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A Tale of Two Tough Characters: Rigel and Tau (τ) Orionis

I’ve lost track of the number of times I’ve looked at Rigel, aka Beta (β) Orionis, but one thing that hasn’t escaped from my memory is the first time I viewed it in a 60mm refractor.  What stands out most clearly about that night was the moment I first caught sight of a flickering spark of light at the blinding primary’s southwest edge, something I really hadn’t expected to see.  That faint flickering spark was the 6.80 magnitude secondary, which is also double, although with a separation of only 0.10” I certainly didn’t notice.

I was reminded of that first 60mm refractor view of Rigel just a few weeks ago when I wandered just north of it to another difficult star, Tau (τ) Orionis.   Both stars are similar in their wide magnitude spreads, 6.50 magnitudes between Rigel and its BC secondary, and 7.30 and 7.40 magnitudes between Tau (τ)’s primary and the two stars that accompany it.  They both also boast of doubled up secondaries, but with the difference that the average backyard double star diehard has a fighting chance to wrestle Tau’s BC pair apart.

So in some ways this is a sojourn down one of the more difficult paths to double star nirvana, but it’s more a steady uphill climb than a hand-over-hand hoist to the top of a steep precipice.  After all, Rigel is very splittable with a 60mm refractor, although I can’t say the same for Tau (τ).  Nevertheless, difficult though they are, neither of these stars are really all that far out of reach, and both share one very important and redeeming attribute: they’ll sharpen your observing skills, provided you’re patient and are willing to persevere under a cold winter sky.

First, for those unfamiliar with the area, a chart displaying our destination:

You'll find Rigel and Tau (τ) holding down the southwest corner of Orion.  (Stellarium screen image with labels added).

You’ll find Rigel and Tau (τ) holding down the southwest corner of Orion. (Stellarium screen image with labels added).

We’ll wrestle with Rigel first and then we’ll give Tau (τ) a try.

Rigel  (Beta [β] Orionis)  (19 Orionis) (Σ  668)  (H II 33)
HIP: 24436   SAO: 131907
RA: 05h 14.5m    Dec: – 08° 12’
Identifier             Magnitudes       Separation      Position Angle     WDS
STF 668  A,BC:  0.30,   6.80               9.30”                  204°              2011
Bu 555       AD:   0.30, 15.40            44.60”                     1°                2008
Bu 555       BC:   7.50,   7.60              0.10”                    30°               2005
Distance: 773 Light Years
Spectral Classifications:  “A” is B8, “B” is B9

The challenge with Rigel lies in learning to see past its zero magnitude glare in order to visually excavate the seventh magnitude secondary buried there.  Every time I look at Rigel, I have to give my eyes time to adjust to that glare.

This sketch doesn’t quite convey the glare surrounding Rigel, nor does it do justice to the sudden visual and emotional impact of first sighting the secondary. But it’s close enough to give you a reasonable idea of what to expect. Nothing matches the real thing, though. So if you haven’t turned a telescope toward Rigel, time’s a wastin’! (East & west are reversed to match the refractor image, click on the sketch for a much better view).

This sketch doesn’t quite convey the glare surrounding Rigel, nor does it do justice to the sudden visual and emotional impact of first sighting the secondary. But it’s close enough to give you a reasonable idea of what to expect. Nothing matches the real thing, though, so grab a coat and a telescope and take a look. (East & west are reversed to match the refractor image, click on the sketch for a much better view).

Your first view is the hardest, not only because of the glaring battle you’re face to face with, but also because it’s hard to visualize what the secondary will look like in that glare.  More than likely your first sighting of it will sneak up on you unexpectedly, meaning you’ll suddenly discover you’ve been staring at it for several seconds without realizing it.  The first time I saw it I remember an eerie sensation – I had an uncanny feeling it had been looking back at me for quite a while before I recognized it for it was.  Weak as the secondary is in comparison to the primary (about four hundred times fainter), once your eyes have adjusted for the glare and that secondarial speck of light has registered in your brain, you’ll find it’s very sharp and very distinct.  There isn’t any guessing about whether you’re actually seeing something or whether your imagination is playing tricks, which has been my experience with Sirius.

Rigel  is frequently described as something of a training tool for visually sighting the secondary spinning around Sirius, which is accurate to a point — that point being the difference in the distinctiveness of the secondaries.  I’ve caught sight of the Sirius secondary (aka the “Pup”) several times this year, but each time I’ve been less than overwhelmed by the experience.  While sighting the Pup is a greater challenge, it’s a faint ghostly presence that plays tricks with your mind, whereas there isn’t any guessing about whether you’ve spotted Rigel’s secondary.  And there’s a good reason for the difference.  As of early 2014, Sirius “A” and “B” are separated by 10.1”, very similar to the separation of Rigel “A” and “BC”, but Sirius “B” is 9925 times fainter than Sirius “A”.

Now after you’ve wrestled Rigel into submission and are basking in the radiant blue-white primarial glory of your secondarial success, stop for a second and think about that secondary’s secondary  —  the “C” part of Rigel BC, the star that’s only 0.10” distant from “B”.  Think about the challenge of detecting an elongation in that BC pairing.

