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Castor – three steps to take you from aw to awe!

Castor can  leave me rapt in awe, but how I get there, especially with the inevitable been-there-done-that feeling that can set in after half a century of observing, is  a three-step process. Oh don’t get me wrong. I appreciate the pure beauty of these stars. But what fascinates me most about this magnificent system is all that we don’t see.   I’ve split Castor quite easily with my little 50mm Tasco F12 achromatic refractor, a refugee from the 1960s, so Castor’s beauty is open to anyone with any size scope. But when you dig beneath the obvious, the real marvel of Castor blossoms.

Castor (Alpha [α] Geminorum)
RA:  07h 34.6m   Dec:  +31° 53′
Magnitudes         A: 1.9    B: 3.0    C: 9.8
Separation          AB: 4.2″      BC: 71″
Position Angle    AB: 62°    BC: 164°
Distance: 50 LY
Spectral Classification: A1, A2, M

It all adds up to make  Castor one of my favorite doubles that I return to time after time.  As with so much in astronomy, as you try to express your feelings about it, Castor can quickly slip into the realm of the ineffable. My challenge is to make it effable – and yes, there is such a word – I looked it up 😉  But more on getting beyond the starry doldrums later.  First let’s find and observe Castor (step 1) , then examine what we don’t see (step 2), and then try to put it all into some more human-sized model (step 3).

To begin with, Castor isn’t a double, it’s a double double – well, a triple double –  well, actually a sextuple! What I can see with the 50mm is a very nice double and with a slightly larger scope, a triple.  And, of course, in mythology Castor is a another double of sorts – one part of the heavenly Gemini Twins, Pollux being the other.

The Gemini twins, castor and Pollux, as depeicted int he 1603 Unraometria by johann Bayer. (Used with permission from the Linda Hall Library of Science, Engineering, & Technology 2005)

The twins are easy enough to find – at mid-northern latitudes they rise in the East during prime time in early winter –  but your first challenge is to identify which of the two stars is Castor. Fortunately, there are three strong clues, for these twins are far from identical.

1. Castor is the dimmer of the two – despite being designated by Bayer as Alpha Geminorum.  Alpha usually means the brightest.  And in one sense Castor is the brightest – it’s the brightest second magnitude star in our sky 😉 At magnitude 1.58 its just doesn’t make it into the first magnitude category, but brother Pollux, at magnitude 1.14, does.  Can you tell the difference in brightness? It’s half a magnitude, certainly more than enough to detect with the naked eye, but still not all that easy.

2. Castor is also the “leader.’ That is, it’s the western-most star and since stars always appear to be moving westward, Castor leads Pollux across the sky.  At mid-northern latitudes it’s far enough north to do quite a circle, though, so while it’s clear which star is leading when they’re rising, when they’re high in the western sky and seemingly side-by-side, it may leave you a bit puzzled.

3. Third, Castor is white (Class A) – Pollux is yellow/orange (Class K). Again, I don’t find this difference dramatic, but it is certainly noticeable.

Given all that, Castor still gives me pause when I go to aim a scope at it. I have to consciously remind myself of one or more of these differences so I’m aiming at the right star. And then a second issue enters. Since they are both very bright and they are separated by less than five degrees, if you’re using an optical finder it’s easy to point it at the wrong star.  My solution to this is simple, though. Castor is neatly bracketed by a triangle that consists of a 4th, 5th, and 6th magnitude star. It’s largest dimension is about two degrees, so it fits nicely in the typical finder field with plenty of breathing room around it. Brother Pollux has only a single 4th magnitude companion in close proximity.

Here’s what you should see in your finder scope.

what you should see in a finder with 5-degree field of view. (Prepared from Starry Nights Pro screenshot.)

What you see in the telescope looks something like the next drawing which is done to illustrate the relative splits of the B and C components, as well as the position angles of each which are marked on perimeter of a 17’ field of view.

Step 1 – Observing Notes

Here are my notes on an observing session using the 50mm Tasco in the winter of 2010.

First out was the 50mm. I’m really enjoying putting this little scope to the test. This morning it began the session by delivering a charming rendition of Castor – as a double. I could not see the 9th magnitude, more distant “C” companion. I used the 6mm Plossl to  achieve a nice split, but I have trouble holding my head in position for that eyepiece. I found I could split it just as well with the 18mm Meade and a 2X Shorty Barlow.

