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A Tale of Two Secondaries: Part One — Bu 193

It’s not easy being a secondary on a short leash.  You live out your allotted hydrogen and helium burning years under the glare of your primarial parent, suffocating in its infernal glow, while yearning to shine separately in some remote dusty sector of the galaxy where you can set up house and raise a few planets of your own.  The best you can hope for is a passing stellar interloper with enough gravitational muscle to cast a few well-aimed gravity waves your way, giving you a chance to grab hold and surf your way to freedom.

And then there’s the impression you leave on that strange breed of earthlings known as Star Splitters.  Heck, most of the time they don’t even know you’re there, and when they do finally catch a glimpse of you, it’s only to record some obscure data that no one looks at for years — or worse, they confuse you with another star.  And even if you’ve managed to eke out a colorful existence, they only compare you to the more brilliant color of your primary nemesis.

To borrow a phrase from Sir Rodney of Dangerfield:  secondaries get no respect.

Maybe not — but they do have a few things to teach us.

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

Come along with me and I’ll take you to the far eastern fringes of Orion, where we’ll immerse ourselves in the mysterious interstellar expanse between Betelgeuse and the Rosette Nebula, and dig out an obscure triple star dubbed Bu 193 by that dedicated devotee of the difficult, S.W. Burnham.

 Here’s the wide view, showing the location of Bu 193 in relation to Betelgeuse and Epsilon (ε) Monocerotis.  Use Betelgeuse as a reference point to work your way to 4.40 magnitude Epsilon (ε) Mon, because that star will be our jumping off point to reach Bu 193. (Stellarium screen image with labels added, click to enlarge).

Here’s the wide view, showing the location of Bu 193 in relation to Betelgeuse and Epsilon (ε) Monocerotis. Use Betelgeuse as a reference point to work your way to 4.40 magnitude Epsilon (ε) Mon, because that star will serve as our jumping off point to reach Bu 193. (Stellarium screen image with labels added, click to enlarge).

And here’s the star hopper’s view.  From Epsilon (ε) Mon, move two degrees northwest to 5.70 magnitude HIP 29860 and 7.15 magnitude HIP 29780.  If you look south one degree from HIP 29780, you’ll see an arc consisting of four seventh magnitude stars (HIP 29618, HIP 29739, Bu 193, and HIP 29594).  Drop straight south from HIP 29780 to 6.60 magnitude HIP 29739, and you’ll find yourself less than half a degree north of Bu 193.  Slip south, center Bu 193, and pull up a chair.  (Stellarium screen image with labels added, click for a larger view).

And here’s the star hopper’s view. From Epsilon (ε) Mon, move two degrees northwest to 5.70 magnitude HIP 29860 and 7.15 magnitude HIP 29780. If you look south one degree from HIP 29780, you’ll see an arc consisting of four seventh magnitude stars (HIP 29618, HIP 29739, Bu 193, and HIP 29594). Drop straight south from HIP 29780 to 6.60 magnitude HIP 29739, and you’ll find yourself less than half a degree north of Bu 193. Slip south, center Bu 193, and pull up a chair. (Stellarium screen image with labels added, click for a larger view).

Bu 193                HIP: 29713   SAO: 113645
RA: 06h 15.5m    Dec: +03° 57’
Magnitudes   AB: 6.99, 12.38     AC: 6.99, 10.02
Separation    AB: 19.5”                AC: 57.8”
Position Angles   AB: 97°  (WDS 2000)    AC: 232°  (WDS 2011)
Distance: 1004 Light Years
Spectral Classifications:  A is B5, B is G3, C is A8

Now our primary focus here is on the secondary, the “B” of the AB pair, which is going to leave a faint, but lasting, impression on us.

Click to enlarge the view.

Click to enlarge the view.

In most cases, a distance of 19.5” from the primary would be enough to allow a secondary to be easily seen.  In this case, though, there’s 5.4 magnitudes of difference between the primary-secondary pair, in addition to which the secondary flickers at an almost futile magnitude of 12.38 — so sighting “B” is not for the faint of heart.  But since S.W. Burnham discovered “B” with his six inch f/15 Clark refractor (see the inset at the right — the link to the book is here), I figured I could do it, too, with my six inch f/10 Jaeger’s-lensed refractor.  So I gritted my teeth and went to work, well aware that I was in for a trying test of visual acuity.   And as we’ll soon see, there’s a lesson to be learned here from Mr. Burnham’s eagle-eyed artistry, and we’ll find it’s worth its weight in gold when we get further along to Tau (τ) Canis Majoris.

