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Zeta (ζ) Geminorum — Another Neat Knee Star

So there I was, trying to enhance my dark-adapted vision by sitting in the dim light of dusk with no lights on in the house, leaning forward in my chair, shoulders hunched over that old and dependable 1844 standby, The Bedford Catalog,  skimming the table of contents for the entry on Epsilon (ε) Geminorum  . . . . . . .  when suddenly my eyes landed on Zeta (ζ) — which came as a complete shock.

Not sixty seconds prior to that I had been looking at Gemini in The Cambridge Double Star Atlas, and had noticed Zeta (ζ) was devoid of a double star designation.  Haas doesn’t mention it all, and you have to look closely in Sky & Telescope’s Pocket Atlas to catch it’s double star status because it’s also identified as a variable star (a Cepheid) . . . . . .  which we’ll circle back to shortly.

Now I’ve never quite figured out why one atlas will identify a specific star as a double, and another will ignore it, especially when the star is as obvious and easy as this one is — but, as Galileo tried to tell the Pope, “It is what it is.”   On the other hand, there are so many double and multiple stars scattered about the galaxy, no one person or atlas can track all of them — although I’m doing my best to get there first.  😉

At any rate, my imaginative attention was completely captured by the fact that Zeta (ζ) is  also a “knee-star” — meaning it’s located on Pollux‘s right (west) knee, opposite its Castorian counterpart, the very attractive Epsilon (ε), shining richly in yellow from Castor‘s right knee.  Kind of “kneat.”

But that led to another thing that caught my attention — which is how the dual knee stars match up well with what I’ve always thought was an uncanny symmetry about Gemini.  Granted, we’re dealing with twins here, but the two sides of Gemini — the Castor side, and the Pollux side — are so darn close to being mirror images of each other, it’s really stretches the boundaries of belief.  And here we have it again, with each of them sporting double stars on their knees.  Rather dazzling of them to dress that way, too.

And then there was one more thing that caught my attention — which is the number of components in Zeta (ζ).  There are a total of five, ranging in magnitudes of 4.1 to 13.5, all widely separated, and thereby eliminating any question of obscuration by omnivorous photons.  Although it did occur to me that the dim 13.5 magnitude “E” beacon-ette could well prove to be a challenge.

At any rate, now that you’ve been introduced to Zeta (ζ), let’s go find it and see what it has to offer.

Zeta (ζ) Geminorum     (Mekbuda)  (H VI 9) (SHJ 77)  HIP: 34088   SAO: 79031
RA: 07h 04.1m   Dec: +20° 43′
*****                      Magnitudes         Separation        Position Angle         WDS Data
SHJ 77       AB        4.1, 10.7                 87.3″                       85°                         1997
SHJ 77       AC        4.1,   7.7               101.3″                     347°                        2008
SHJ 77       AD        4.1, 12.6                 67.8″                     354°                        2008
TOB 46      CE        7.7, 13.5                 97.1″                     322°                         1997
Distance: 1169 Light Years
Spectral Classifications:  “A” varies from F7 to G3, “C” is classed as G0

Since the illustrious and industrious Admiral Smyth is the one responsible for our being here to start with, let’s see what he has to say:

A coarse triple star, on the right knee of Pollux.  “A” 4, pale topaz; “B” 8, violet; and “C” 13 Grey.  . . .  It is easily seen on running a line between the cluster in Orion’s sword and Pollux, for it passes over ζ at 9° from the latter star; and it is near the mid-distance between ζ Tauri, the tip of the southern horn, and the Præsepe in Cancer.”

(The Bedford Catalog: Willman-Bell, 1986: p. 169)

I’ve labeled M 42 (“the cluster in Orion’s sword”), Zeta (ζ) Tauri, and M 44 (“the Præsepe, or Beehive Cluster”) in yellow on the chart above, so if you follow either of the Admiral’s directions, you can’t miss. And, if nothing else, you can always leap southeast from Castor’s yellow knee to get to Pollux’s yellow knee. (Stellarium image with labels added, click to enlarge)

Let’s start with the Admiral’s lettered designations first, since it seems the identities of two of the stars have been reversed.  The one he refers to as the eighth magnitude “B” star is now the 7.7 magnitude “C” component, and the 13th magnitude star he refers to as “C” is now identified now as 10.7 magnitude “B”.  And as for his colors — well, we’ll go take a look and see for ourselves in just a minute.

The 76mm Tasco surveying the darkness.

Now the last time out, I invoked the aid of Homer, that great 700 B.C. Bard of  Greece, in hopes of breaking the weather curse cast upon me by the cloud creating Sky Gods.  And, bless his ancient heart, he heard my desperate plea and succeeded in scattering the snickering omnipotent ones for long enough that I had one good night.

So with clear skies and very few clouds in sight, but banished from my second floor observing deck by a thin layer of ice that should have been on top of a skating rink, I grabbed an old 76mm Tasco f/15.8 refractor and headed for my gravel driveway, where the surface was richer in friction.  I plopped an old Vixen Polaris mount down in the drive, attached the scope, let it cool while I found some warmer clothes, and then re-emerged into the balmy thirty-five degree second-day-of-Spring-air, and aimed the Tasco at Zeta (ζ) per the Admiral’s directions.