The 6 mm would have delivered 100X – the other combination, 66X. From other experiences somewhere between 50X and 100X should deliver a nice split. Splitting the “C” component is no issue – but seeing it is. Because it is magnitude 9.8  is it hard to pick up in a small scope. Under good conditions you certainly should be able to see it in a 60mm  and looking at  my past notes I know I found it easy with an 80mm Celestron Onyx refractor at 72X. My more recent observations have been with a 8-inch SCT – one of the new Celestron EdgeHD scopes – and it does a great job on Castor.  Using a Takahashi LE 30mm it splits nicely at 66X.  At higher power I picked up some color: Pale blue for the primary, pale yellow for Castor B, and a greyish-red brick for Castor C. Sissy Haas calls A & B “lemon white” and leaves C  alone.

What I did notice recently is that there’s a field star that someone with a small scope might mistake for the C component. While almost the exact same brightness as “C,” it is at PA 221 and about three times as far away from Castor A as Castor C is.

(Note: Had the 50mm out to again to tackle Mizar and once done, I took on Algieba and then Castor again. This time I pushed it with  a 6-3 Nagler zoom and I used it at the 3mm click stop giving a ridiculous – for this tinys cope – 200X. That’s 100X per inch, but it still yielded a useful image which says something both for the seeing in this morning and the scope.)

Step 2 – What we don’t see

But as I said at the outset, what grabs me about Castor is what I don’t see. Each of the three stars we do see is a double so close that we can’t split them with any telescope.

Castor A has a companion that rockets around it in 9 days. It is just .12 AU from it – that’s about 11 million miles, so it’s less than half the distance of Mercury to our Sun.  And Castor B? It’s companion circles it in less than three days and is less than 3 million miles from it!

To put this in perspective, the most obvious thing we see when we look at Castor is the delicate split between two bright star pairs, Aa/Ab and Ba/Bb.  Yet that delicate split is really in the order of 10,000 million miles – more than 100 times the distance between Earth and Sun. That’s what’s represented by the slither of black sky we can see between these two beauties and that’s why it takes Castor Ba/Bb 445 years to orbit Castor Aa/Ab.

All of this pales in comparison with Castor C, however. Castor C is a pair of “M” class dwarfs -which explains the dull red I detected. These are circling a common center of gravity every day – in fact in 19.5 hours according to the description on Jim Kaler’s web site.   So they too are extremely close to one another, but that huge gap – it’s more than a minute – that we see between A and C? That’s in the order of 1,000-plus AU – or about 100,000 million miles.  It’s still close, however, as stellar distances go. One thousand AU would  put it about 1/63rd of a light year from Castor A. But that’s enough of a gap so it takes some 14,000 years to make a revolution -about twice all of recorded human history.

As I  contemplate all this what pops into my head is hirling dervishes doing one of their dances where they seem to hold their form and spin continuously while also following some complex pattern.

Step 3 – “human-size that one, please!”

As in all astronomical observing, I’m always reaching to get my mind around what I see – to somehow internalize this information so I can experience what Einstein meant when he said:

The most beautiful thing we can experience is the mysterious. It is the source of all art and science. He to whom this emotion is a stranger, who can no longer stand rapt in awe, is as good as dead; his eyes are closed.

And so I find myself in a never ending battle to open my eyes – to discover how to add the “e” to “aw.” The truth is, no formula works, at least none I have discovered. But I think you can prepare yourself so that awe has a better chance of finding you.  The first problem is the visual information we receive when we look at these incredibly distance objects is so scant. So I turn to abstractions – the incredible numbers that describe the system we’re observing – and they’re sufficient to leave me mentally amazed, but that’s not awe. My left-brain is intrigued, my heart is still.  I get a rational recognition in words and numbers, but there’s no magic punch to the solar plexus. And this leads me to a third stage of observing a system such as Castor where I try to build a model of the system on a human scale.  That’s what this next image begins to represent -a very crude attempt at a model using clay and beads of appropriate sizes.

