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Alnitak (Zeta Orionis) – A Flaming Delight – and its Neighbors, Sigma Orionis and Σ761

Alnitak, with the “B” and “C” components lost in the glare. Click on the photo for a larger version. (STScI photo)

There is something about that name – Alnitak, from the Arabic Al Nitak – that evokes an image of restrained elegance.  In my eyes that elegance is seen in the bluish-white glow it radiates into the eyepiece of a telescope.  In Arabic, it means “the girdle,” which I believe is a reference to the belt of Orion, not the confining modern day garment that leaps to mind.  Despite being 817 light years away, it’s very close to being a first magnitude star, which is no surprise given that it’s luminosity in the visible spectrum is 10,000 times greater than our sun.  As the attached photo shows, it’s more than capable of lighting up the clouds of interstellar gas in the area surrounding it.  And, as James Kaler discusses here, it has the distinction of being the brightest class O star in the sky.

Plagued by the expected rainy November which is normal for the north Oregon coast, I was thrilled to have a clear night about a week ago — even though the moon was full.  In this part of the world you take what the weather gods offer.  However —  the full moon of winter is not the full moon of summer.  In those warmer months it sits much lower in the sky, radiating a soft yellow light that actually can make an evening of double-star observations very pleasant if you keep it at your back.  But the winter moon is like turning on a 1000 watt flood light in a small closet lined with aluminum foil.  Because it makes a higher arc through the heavens, the entire sky is flooded with its brilliant white light.  Add in the atmospheric scatter caused by the dampness which is typical of this time of the year, and it becomes downright obnoxious.

So there I was, under a clear bright sky, wondering what to look at.  My plan was to stay as far away from the moon as possible, so I toured some of the doubles in Cassiopeia, Cepheus, and Draco that Greg and I have written up over the past few months.  But even though I was 180 degrees away from the moon, they were darn hard to pick out of the bright sky.  Kappa (κ) Cephei kept doing a disappearing dance while I tried to sight it through the Telrad, and I managed to stumble across Epsilon (ε) Draconis by pure luck.  But the seeing was reasonably steady, and the moderate magnification I was using provided some rewarding sights.

After a couple of hours of that, I had developed a stellar appetite for something more thrilling, something with a bit of a challenge, something bright and faint at the same time, something with a bit of gleaming interstellar gas even.  And then I saw it — rising over the roof of my house  —-  Orion.

Betelgeuse —  Rigel — those three gleaming belt stars — the cloud of glowing gas just below them.  Irresistible.

Stellarium screen shot with a few labels added. Click once for a larger view and then a second time for a still larger view.

I don’t care how many times I look at the huge, sprawling outline of this constellation, I can never get enough of it, full moon or not.  There just isn’t anything in the sky that grabs my attention like that configuration of gleaming stars.  If Orion didn’t exist, it would have to be invented.

So of course I took a look at the Trapezium — thought I could just get a hint of “E” —  worked up through the open cluster north of it — NGC 1981 — and bumped into Alnitak as I continued north to the three belt stars.

Alnitak  (Zeta [ζ] Orionis)  (Σ 774)  (AC is H IV 21)        HIP: 26727    SAO: 132444
RA:  05h 40.7m   Dec:  -01° 57′
Magnitudes         A: 1.9    B: 3.7    C: 9.6
Separation          AB: 2.2″      BC: 58.0″
Position Angle    AB: 166°   (WDS 2012)  BC:  10°  (WDS 2006)
Distance: 818 Light Years
Spectral Classification: O9.5
Status:  AB is physical, orbital chart and data can be seen here.

Rich, one of the people who have commented on a few of these posts, mentioned Alnitak in one of his replies.  I had forgotten it was a double until I read his comments, so I took a close look at it on that full moon night, and had no problem seeing what turned out to be the “C” component situated about a full arcminute to the north of Alnitak.  I’ve seen it numerous times before – despite the glare, it’s far enough away to not really be that much of a challenge.   The next day, I was checking the Haas book and found the much closer “B” component, which I hadn’t noticed that night.  I went back through some sketches I did a few years ago, and sure enough, there it was — got it on January 16th of 2008 with a 7mm Nagler (126x) in a four inch refractor, as shown in the accompanying reproduced sketch.  My notes show “A” and “B” were touching, so I didn’t actually get a clean split — but I saw it, which was what interested me the most.

