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A Tale of Two Secondaries: Part Two — Tau (τ) Canis Majoris (h 3948)

If you wandered to this point without reading the first part of this two part post, you can get to part one by going  ➡   HERE.

Meanwhile, we’re leaving our former location east of Orion’s Betelgeuse and taking a trip to the warmer southern regions of Canis Major, where we’ll take a look at a complex multiple star basking in the center of a beautiful open cluster.

To reach (τ) from Delta (δ) Canis Majoris (aka Wezen), move three degrees due east to 4.85 magnitude HIP 35412 (aka 29 Canis Majoris).  You can’t miss Tau (τ), which is about half a degree south of it surrounded by a shimmering glow.  (Stellarium screen image with labels added, click for a larger view).

To reach (τ) from Delta (δ) Canis Majoris (aka Wezen), move three degrees due east to 4.85 magnitude HIP 35412 (aka 29 Canis Majoris). You can’t miss Tau (τ), which is about half a degree south of it surrounded by a shimmering glow.  Center it in your eyepiece and make yourself comfortable — we’re going to be here for a spell. (Stellarium screen image with labels added, click for a larger view).

Tau Canis Majoris  (h 3948)                          HIP: 35415         SAO: 173446
RA: 07h 17.8m   Dec: – 24° 57’
.                                    Magnitudes      Separation    Position Angle       WDS
.FIN 313      Aa, Ab     5.33,   4.89               0.1”                 129°                 2011
HJ 3948            AB     4.42, 10.20               8.6”                   93°                 2002
HJ 3948            AC     4.42, 11.20             14.2”                   87°                 2002
HJ 3948            AD     4.42,   8.22             84.8”                   77°                 2002
.TOK 42         Aa, E     5.33,   9.70               0.9”                   88°                 2011
Distance: 3198 Light Years
Spectral Classifications:  A is O9, D is B2

As you can see from the data above, this is a complicated multiple star.  In addition to the four components we’re going to look at, there are two more, at separations of 0.1” (Aa, Ab) and 0.9” (Aa, E), which are well beyond our reach.  But since Tau (τ) resides right in the middle of an eye-opening open cluster, NGC 2362, the complexity isn’t surprising.

East and west are reversed here to match my sketch.  (STScI photo, click for a larger view).

East and west are reversed here to match my sketch. (STScI photo, click for a larger view).

As alluring as the cluster is, it creates a lot of star glow around Tau’s components, making it more difficult than it is anyway to pry them out of the glare.  In fact, there’s so much glare here from the cluster’s many stars, that in a four inch refractor or smaller it looks like Tau (τ) is immersed in nebulosity, which was precisely the impression it made on me when I first viewed it several years ago in a 102mm refractor.  Most of that nebulous impression is resolved into individual stars, though, in apertures of six inches or greater.

The photo above gives you some idea of the nature of the area, although nowhere near that many stars are visible even at ten inches of aperture.  And of course all those stars in the cluster have an additional effect:  they make it difficult to determine which stars belong to the cluster and which belong with Tau (τ).

Let’s start first by looking at my sketch:

I caught most of the stars in the field of view in this sketch, but since I only had thirty minutes before losing the view to a cluster of star-eating coastal pines, a few around the outer perimeter are missing.  (East & west reversed, click on the sketch for a larger view).

I caught most of the stars in the field of view in this sketch, but since I only had thirty minutes before losing the view to a cluster of star-eating coastal pines, a few around the outer perimeter are missing. (East & west reversed, click on the sketch for a larger view).

Now the big question confronting us is this:  which stars are which components?  If you go back to the data above for Tau (τ), you can see from the position angles of the “B,” “C,” and “D” components that all three stars are lined up within sixteen degrees of each other.  It was obvious that “B” and “C” were going to be the tough ones, so I decided to start with “D” and work my way in to the primary from there.  With several stars competing for attention, I knew I had to pin down “D” precisely, so I put the plotting ability of Vizier to work on Tau and came up with this:

The line labeled 1.425” identifies “D”, which also matches it’s 77 degree position angle. (The software also plots that angle for you, but it isn’t visible here — click for a larger view).

The line labeled 1.425” identifies “D”, which also matches it’s 77 degree position angle. (The software also plots that angle for you, but it isn’t visible here — click for a larger view).