What I did was reduce the six stars to a scale where a million miles is represented by a quarter of an inch. This was instructive and roughly reflects the sizes listed on Kaler’s web site,  so it begins to give me a feel for the system.  But, of course, there’s one huge problem here: I’ve really used two scales – one to represent the size of the stars and the other to represent the distances between them. If I put the distance onto the same size scale as the stars themselves, then we would have something I couldn’t show you on this screen – something like this:

Castor Aa is 1/4-inch in diameter and is orbited by Castor Ab at .12 AU – about 11 million miles – roughly 2 3/4-inches on this scale.

That’s certainly easy enough to depict.

And Castor Ba has a companion, Bb at .03 AU – about 2 3/4 million miles or slightly less than   3/4 of an inch.

Again, easy to depict.

BUT… the distance between A and B is, on average, 104 AU – or 9,672 million miles. That’s 2,418 inches 202 feet on our scale!

Ouch – my paper just isn’t big enough. But my head is. My yard is about  250 feet long. So it’s easy enough for me to imagine placing a pair of these beads at one end of the yard to represent Castor Aa and Ab. And near the other end of the yard I can put the other pair of beads to represent Castor Ba and Bb. But could I even see them 202 feet away? I doubt it.  And now where does the Castor C pair of beads go – those tiny, rosy ones in the picture? Nearly half a mile away on this scale!

And while we’re at it, let’s get our own Sun into the picture. It’s 50 light years away. So where would be put it? Well on our scale one light year is about 23.2 miles, so 50 light years is 1,160 miles.

So when you look at Çastor in your backyard telescope, you might try to get this in your head:

In our imaginary scale model we are sitting on a micro dot that is revolving around a star that is one quarter inch in diameter.  To get to this sextuple Castor system we’re looking at – this system of six stars – remember,  they are  roughly quarter-inch globes in our model that are spread over a couple thousand feet of space – we have to go 1,160 miles.

And then  try to imagine this: How in the heck could something smaller than a marble radiate enough energy so we can see it from more than 1,000 miles away – and continue to radiate that energy from that tiny globe for billions of years?

If nothing else, it must have a battery that would leave the Energizer’s pink bunny green with envy!   And it sure as heck leaves me wrapped in awe – though not while I sit here and play these mental games – no, the only way I can experience that awe is to get to the telescope with these ideas floating in the back of my mind, then sit quietly and wait for it all to overwhelm me. And on the best nights, it does.

6 Responses

  1. I think I’m “wrapped” in awe by the sheer amount of information in this piece, Greg! I have a feeling I’ll come back to it quite a few times for information.

    My first really detailed look at Castor was with an 80mm f15 Sears refractor I bought about a year ago. Before that, my Castor-ial experience was pretty much limited to using it as an alignment star on a CGE mount. Usually, it would be in the west, which from my location is out over the ocean, where the seeing is usually poor. In the eyepiece of my C11, it would be boiling so badly that it would be impossible to detect any hint of two stars.

    But to return to the 80mm scope, that first night with it was black as coal, and both the A and B components of Castor were really beyond anything words can touch. I remember working my way up to a 6.7mm Plossl, which was about 180x, and sitting there for quite a while totally lost in the beauty of the shimmering diffraction rings. Castor C was easy to pick out of that glare, but my attention was riveted on A and B.

    I’m still waiting to get my first look at it this year – if the weather doesn’t cooperate soon, I may take to the road to find a clear hole in the sky elsewhere – anywhere!

  2. Lucky to see Castor at the end of a brief “rain stopped play” session 3 nights ago. Always a pleasure and a reason to wonder. Two stars in orbit around a common centre of gravity-fine. Three stars? more stars ! Do like the diagrams and drawings-very helpful when it matters. Your reduction of part of Gemini to fit in your yard brings to mind the Pharaos of Egypt who supposedly built the Pyramids to represent Orion. The Pyramids originally were painted white. Just imagine a WiseMan seeing them under a crescent moon. Could start a whole new religion. Your model is far more complex and i`m going to have a long look with a strong coffee in hand. Thanks for all the stuff you put online for us to read and re-read. Hope your sky is available to view. regards rich.