Let’s not ignore 10.02 magnitude “C”, though.  If you look closely at the data above, it would seem “C” should suffer from a relative disadvantage also, since it’s three magnitudes fainter than the primary.  But because it’s almost a full arc minute away, it manages to shine rather solidly on its own.  Or at least it does with four inches of aperture or more — I suspect it could be difficult with less.

Let’s take a first look at our target through an 84x lens:

 In a 40’ field, Bu 193 is surrounded by an attractive scattering of field stars, but in contrast, the area immediately around it is rather barren.  (East & west reversed to match the refractor view, click for a larger view).

In a 40’ field, Bu 193 is surrounded by an attractive scattering of field stars, but in contrast, the area immediately around it is rather barren. (East & west reversed to match the refractor view, click on the sketch for a much better view).

 Photo used with permission of Chris Theumen, click for a better view!

Photo of Bu 193 taken by Chris Thuemen and used with his courteous permission.  Click to enlarge the image in order to see “B”!

“C” is the first star immediately to the southwest of the primary, but “B” is nowhere to be seen in the sketch.  I looked long and hard, and finally managed a glimpse of it flickering at the edge of an averted gaze, but it kept drowning in the primary’s intense glare.   Judging from the visual distance separating “C” from the primary, I knew “B” was going to be uncomfortably close in my 84x view, but I was still surprised at how little space there was between that flickering ghost and its parent.  To give you some idea of how difficult it was to see the secondary, look very closely at Chris Thuemen’s photo of Bu 193 above (you’ll have to enlarge it).  You should be able to catch the weak glimmer of the secondary immediately to the right of the primary, which is slightly less difficult than my experience of it at 84x.

So let’s apply some magnification now and narrow the field of view down to about sixteen minutes of arc:

Insert Steve’s sketch:  And now “B” comes out of hiding!  It’s amazing what 9.25 inches of aperture will do (not to mention 261x).  Thanks again to Steve McGee for permission to use his sketch.  (East & west reversed, click on the sketch for a larger view).

And now “B” comes out of hiding!  It’s amazing what 9.25 inches of aperture will do (not to mention 261x). Thanks again to Steve McGee for permission to use his sketch. (East & west reversed, click on the sketch for a larger view).

And next, let’s zero in on a ten arcminute view and force the stars further apart:

 We’ve gone from 9.25 inches of aperture down to six, so “B” is a bit fainter now, but the view is tighter in this smaller field.  (East & west reversed once more, click for a larger view).

We’ve gone from 9.25 inches of aperture down to six, so “B” is a bit fainter now, but the view is tighter in this smaller field. (East & west reversed once more, click for a larger view).

We’ve eliminated most of the distraction from the field stars now and enlarged the view, which provides us with a rewarding contrast of the spacing between Bu 193’s three components (remember how close they were in Chris’s photo).  And we also have an opportunity here to gauge the visual impact of the primary’s seventh magnitude glare on the 12.38 magnitude secondary.  If you look to the left of the primary in the sketch above, you’ll see there’s another 12.4 magnitude star in this field (marked by the arrow at the left).   Although that star wasn’t visible in the first sketch above, you not only can see it here under the increased magnification, you can also see it stands out more clearly than the secondary.

And on the other side of the field, at the extreme edge (just slightly south of east), you’ll find two faint stars.  The fainter of the two (at the extreme right) is a 13.1 magnitude star which I found was slightly easier to see than the 12.38 magnitude secondary.  So in this particular case, as a rough approximation, the glare of the primary, when combined with the intervening 19.5 arcseconds of separation, reduces the magnitude of the secondary by about a whole magnitude.

Keep those comparisons in mind now, because they’re going to be very useful as we move on to Tau (τ) Canis Majoris, aka h 3948, which you can find  ➡  HERE.

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