When I look at Alnitak, the color I see is bluish-white — period.  But apparently a lot of other people don’t — or haven’t.  Haas decribes the two brightest components as “yellow and silvery yellow.”   Burnham wrote that they have been called “topaz yellow and light purple,” “yellow and blue,” and finally — at last someone who almost agrees with me — “brilliant white.”  I’ll just leave you to work it out for yourself. 😉

To digress a bit, if you’re not familiar with this area of Orion, you’re in for a treat if you happen to be sitting behind a telescope on a dark, moonless night — certainly not this one!  If you move Alnitak to the west edge of your eyepiece, and then push it beyond the field of view, you should be able to glimpse the Flame Nebula (NGC 2024), a dark glowing cloud of gas that is pretty much split down the middle.  When the moon is turned off, my skies are about sixth magnitude, and rare is the dark night when I can’t see that dark, mysterious cloud.  If you live in a light polluted area, I suspect it’s probably rather elusive unless you use a filter of some kind — either an OIII or a UHC is recommended.

Sigma (σ) Orionis (Σ 762)              HIP: 26549    SAO: 132406
RA: 05h 38.7m   Dec: -02° 36′
Magnitudes       AB: 3.8    C: 8.8    D:  6.6     E: 6.3
Separation         AB-C: 11.4″          AB-D: 12.8″        AB-E: 41.2″
PA     AB-C:  238°  (WDS 2008)    AB-D:   84°  (WDS 2011)    AB-E:   62°  (WDS 2011)
Distance: 1148 LY
Spectral Classification: O9.5 (A)  B2 (B)

Σ 761           HIP: Not assigned in Simbad    SAO: 132401
RA: 05h 38.6m   Dec: -02° 33′
Magnitudes:         A: 7.9    B: 8.4    C: 8.6
Separation –        AB:  67.8″     AC: 71.8″   BC:  8.5″
Position Angle –  AB: 203°   AC: 209°    BC: 269°  (All WDS 2011 data)
Distance:  ?????
Spectral Classification: B5

Now, if you move one degree south of Alnitak to the area of the elusive Horsehead Nebula (B33), and then west another half degree, you’ll come to the quintuple star Sigma (σ) Orionis, and in the same field of view you’ll find the triple system Σ 761.  Sigma (σ) dominates the view, but the close “BC” pairing in Σ 761 will also catch your eye.

When I’m traveling through the territory of the Horsehead or the Orion Nebula, I always pull over for a few minutes and take a look at these two systems.  This night was no exception.  Despite the nearby moon, I had no problem seeing all of the components of these two multiple stars in my five inch refractor, except for the AB pairing of Sigma, which is a mere .25″ apart – for that you need a much larger scope than 99.9% of us have.

The seeing was steady enough to support a 12.5mm Ortho (94x), yielding a very well-resolved view of Sigma’s (σ)  four components, similar to what is shown in the accompanying sketch I made a few years ago.  As I looked at them, they reminded me of a small open cluster, and according to Kaler, they actually are part of a cluster that lies at a rough distance of 1150 light years.  The higher magnification of the Ortho also gave me a much wider view of the close “B” and “C” components of Σ 761 than I am accustomed to.  Normally if I’m in this area, it’s because I’m trying to pry the mysterious dark Horsehead Nebula out of the never-quite-dark-enough background sky, which calls for quite a bit less magnification.   A good test of your visual acuity and the optics of your scope is to see how low a magnification you can use and still split this “BC” pair.

If you get a chance to look at these two systems under moonless skies and away from city lights, you’ll find the brighter components of Sigma (σ) are like white jewels gleaming on black velvet, and the fainter components of both it and Σ 761 are sharp pin holes of light shining through from the other side.

As I normally do when I’ve wrapped up for the night — usually morning, actually — I grab my four-legged companion’s leash and let him take me for a walk.  So off we went at 2AM — he was sniffing the ground, and I was looking skyward sniffing for moon dust.  By now the moon was high overhead and it was almost as bright as daylight.  Even though I’ve described the winter full moon rather harshly here, I’m not so hopelessly hypnotized by starlight that I can’t recognize the beauty of a night like this.  There are times when you have to remind yourself to get away from the magnified view for a while and enjoy the night by taking a look around —– at the soft moonlit shadows, the hazy indistinct outline of the hills in the distance, the stands of trees that merge into a single dark silhouette projecting skyward, and the other-worldly beauty of a moonlit sky punctuated by a few flickering bright stars.

This was a night that was made for walking, not trying to ferret out faint stars lying impossibly close together in a bright sky.

And that’s what we did for about thirty minutes.


6 Responses

  1. Hi John ,thanks for your detailed reports on parts of everyone`s favourite-Orion. I will look at sigma Ori. more carefully next time. Re. Alnitak I have no success yet with AB. On Porrima I have got some recent data which puts its sep. at 3.5″ approx. in 1989 which would have been the last time I saw it as two round stars. Have been trying to copy your excercises in Cepheus which is was a constellation known to me only by name and and by ref. to Cassiopeia . Unfortunately your description of the recent moonpolluted sky is spot on. Also we now have rain-AARgh ! By the way, is the Stellarium view of Orion which has East on the compass to the right as above, actually how you see it? From here looking at Orion ,East is left; i.e. Alnitak is the most eastern of the belt stars. It doesn`t take much to disorient me and that happens often near the zenith even with an alt-az mount. Hope your weather settles. regards, rich.