Impossible, right?  Not quite.

Not only was it done with a six inch refractor, but it was done twice.  In fact, the first hint that Rigel “B” was duplicitous was actually provided by a six inch f/15 Clark achromatic refractor.  And, not surprisingly, at the eyepiece of that instrument – both times – was none other than Sherburne Wesley Burnham.  He did get a slight break – apparently the separation of the BC pair was close to 0.20”, as opposed to 0.10”.

Click to enlarge.

Click to enlarge.

The full account of S.W.’s adventure is shown at the right, and it’s well worth reading.  As it turns out, the first solid confirmation came from the 36 inch Clark refractor at Lick Observatory (also an achromat), and even then it took several attempts, first because of poor seeing conditions, and second because the separation between the two stars apparently had narrowed to below 0.10”.  Note the magnifications required with that instrument, though – 2000x to 3000x!

So before you strenuously pat yourself on the back after your first success with Rigel “B”, pause to consider the level of visual skill wielded by Burnham.  Not that any of us will ever quite get there – and the pat on your back is well deserved, by the way – BUT, remember, there’s always another level of achievement to be had beyond your most recent one.

Like Tau (τ) Orionis.

Tau (τ) Orionis  (20 Ori)         HIP: 24674   SAO: 131952
RA: 05h 17.6m   Dec: -06° 51’
Identifier              Magnitudes       Separation      Position Angle     WDS
H V 25        AB:    3.60, 11.00             33.30”                   251°             2011
HJ 2259    AD:    3.60, 10.90             35.40”                     60°              2011
Bu 188      BC:  11.00, 10.90               3.80”                     51°              2011
Distance: 555 Light Years
Spectral Classifications: “A”is B5, “D” is G6

Located a mere one and a half degrees north and slightly east of Rigel, Tau (τ) Orionis is an experience of a different sort.  There’s less glare since the primary is over three magnitudes fainter than Rigel “A”, but there’s a wider magnitude separation between the primary and its two companions, “B” and “D”, which makes them about a thousand times fainter than the primary – and one of the stars is actually fainter than that.  The fact that they’re both almost four times further from the primary than the distance separating Rigel’s A-BC pair doesn’t seem to count for much in the advantage column.

On first glance, you’ll more than likely find a single star staring back at you.  But with persistent use of averted vision – and five or six inches of aperture – you’ll find this image gradually emerging:

 “D” will emerge first from the glare surrounding the primary, and with persistence and at least average seeing conditions, “B” will gradually make itself known on the primary’s opposite side.  The primary, by the way, is brightly white.  (East and west reversed, click on the sketch to bring “B” and “D” to life).

“D” will emerge first from the glare surrounding the northeast side of the primary, and with persistence and at least average seeing conditions, “B” will gradually make itself known on the primary’s opposite side. The primary, by the way, is brightly white. (East and west reversed, click on the sketch to bring “B” and “D” to life).

Even though Tau (τ)’s primary isn’t as bright as Rigel, it still throws off enough glare to make things difficult.  And despite the WDS data which shows only a tenth of a magnitude of difference between the two eleventh magnitude companions, “D” is noticeably easier to see than “B”.  I had to scoop up every photon I could find with my averted vision in order to keep “B” in view, and even then it had a nasty tendency to fade from sight.  Simbad shows “B” with a visual magnitude of 11.8 (compared to 11.0 for the WDS) and “D” at a visual magnitude of 10.9, which is more in line with what I saw.

And right there is the crux of a problem.

The WDS data shows the significantly harder to detect “B” companion as a William Herschel discovery, H V 25 – and it’s a little difficult to believe he would have cataloged that companion and ignored “D”, which is so much easier to see.  In fact, as it turns out, Herschel didn’t provide a position angle for the star he included in his 1780 description (twelfth title down):

Wm. Herschel on H V 25

In his 1906 Catalog, Burnham states the senior Herschel failed to see “D”, but that it was added by the junior Herschel, John, in his Fifth Catalogue . . . . . . .

Burnham on Tau Orionis

. . . . . . . so I dug out the entry in John Herschel’s Fifth Catalogue and found this . . . . . .

John Herschel on Tau.
. . . . . . . which isn’t a lot of help.   Even though that entry shows position angles and separations, none of them are reasonably close to either Burnham’s data or the current WDS data.  However it does show that John Herschel noticed the difference in magnitudes of the “B” and “D” companions, which again leads me to conclude the star William Herschel recorded in his observation is the brighter “D” on the east side of the primary, not the fainter “B”.

Enough of that.  There’s still a visual challenge we haven’t wrestled with yet, and that’s the BC pair.

Given how difficult it was for me to keep “B” in view, I wasn’t sure it was even worth the effort to attempt to pry it apart.  The seeing was poor – about a notch below average (a II on this chart) – and the glare from the primary was being magnified by a surplus of moisture in the air.  But with nothing to lose, I armed my six inch refractor with a 6mm AT Plössl  (253x) and finally detected a hint of an elongation – a very weak hint.  At that magnification, the primary was primarily a blob of unfocused light, and the image wasn’t helped in the least by a strong east wind that was making the visual observing life difficult.