I had hoped to identify the “B” and “C” companions with Vizier, but as you can see in the photo, they’re hopelessly buried in the primary’s glare.  But I did plot their positions as closely as possible (the 14.12″ and 9.401″ labels), and even though there aren’t any stars visible at the plotted locations, it gave me an idea of where exactly on my sketch to look for them.

 And this is where our experience with Bu 193’s secondary becomes invaluable.

With “D” now labeled, let’s look at my sketch again.  You’ll see two faint stars southeast of the primary (enclosed below in a box) which I found very intriguing.   Could they be “B” and “C”?

Same sketch as earlier, with "D" now labeled, and the possible "B" and "C" candidates boxed in. (East & west reversed, click to enlarge the view).

Same sketch as earlier, with “D” now labeled, and the possible “B” and “C” candidates boxed in. (East & west reversed, click for a full size view).

Something made me hesitate — I think it may have been S.W.’s voice warning me away from making a rash decision.  On the one hand, I was mystified by the fact that the position angles of those two stars are greater than the 87 and 93 degrees listed for them in the WDS figures from 2002 — and on the other, when I studied the relative position angles and distances between the two stars, I could see enough similarity to the WDS figures to slap “B” and “C” labels on them and call my chores done.

But think back to Bu 193 and the 19.5” separation of its secondary.  It was further out than the two companions of Tau that I was searching for (8.6” for “B” and 14.2” for “C”), and although it could only claim an anemic magnitude of 12.38 (compared to 10.20 and 11.20 for Tau’s “B” and “C” components), it wasn’t drowning in a sea of open cluster glare, either.  So I came to the reluctant conclusion that my “B” and “C” chores really weren’t done at all.  I knew where they were now, and they weren’t going to be easy to see.

I mulled over the wisdom of my next step — it had as much chance of succeeding as a marshmallow has in the middle of a bonfire — but I finally closed my eyes and leaped into the flames, telescope and all.  After waiting patiently for a clear night to arrive, I raced outside and set up my six inch f/10 refractor and pointed it into the middle of that glowing open cluster surrounding Tau (τ).  Unfortunately I was late in getting set up because dinner detained me longer than I had expected, which meant I lost about half of the thirty minute window I had for catching Tau (τ) while it was in the open sky between the coastal pines.

Once I had the primary centered in a wide angle eyepiece, I went for the murky heart of it right away with a 4mm Astro-Tech Plössl (380x).  And there were some heavy odds stacked against me: first, the seeing was poor to start with, and of course it was a whole lot worse at 380x; second, there was almost enough murk in the sky to make Sirius look like a second magnitude star; third, Tau was even lower in the thick atmosphere of the southern sky than Sirius; and fourth, there was all that white light rushing into the eyepiece from both the primary and the cluster’s glare.

Well aware that I was running a race against time, I persisted for about ten minutes, delicately tweaking the focuser’s fine focus knob first this way, then that way, then back again, pausing each time to see what would happen — and finally I had a glimpse of something.  I could feel the breath sucked out of me like a wandering star yanked into the heart of a black hole.  I became rigid as a rock, locked solidly into a frozen crouch, waiting to see what would happen next, and suddenly I caught another glimpse — two ghostly egg-shaped smears of light swimming in an overwhelming glare, desperately trying to break free of the primary’s glowing grasp — and then they were gone again, as fast as they had appeared.

I had one more tantalizing glimpse of them, then the trees swallowed everything, the view in the eyepiece went dark, and they vanished — probably forever.  And every bit as tantalizing as those glimpses was the thought that Tau’s “E” companion may have contributed it’s 9.70 magnitudes to that bleary egg-shaped smear of light.  Chalk one up for the marshmallow.

Those two ghostly stars were one of the most fantastic things I’ve ever seen.  Try to imagine looking through the blinding light at the center of a white-hot flame and suddenly catching sight of a wavering ghost-like face on the other side.  It disappears almost immediately, then it re-appears for a few tenths of a second longer, vanishes once again, and quickly flashes into existence once more for about as long as it takes you to blink your eyes in surprise — and then it’s gone.  Completely.  Everything goes dark.  And all you have to hold on to is a very brief, but amazingly vibrant, image of that bleared shimmering face being suffocated in a rush of white-hot flame.  You don’t quickly forget it.

** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** **

Now there’s one sure way to make the frustrations of identifying short-leashed secondaries less frustrating, and that’s to use the same scope all the time for all your searches of that kind.  That way an observer becomes familiar enough with distances at various magnifications that it’s possible to become reasonably adept at estimating both separations and magnitudes.  But I’m like a kid in a candy store when it comes to using refractors of different apertures — and removing the temptation from the candy store is like separating Laurel from Hardy.  It can’t be done — at least I can’t do it, anyway.

Speaking of which, I wonder if I can ferret Bu 193 “B” out of the primarial glare with a 90mm refractor?  Gotta go – I hear the candy rattling in the jar.

Clear Skies!  😎

UPDATE:  This recent photo (about March 20th, 2013) by Steve Smith captures both the “B” and “C” companions of Tau, which really is an amazing feat given the glare from the primary.  The glimpses I had of the two inner companions were nowhere near this clear, so feast your eyes on this carefully since it’s a beautiful, not to mention, rare view!  First, a view with the companions labeled:

Photo taken by Steve Smith through a 100mm refractor at prime focus, exposure of four seconds, ISO set at 200. I've flipped east and west to match my sketch. Click to enlarge both this and the photo below.

Photo taken by Steve Smith through a 100mm refractor at prime focus, exposure of four seconds, ISO set at 200. I’ve flipped east and west to match my sketch and added the labels for the three companions, plus the directional indicator.  Click to enlarge both this and the photo below.

If you compare Steve’s photo with my labeled sketch above (here), you can see the two stars I had originally thought might be “B” and “C” are located in the photo to the right and below the primary at about a thirty-five degree angle.   The two inner companions are completely hidden in the primary’s glare in both my sketch and the STScI photo above.  Really, it’s remarkable to see those inner companions so clearly!

Here’s the photo once more, with the labels out of the way:

Photo again by Steve Smith, and used with his kind permission.

Photo again by Steve Smith, and used with his kind permission.

Post Script:  Thanks go to Steve McGee and Chris Thuemen for waving Bu 193 in front of me.  I probably never would have discovered it otherwise, and as unlikely as it seems, it led me back to Tau Canis Majoris, a trip I’ve had planned for the last couple of years, which also happened to be a favorite of theirs, too.  And many thanks to Steve Smith for permission to use the two photos above.   This has been a heck of collaborative effort, which has made it all the more enjoyable!

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5 Responses

  1. Hi John!
    John you’ll have a laugh at this…less than 5 minutes after sitting down at my computer with an oversized mug of peppermint tea, your email arrived. I had only 2 more systems to research & confirm, of a set of 10, to reach my goal for the evening. When that was done I sat down with my tea and devoured your two cookies. As always a really fine read. I appreciate how you keep subtly hammering into the reader the need to be patient while at the eyepiece. The notion of constantly changing seeing conditions is something that I have not yet come to a full appreciation of. While on the summit of Halekala with Steve, trying to tease out the secondaries of Tau CMa and inspite of exceptional seeing conditions, these faint components were constantly showing themselves and then disappearing. I suspect a comfortable observing position is the key to a relaxed concentrative session at the eyepiece. I sure wish that all these clouds would go away so that I could perfect my patience.

    Cheers, Chris.

  2. Thanks, Chris!

    Yes, patience and a comfortable observing position are the two key ingredients in improving what you can see. Some people do just fine standing, but I suspect they’re still missing some of the detail they would otherwise see if they were seated.

    There’s no provision for posting pictures in comments, so this will have to do instead: Steaming Tea!

    John

    • Hi John!
      I have never seen a more enticing cup of tea in all my life.Makes me want another one. That rich golden yellow….hmmmm, with just enough honey..sacreligious I know, but I’m from the mainland!! LOL.

      Cheers, Chris

      • I’ve just added a remarkable photo of the inner companions of Tau Canis Majoris — something I never expected to see in a photograph. Take a look (at the end of the post)!

        John

  3. I’m waiting for another clear night to observe this system from the summit with my C925. If I can get at it around 10 or 11pm this month, it will be low enough to view without a diagonal. The challenge of sketching for me is dealt with by a variable height stool and a clipboard tucked into my belly. A more compelling challenge is fumbling with my glasses on a leash around my neck to place the tiny dots on the log. Letting the F.O.V. drift at sidereal time helps, as fait glimmers of >10th mag. stars pop out of the darkness from time to time. A hot cup of Earl Gray tea finishes it off after about an hour of viewing. Give me MORE!!! ;-]

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