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.
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.
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:
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:
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”?
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.
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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:
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:
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!