So the challenge is still there to get a definite split from the BC pair, and it should be within reach of both a five and a six inch refractor given an atmosphere that’s willing to cooperate.  As temptation for what’s possible, here’s a photograph of the Tau (τ) system taken by Steve Smith which provides a good view of a distinctly separated BC pair.  It also shows quite clearly the disparity in magnitudes between “B” and “D”.

Photo used with the permission of Steve Smith, click to enlarge.

Photo used with the permission of Steve Smith, click to improve the view.

Credit for the inspiration to visit Tau (τ) goes to both Chris Thuemen, who sent me a photo of it hinting at an elongation in the BC pair — which in turn launched me on a long search for a good photograph of Tau BC — and to Steve Smith who went out and took the photo I was looking for.

As soon as the weather improves, the moon disappears, and the seeing cooperates, I’ll be back to tussle with Tau (τ) again.  If I can’t match the visual acuity of eagle-eyed S.W. Burnham, the least I can do is subdue Tau (τ) in order to get a taste of the satisfaction he must have felt when the elongation he saw in Rigel BC was confirmed.

Next stop – somewhere in the vicinity of Tau (τ).  Until then, clear skies!  😎


Update: Giving into the “temptation for what’s possible.”

In the winter you’ve got to take the clear nights when they’re there, and when I finally found an unexpected one looming on the last day of February, Tau (τ) Orionis was still on my mind.  The seeing didn’t look any too great.  Sirius was doing what it’s done every clear night this winter, which is a polished imitation of an airport beacon flashing in red, green, and white — and Rigel was hard at work imitating it.  But I had already brought my 9.25 inch SCT out before dusk so it could cool down, and knowing Tau’s BC pair was going to demand plenty of aperture, I decided to see what would happen.   In the back of my mind, I was hoping a low magnification view would be enough to drive a wedge between “B” and “C”.

The seeing was pretty much what I expected — erratic, nervous, and even a bit blurred.   But with a 26mm Celestron Plössl at 94x, I could detect an elongated smudge of light radiating from the BC side of Tau (τ).  What really surprised me, though, was how difficult BC and D were to see in the primary’s glare, even with 9.25 inches of aperture.  But despite their battle with the glare, they were persistent enough to stay in view without resorting to averted vision.

Since I was having mixed success at 94x, I tried a 20mm AT Plössl (123x), which resulted in the same blurred elongation, but at least it wasn’t any worse.  Then I reached for an 18mm Radian, but somehow I managed to pick up the 14mm (175x).  “B” and “C” were now clearly separated — certainly not quite what you would call pleasingly precise points of white light — but once I realized I was seeing them at 175x, I decided to make one more determined seeing-defying leap to a 10mm Radian.  Using 245x on a night like this would normally be a sign of sitting in one place under a cold dark sky for too many hours, but since the night was still young, I convinced myself that since I didn’t expect anything more than unfocused blobs of bouncing light, I was still on the safe side of insanity.

Anyway, if you don’t try, you don’t know . . . . . . .

. . . . . . . and darned if those two blobs of light weren’t impossible to unscramble.

Esthetically pleasing they sure weren’t, but they were clearly separate:

This is what I saw, minus the blog effect, so don't be misled into thinking the view was quite this comfortable.   What it does do, though, is convey how much BC and "D" are subdued by the white primary's intense glare.   (East & west reversed to match the SCT view, click on the sketch to relive the experience).

This is what I saw, minus the blob effect, so don’t let the sketch mislead you into thinking the view was quite as comfortable as it appears. What it does do, though, is convey how much BC and D are subdued by the white primary’s intense glare. (East & west reversed to match the SCT view, click on the sketch to re-live the experience).

I had just a hint of what the inimitable eagle-eyed S.W. Burnham must have felt when he saw the elongation in Rigel’s BC pairing.  Subtle though it was, I stared at that dance of white light until my eyes began to cross.

And savored every single bouncing photonic moment of it.


4 Responses

  1. Hi John, like you I have been looking at Rigel and Sirius this
    past month and like you I found Rigel B a lovely sight in the
    80mm refractor at 120x as you say there is something very
    pleasing about the difference in magnitude plus the separation
    of this pair . Now Sirius is another matter I have attacked it a
    number of times with the 150mm reflector and the best I can
    say is maybe I saw :something; but not very exciting at all.
    I did have a look at Tau Orion once but it was a night of a
    almost full moon and lots of moisture in the air so even with
    averted vision I could just about see the D star, Must try it
    again when the moon leaves the sky. I enjoyed the read
    keep im comeing.


    • HI Pat,

      Yes, the difference between Rigel and Sirius is surprising — I guess you could even call it serious (poor pun). But I really was disappointed in the view when I caught sight of the Pup — not at all the same experience as Rigel. Sirius is widening over the next few years, so maybe the view will improve soon.

      Hope your weather is better than here!


  2. Just added a new sketch of Tau Orionis to the bottom of the post on Rigel and Tau because . . . . . .

    I split that Burnhamesque BC pair!

    John 😀

  3. John, great job on splitting the BC pair, we have just had 3 clear
    nights in a row and guess what I have the flu.


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