    • Rich – I owe you one ! Thanks for catching that error – I’m so accustomed to reversing east and west for the eyepiece views in refractors that I just did it out of habit on that one. Alnitak is the easternmost of the three belt stars on this side of the Atlantic, too! At least it is now that we’ve got that corrected.

      Still can’t believe I did that.

      By the way, the correct Bayer designation is Zeta, not Theta – I had that mixed up, too, but it’s corrected now.

      Anyway, that post is for you! Thanks for mentioning it a while back. That was what prompted me to write it up.

      Good luck in Cepheus and Cassiopeia – there are some very rewarding sights in those constellations, provided your weather cooperates.

  2. Thanks again for your post. Why i wanted to get east west confirmed John, was mainly due to your sketch of Alnitak in Jan 2008;particularlarly the PA of “B”. Two weeks ago i saw “something” . It was more like an extra thick bit of the diff. ring around A or part of a diffraction ring . No way anywhere near to a result but enough for me to check the angle which was close with Orion being on its side then so that the line to the pole was nearly horizontal. If i take your PA as “4 o`clock”, mine would be approx. 7.30. So with Orion in mid Jan appearing already on his feet ,i.e. nearly 90 degrees further round you might understand my interest. BTW i only recently got to understand how PA is measured from Wikipedia,previously it was only the separation i considered. The learning curve inches upwards. Ive been able to get a an hour or so in Cepheus, trying to follow your detailed tour. So far i can say that delta is a wonderful colour contrast at 20X in a 2″ eyepiece, and getting easily lost in this constellation is pleasure. Hope to get the “C ” star in sigma oronis tonight-bit of a puzzle why not before now. Thanks again for the post, rich.

  3. Got a decent look – my first, really, at Alnitak this morning – very nice. I was using the 8-inch Celestron EdgeHD and seeing conditions were poor, but not so poor that I could not get an easy split on Rigel at 111X.

    I moved up to Alnitak and the first thing I did was look for the Flame using the 30mm Tak (66X) and as soon as Alnitak drifted off stage the flame jumped out at me with the obvious darklane separting the two major portions. I explored this region extensively a few years ago with a deep sky video set up that I was fond of back then. I can’t recall if I have ever seen it visually – I don’t think I have. It’s an area I’ll have to explore more another night. (I did not attempt the Horsehead.)

    The second thing that hit me was the Zeta “C” star – it was obvious, though widely separated, and the PA confirmed I was looking at the right star. So i doubled the power – no joy – then went to 200X with a 10mm Tak and using the technique of keeping my eye well off – about two inches away – from the eyepiece was able to get a nice clean split.

    While there was a slither of dark sky between primary and secondary, they were dancing, the primary repeatedly grabbing at it’s duller companion. I saw the primary as white, John, the secondary as a variation of steel blue where the typical shine is missing, but the color is there.

    This – and my earlier experience with Rigel – confirmed the concept that at least under marginal conditions, backing off the eyepiece significantly can result in a cleaner star images. I still haven’t a clue why and from recent experiments it doesn’t seem to work for me when seeing is average or better. But it sure seemed to help with both these stars in marginal conditions this morning.

  4. Very interesting! To coin an old phrase. I use Jerry Perez-“Belt of Venus” website to confirm/see what a photo view of a double looks like. His drawings just look like the image you want to see. I noted particularly that :-
    For Zeta and Sigma Ori. on his site ,only “C”; and “D” and “E” respectively are pictured. Despite his using a 6″ F8 reflector @200X-his normal rig . At times i would love to turn to an 8″ SCT for the extra light and colours that brings and to confirm stuff. What you described in the the dancing arena is just how it looks. All too often. Will try your “backing off” technique -soon, i hope. regards, rich

  5. Hi John , a belated happy New year. Finally managed to split Alnitak. On the night of 1.1. 2011-great start to the year. Just went straight for it at 280x. A steady white disc completely surrounded by one solid diff. ring with a”hard” light gray ball-bearing smack in the ring at approx 160 degrees. Just wonderful. So definite it seemed to say I`ve always been here-what`s been your problem? Yeah well , lousy eyesight and boiling air are two big problems. Anyway i now have a visual image to remember. Both you and John mention in your posts that the atmosphere always dictates what you are able to see and that relatively inexpensive equipment can provide great views when skies are steady. From recent experience i can totally agree. The image even of a single star at high power in steady air is something literally to wonder at: a close double becomes modern art for me-light ,physics, celestial mechanics. Anyway i need something to remember as total cloud cover is going to alternate with T-storms over next 5 days, or more importantly for me -nights. Hope your skies are much better,and thanks again for the great detailed info. you make freely available. regards,rich.

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