  3. Hi Greg, i was lucky to get another look at Castor last night and and in my 80mm Aa and Bb as i now regard them both look Bright White. What really makes this double unusual for me is that the secondary does seem 2 and 1/2 times smaller than the primary BUT it is just as bright.

  4. It hasn’t been clear here since the last full moon – which is a bit more than three weeks – but at midnight last night, I could see the moon trying to wedge itself between the holes in the clouds, and it looked like they might actually start to break up – so I set up a pair of scopes and decided to wait it out.

    About the time I was ready to go, the sky opened up in several places, revealing a stretch of sky from Orion to Gemini. I spent a few minutes aligning both scopes to each other using Polaris, then turned towards Orion – and it was gone. Figures.

    But Castor and Pollux were still visible, so I quickly zeroed in on Castor, and got my first look at it this fall. I had a 17mm Plossl (25x) in an AT72ED and a 14mm Radian (56x) in an AT111ED. The first was barely enough to split Castor A and B, and in the AT111 the split was very close.

    So I replaced the 17mm with a 10mm Plossl (43x) and substituted an 8mm Radian (97x) for the 14mm in the AT111, both of which were much more to my liking. By now, though, the clouds had stretched themselves back over that area of the sky, so I was looking through a fairly thick haze.

    The clouds were moving quickly – which meant turbulent air – which meant the seeing was absolutely horrible. Both stars were hopping around like Mexican jumping beans at a Mariachi Band Recital. They would merge, separate, merge, jump, hop – more than once I expected to see them swap places.

    Still, there were moments when the seeing would calm down for as much as thirty seconds or so, and when that happened, I was rewarded with the very pleasing sight of two sharply defined white globes very close to one another. For a pair of photon starved eyes, it was a riveting sight. I didn’t move for about 45 minutes except to change eyepieces or switch views between the two scopes.

    The star of the night turned out to be a 7.5mm Celestron Plossl. I don’t know much about this eyepiece – it was given to me by a friend who had a box of astronomy related items he didn’t need . The top of the barrel is black with orange printing, and according to the label it was made in Taiwan. In the AT72ED, it only gave me 57x, but it was a beautiful 57x – the image was razor sharp when the seeing calmed down. Since I acquired these two scopes, the weather has been very uncooperative, but if what I saw tonight was any indication, I believe the 7.5mm and the AT72 were probably made for each other.

    The clouds began to get thicker and Castor continued to get fainter, until it was just about impossible to see it at all by 1AM. Looking up, I discovered the moon was totally hidden behind the clouds. All my attention had been so focused on Castor that I had no clue a thick cloud cover had blanketed the entire sky.

    Amazing what you can see through the clouds!

  5. Thank the Sky Gods for Castor!

    Clouds have been a persistent pain for the past three weeks, and they just don’t seem to want to move on to some place that could use them – like the Sahara desert, or maybe Mars.

    I’ve tried to catch a few clearing periods several times, and each time I’ve been ready to go, the clouds have taken over the entire sky. I’ve been suckered by sucker holes more times than I can count. But I never give up — which could also mean I’ll never learn. 😉

    Tonight was no exception — but — I managed to lock onto Castor before the clouds covered it, and when it disappeared, I had the last laugh. I still had Castor in view!

    It always amazes me what can be seen through the clouds, provided they’re not really thick. Castor is bright, and that makes a huge difference. I had an 80mm f6 locked onto it, so I just sat there and watched it for about fifteen minutes.

    I was able to see both of those shimmering white globes of light the whole time, although their photons almost faded from sight on several occasions. And every now and then both of them would suddenly start to brighten as a thinner area of clouds moved past ….. and every time, I would look up expecting to see clearing, and would find I could barely pick Castor out of the cloud cover.

    Eventually, the clouds overwhelmed the dual stream of Castorial photons and I had to yield the sky to them once more.

    But for fifteen minutes, Castor and I had them whipped.

  6. Great post! I looked up Castor in Stelle Doppie and didn’t find a Struve number. I’ll look it up in the WDS. I want to observe it with my Vixen 104mm. I can pick up the C companion easily with my C925. Your sketch is helpful and the Lit. you provide should suffice. I’ve got a star party coming up with some 8th graders and I want to try to show it.

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