastron. To the early astronomers, the stars comgealed into one. Alas, like for a comet, the section of orbit close to periastron is the most sensitive to errors. Be wary of articles still citing the Strand orbit from 1935. This offers a periastron in 2008! If you're not with it, you may sleep off the years until then and miss completely the periastron right now in progress!! Even the Soderhjeim calculations are already out of spec. The closest separation is 0.35 arcsecond. Measurements in 2004 showed the stars were nearer than 0.30 arcsecond! Such a small periastron, and a well documented apoastron, pushes the excentricity to the Moon. The extant orbits posit excentricity of 0.85 or so. It looks like it may be as high as 0.92!
Proximity vs periastron --------------------- Many authors treat these two as alternate terms for the closest approach of the two stars. In the general case this is totally wrong. The dip and strike and sweep of the orbit on the celestial sphere puts the physical closest approach, periastron, away from the angular closest approach, proximity. Think of a comet in our sky. Its smallest angular separation from the Sun is not at all its perihelion passage. We have to work with TWO orbits. The first is the apparent path on the sky, which is the projection of the other, true, orbit in space. The latter is not directly observable, unless by chance we happen to view it face on from its pole. Elements in binary star catalogs are always for the true orbit. It takes a bit of fancy maths to convert the plotted apparent orbit into a path in 3D space. This exercise occupied a whole weekend in the age before home computers and calculettes. Today, I tickle a handheld computer, while I type out this article, with a binary star orbit calculator program. A couple pecks with the stylus and I can play with gamma Virginis to my heart's content. Due to the high excentricity of gamma Virginis and the as-yet still fuzzy true orbit, proximity and periastron are near together, within 1/10 year. The Soderhjeim orbit offers a periastron of 0.40 arcsec on 2005.30 and a proximity of 0.35 on 2005.37. Lo here the details:
year proxim perias | year proxim perias ------- ------ ------ | ------- ------ ------ 2005.28 0.3476 0.4049 | 2005.35 0.3453 0.4053 2005.29 0.3471 0.4048 | 2005.36 0.3452 0.4055 2005.30 0.3467 0.4048 | 2005.37 0.3452 0.4057 2005.31 0.3463 0.4048 | 2005.38 0.3452 0.4060 2005.32 0.3460 0.4049 | 2005.39 0.3453 0.4063 2005.33 0.3457 0.4050 | 2005.40 0.3454 0.4067 2005.34 0.3455 0.4051 | 2005.41 0.3456 0.4071
The 2005 periastron ----------------- Some binary star enthusiasts, in the lack of a valid orbit, took to brute comparison of the 1836 experience with theirs of the 1990s and early 2000s. From this it appears that the periastron occurs in mid May 2005 with a proximity of about 0.25 arcsecond. As the stars closed up in the 2000s, they fell out of range of ever larger home telescopes. For my own experience with a variety of instruments of my own and at starviewings, I followed Porrima as two stars thru early 2003. Then they merged in my totable scopes. By early 2004 I lost them in almost every home telescope that crossed my path. Never the less, I was thrilled as the months rolled by that I could see for myself two suns in dynamic motion. In 2003 and 2004 gamma Virginis required not only major aperture but also stable air and sharp eyesight. On a typical turbulent night it was impossible to tell that the smeared blob had two stars in it. By mid 2006 the stars are receding enough for the midsize scopes to discern. Then year by year, the star returns to duplicity in ever smaller telescopes. By the New York Super Bowl, in 2010, Porrima is a close but resolvable star in 100mm scopes. By 2012, the New York Olympics, it is a trivially easy binary in the smaller scopes. There after, until the next periastron in 2174, gamma Virginis is a pleasing pair to admire, along with others in the spring sky.
A star on the move ---------------- A typical binary star that is seen as two stars in home telescopes has a period so long that during a lifetime there is little motion of the components. This causes some observers to keep old figures for the distance and orientation thruout their astronomy life. The long period comes from the necessary immense linear separation of the stars to be seen as two in small scopes. Kepler's laws and all that, you know. Porrima is one rare exception. Within a lifetime (provided it's not centered around apoastron!) real spatial motion of stars can be witnessed within a couple years. Near periastron the shift in position is discernible within months, and, now, weeks. The downside is that you will need some huge scope, like the largest Dobsons or a major observatory. By late 2006 more and more telescopes will start seeing the return of the double Porrima. During this phase, the stars are still in revolution, so they will continuously change their orientation. Be sure to watch for that!
Observing tips ------------ Naively the stars at periastron should be resolvable in a scope of 480mm aperture, based on the (120)/(diam) rule. This is an ideal value with the stars as perfect optical images. In fact, the atmosphere never allows such perfect images. The stars may blur to a half to a full arcsecond diameter, thoroly smearing them together. It may be a challenge project to capture digital videos during instants of good seeing and combine the best of the frames. This is routinely done for planet and lunar pictures, so why not try it for double stars? When the stars start reproaching and become a pair in the eyepiece, inspection should be made with some particular magnification repeatedly to experience the widening separation at the same visual scale. If you use various scopes and oculars, keep within 10% of this nominal power. The angular rotation is harder to appreciate because of field and optical rotation. Unless you fix the field orientation, you won't know which way the stars are turning. For sure, here's a star that can justify a permanently set up rig. I haven't heard of any ongoing observations from space observatories. I expect at least a couple of poster or publicity shots by Hubble, no? Same wish with the new supersize machines in Chile with their adaptive optics.