Porrima (Gamma [γ] Virginis)

Porrima  (Gamma {γ} Virginis)   (Σ 1670  [STF 1670] )   HIP: 61941   SAO: 138917
RA: 12h 41.7m    Dec: -01° 27′
Magnitudes:  3.48, 3.53
Separation:   3.373  (WDS 2024)   (See this source, scroll down to orbital data!)
Position Angle:    352.8° ( WDS 2024)
Distance:   38 Light Years
Spectral Type: F0, F0

Virgo, including “The Retreat of the Howling Dog!” (Stellarium screen shot with labels added – click on the chart for a larger view)

Special note for May/June 2011

This is what Porrima and Saturn looked like on May 23rd, 2011, at 2300 PDT, when they were a little less than a degree apart. For data on the faint D, E, and F components, see below.

Saturn kisses Porrima, moving so close in early June 2011 that you may just be able to squeeze the two in a high-power, wide field eyepiece enjoying the beauty of Porrima’s nearly identical white globes and Saturn’s always spectacular rings at the same time. For details, see this post on “Prime Time: Your Journey through the Astronomical year.“

Porrima!    —-    A name that conjures up images of dark nights spent at the eyepiece of a telescope trying to coax this most stunning of doubles into duality.  The name itself is firmly imbedded in the lore of early Mediterranean and Chinese astronomy.  In volume three of Burnham’s Celestial Handbook (p. 2067), Robert Burnham writes that this star was named in honor of the “Goddess of Prophecy.”   Other sources indicate there were two  goddesses of prophecy who were twin sisters — which is a pretty darn good prophecy considering they had no way of knowing Porrima consisted of two twin-like stars.  An excellent site that Greg came across for star lore is this one which contains excerpts from R. H. Allen’s Star Names — scroll halfway down to get to the section on Porrima.

Burnham also mentions the Arabian name for Porrima, Zawiat al Awwa, which is a bit more interesting.  It means “The Angle (or corner) of the Barker” and refers to the area lying within the curve formed by Epsilon (ε), Delta (δ), Porrima, Eta (η), and Beta (β), which can be seen on the chart above.  That area is known as  — are you ready for this? — the Retreat of the Howling Dog!”

As Klaus, my four-legged observing companion would say:  WOOF!

But what Porrima really is   ……..   is two beautiful white globes with shimmering diffraction rings!

Or, at least it is once you finally manage to pry it apart.   Fortunately, that will become easier over the next few years because the two components are slowly getting further from one another, which this diagram shows rather well.

It took me a couple of times on this warm June evening of 2010 — the seeing was not cooperating in the least — but when my efforts were finally rewarded with a clean split  in a 102mm refractor, I was in awe of the sheer beauty.  These two stars are virtually identical in size and brightness, so when you see their elongated form finally separate cleanly into two identical stars, it really is stunning.

A closer view of the central portion of Virgo. (Stellarium screen image with labels added)

Located about fifteen degrees to the northwest of bluish-white first magnitude Spica, Porrima “A” and “B” are two of the fastest orbiting stars you’re likely to encounter.  They orbit each other in 169 years, but because of the extreme elliptical shape — it ranges from five astronomical units (AU) to eighty-one AU’s — the separation between the two stars changes relatively quickly (see below for an orbital diagram). The good news, as I mentioned above,  is they’re gradually drawing apart, so they’ll become easier to split in the nights and years ahead.  At their closest, which was in 2007/2008, they were beyond the range of most scopes not housed in a large observatory.

I was able to get another stunning view of this pair on June 5th, 2010, with a 152mm refractor in average seeing.  In a 10mm eyepiece (122x) they were a stretched out oval of light; in a 9mm (136x), they were more defined, still touching, not quite separate yet; and with a 7.5mm eyepiece (163x), they were transformed into two distinct globes — pure, white, shimmering, fantastic — words just don’t quite describe it!  You really have to see this for yourself!

And don’t forget the fainter components of Porrima, either:

Σ1670    Magnitudes        Separation      Position Angle      Date of WDS Observation

AC:        3.48, 15.10              93.7″                121°                           2007

AD:        3.48, 12.05            171.0″                  90°                           2007

AE:        3.48,   8.94            258.6″                168°                           2011

AF:        3.48,   9.53            421.0″                 268°                          2011

I had no problem seeing “E” and “F” in my five inch refractor, and I found “D” fairly quickly with averted vision — although once I found it, I could see it most of the time with direct vision.  With a magnitude of 12.05, I expected it to be harder than it was.  Fortunately, at 171″ it’s far enough away from Porrima “A” and “B” that it escapes most of the glare.  (See the sketch above if you haven’t already).

And that 15th magnitude “C” speck of light at the top of the list above?  I don’t have anything large enough for that one, but I would guess it would take at least twelve inches or so of aperture to pick it out of the glare.  The difference of about 12 magnitudes between “C” and “A” means “C” is 62,529 times dimmer (2.51 to the 12th power) than its brighter parent, although its distance of 93.7″ offsets some of the effect of the glare.  So best of luck if you try for it — and by all means, post a comment if you happen to catch it!

There are numerous additional observations in the comments below, so don’t quit reading yet.  🙂

Update (Feb 14th, 2011)

I’ve been trying to verify the current separation and position angle of the Porrima pair recently, and have found  much of the data that exists is in error simply because it becomes outdated so quickly, which is attributable to the relatively rapid orbit of “B” around “A.”

If you look again at the orbital diagram,  you’ll see how quickly “B” is moving around the primary, which should make it a bit more clear as to why confusion can so easily arise with regard to correct information.  Much of the information on Porrima conflicts even as to the date of periastron — which in plain language is when the two stars were at their closest.   Kaler, for instance, cites 2005, as do some older Sky and Telescope references, but most other sources cite 2007/2008 as the correct date.  Similar discrepancies exist for separation and position angle.

The April, 2011, issue of Sky and Telescope has an excellent article on Porrima (pp. 56-57) which is the best source I’ve found so far for new information.  That article puts the separation of the two stars at 1.7″ as of Spring of 2011, which is the figure I’ve used in the data at the beginning of this post.  That article mentions the use of an adaptive optics technique known as speckle-interferometery for measuring the separation of multiple stars, which is a new and more precise method than anything that has been used in the past.

BUT —- if you happen to have a bilfilar micrometer handy, don’t despair.  Put it to work — you never know what you might find!

One More Update (June 23rd, 2011)

If you haven’t looked at Porrima and Saturn yet this month, time’s a-wastin’ — better take a look soon before they drop low into the southwest and become difficult to see well through the thicker atmosphere.

From my observing location, Porrima and Saturn are about an hour away from a large cluster of coastal pines that block the view to the west of them.  It’s 10 PM before the light fades enough to see the two objects, and it’s not completely dark until 11 PM — and by then, the trees are tugging at them.  As much as I hate to lose these two beautiful objects, they’ll be gone for all practical purposes in another few weeks.   But they’re a stunning sight in a telescope — actually, rather stunning visually, too — and it’s been a real pleasure to have had the opportunity to see this stellar rendezvous.

Right now — provided the atmosphere is cooperating — an 80mm to 85mm scope  is about the minimum aperture you can use to pry apart the two bright components, and you’ll need something on the order of 240x or better.   After Porrima makes its circuit around the sun later this summer and emerges on the other side, it will have widened a bit.  Maybe next year it’ll be in reach of a 70mm scope — and in  a few years as it widens further, no doubt a 60mm scope will be able to resolve it.   Does that mean the challenge will be gone?  Perhaps, but the beauty will always remain.

LAST CALL!   If you haven’t looked at Porrima and Saturn — get going!