“A coarse triple star, on the right knee of Pollux.”  Click to lose this caption, and to get the best effect, turn out the lights!  (East & west reversed to match the refractor view)

What I found looking back at me at 60x was a pale, yellow-white primary shadowed by an obvious companion to the north — and after roving around the east edge, I spied a third star with averted vision.  I recognized it immediately.  There was no doubt in my mind that was the star the Admiral described as “grey” — it was as if I had seen it before.  Thirteenth magnitude it is not (the current 10.7 is a better match), but at the time the Admiral made his observation, magnitudes were consistently under-estimated in comparison to today’s numbers.   It’s a tricky little devil in a 76mm scope, but a bit of magnification (80x) brought it out of it’s aversion to my vision with reasonable distinction.

Getting back to the color of the primary, if the Admiral’s “topaz” refers to yellow-white, that would correspond pretty well with what I saw — it certainly wasn’t the eye-catching deep yellow I saw glowing in that other “knee star” to its west, Epsilon (ε).    Sirs James South and John Herschel scrutinized Zeta (ζ) on March 24th, 1821, and described it as “large yellow; small ash color.”  And Sir William Herschel, who discovered this one on October 7th, 1779, described it as “Double. Very unequal. L. reddish w[hite].; S. dusky r[ed].”

As I mentioned in the first paragraph, Zeta (the primary) is a Cepheid variable, which has a bearing on its color.  Kaler states that its magnitude fluctuates every 10.2 days, between 3.7 and 4.2, and its surface temperature ranges from 5300 to 5800 Kelvin (making it a close match to the 5777 Kelvin of our sun) — and that causes its spectral classification to vary from F7 to G3.  All of which keeps its apparent color in the yellow-white range.

So yellow-white seems to be the majority opinion on the primary, and I’ll gladly agree with the Admiral on “grey” as a good description of 10.7 magnitude “B” — but we’re all over the galaxy on 7.7 magnitude “C”.  The Admiral says violet, James South and John Herschel went with ash, and Sir William saw “dusky r.[ed].”  I saw pale blue-white — yet it’s stellar classification, G0, says it should be yellow-white.   So take your pick, or better yet, take a look.

Now when I took my look, I was also looking hard for some sign of 12.6 magnitude “D” and 13.5 magnitude “E.”  I really didn’t expect to see those in the 76mm Tasco, but having familiarized myself with the photonic framework of Zeta (ζ), I attacked again the following night with my five inch f/15 refractor  — and found myself subjected to the revenge of the Sky Gods.  They conjured up a dry and strong east wind — always a guarantee around here of atrocious seeing — and then one of those chuckling characters began slowly pulling a blanket over the sky, from south to north.  I could swear I heard a sinister giggle or two as the clouds advanced, but maybe it was just the wind.

Anyway, I managed a glimpse of Zeta (ζ) at 64x and 95x, but the leading edge of the clouds had advanced so far I could barely catch an averted glimpse of 10.7 magnitude “B” — heck, I had a better view the night before in the 76mm Tasco!  If I thought it would help, I would gladly sacrifice another eyepiece to those over-bearing rulers of the sky, but I’ve already done that so many times they have more hardware now than I do.  So I guess I’ll holler for Homer again.

But to avoid delaying this for three months while I wait for the Sky Gods to go torment some other poor soul, I’ll post what I have now.  With any decent change in luck, accompanied by a decent change in the weather, I’ll zero in on Zeta (ζ) with a five or six inch refractor and see if I can ferret either, or both, of those two fainter companions, “D” and “E”, out of the depths of interstellar space.  I see nothing but rain and clouds in the forecast for the next week, and then the waxing moon is going to be too close to Gemini for me to see down to 13th magnitude, so it looks like a delay of at least several weeks.

I suspect that because “D” is wedged about halfway between the primary and “C”, it might be a real challenge to pick it out of the glow.  On the other hand, even though “E” is almost a magnitude dimmer, it’s northwest of “C” and far enough away from all of that starlight that it could well prove to be the easier of the two.  In the meantime, if anyone out there catches a glimpse of either the “D” or the “E” component — or both (!) — post a comment here.  If you have a sketch, I’ll be glad to add it here as well.

Oh — and before I forget — Mekbuda is an Arabic name for “folded paw,” referring to that of a lion.  Once upon a time this area of the sky was home to a lion who has since migrated elsewhere, possibly into Leo’s domain.  At any rate, he’s moved on and the Gemini twins have staked a claim to this sector of the sky, yellow knees and all.

Meanwhile, to the folks at asociacionhubble.org  —  Saludos!
. . . .  and to those at astronomy.ro  —  Salutări!   😎

The Tasco family on a warmer night. The 76mm is on the left, the 60mm on the right. Click for a larger view.


3 Responses

  1. Really enjoy seeing the scopes you are using. Just a note, Mekbuda is on the Astroleague’s binocular double observing program that I’m slowly working on. I saw Zeta with my 20×80 binocs as “whitish blue primary and blue secondary” on 21 Mar 12 (UT). Also on the list is 20 geminorum and upsilon geminorum as binocular doubles. Not only are the twins’ knees of interest, but so are their feet!! My next target is upsilon with my binocs. By the way, the views in my 78 mm refractor are much to be preferred to those in my binoculars!

  2. Looks like I need to change “upsilon” to “nu” in my last post. My Greek language skills need could use some improvement.

    • Thanks, John — I’ll have to take a look at both Zeta (ζ) and Epsilon (ε) with binoculars. Hadn’t even thought about that, but Zeta “A” and “C” should be easy — Epsilon will probably need a dark sky, though.

      I can understand the confusion between Upsilon (υ)and Nu (ν)– it’s easy to do!

      Just discovered that Zeta actually has a line through it (the usual double star indicator) in The Cambridge Star Atlas, but still no lettered designation to identify it. But that’s what you get when you look at an atlas when it’s half dark!

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