Background Info on the Theta Orionis Complex
SCAC Star Party Observation: 9/15/01
Aperture Springs the Trap!
Mak Attack!
Little Uns Too!
Jeff Takes All the "Fun" Out of It!

Background Info on the Theta Orionis Complex

This is one of the most intriquing images of the Hubble space telescope, right at the core of M42 sits this magnificent group of stars: The Trapezium. The remarkable "comets" are proto-planetary discs that are evaporating.

The Asterism:

Table of Primary Theta Orionis Complex Components

Star A, also known as V1016, from the General Catalog of Variable Stars, and is an eclipsing binary star system, with a companion star that blocks out some of the light from A every 65.43233 days. A normally shines at magnitude 6.72, but fades to magnitude 7.65 during eclipse, which lasts 20 hours. Mid eclipse, when it is at minimum, lasts about 2.5 hours. Mid Eclipse dates (mag 7.65) are:

Table of Star A Eclipses Through 2002

Year	Date	Time
2001	21 Sept	18:00UT
2001	26 Nov	04:00UT
2002	1 Jan	15:00UT
2002	6 Apr	01:00UT
2002	10 Jun	12:00UT
2002	14 Aug	22:00UT

Star B, also known as BM Orion, is also an eclipsing binary system, with a period of 6.470525 days, with a magnitude range of 7.90 to 8.65.

The Trapezium was first drawn as a triple system ( A, B, C) by Hodierna before 1654, and Christen Huygens described it in 1656. Star D was discovered (independent) by both Abbe Jean Picard and Huygens in 1684. William Struve discovered star E in 1826 with a 9.5 inch refractor, and F was only discovered in 1830 by John Herschel. Star G was discovered in 1888 by Alvan Clark while testing a 36 inch refractor that he made for Lick Observatory, and E.E. Barnard discovered star H later in the same year using that same refractor.

~Cor

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SCAC Star Party Observation: 9/15/01

Hi All,

While at this last Saturday night SCAC star party, stayed around long enough to visit with the Hunter as he strode upward from the east. Time was maybe 3:00 am Sunday morning. Orion's belt and sword probably 25-30 degrees above the southeastern horizon at the time. Had three studies in mind. One was to determine if NGC2024 (The Flame Nebula) would be well presented. And of course the other two centered on M41 - The Great Nebula itself, and THeta Orionis in its midst...

Sky conditions were impressive. ZULM must have been 6.0. Stability very good to. Earlier that evening Dan Grabek and I had checked for resolution on Zeta Bootis (well below the middle third on the far side of the sky). Dan's FS102 showed this matched pair distinctly elongate while Argo gave an hourglass both at 360X. So seeing stability in the 8/10 region overhead and 6/10 down low where Orion was situated.

I had earlier viewed the Plieades. No filter needed. The Merope Nebula direct and obvious at 50x.

So I figured I had a good chance to see The Flame Nebula and was not dissapointed. Not wuite as well defined as I have seen it - still its tripartite structure was discernable without imagination...

What can I say about M41 - simply the finest nebula in the heavens, tenuous, rifting, folds of future starstuff.

But could not resolve any other than the four main components in the Trapezium.

Wandered over to visit with Jack and his XT-10 Dob. Jack is an engineer at Orion and has made huge progress with his "seconded" 10 inch Dob. The E member of the Trap was obvious... No sign of F however.

Returning to Argo, I remembered where the E member was seen in the 10 inch. Allowing for all the usual field reversals was able to catch the little darling momentarily on the limits of perception.

Just don't think it should have been that tough on such a good night.

No question, the brightness of the nebular field makes Trap E and F very difficult in an obstructed six inch instrument such as Argo.

Would have been intriguing to have made a run on the Theta E with Dan's FS102...

Clear and Steady,

jeff

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Aperture Springs the Trap!

OK, you two...

I ran across this thread while puttering around waiting for it to get dark tonight.

"I looked at the trap, Ray..."

Not more than 14 hours ago... and I'm here to tell you that with the 12.5 and a 17mm eyepiece, the E and F stars make a clean, perfect breakaway. There is no touching... no airy disc... no elongations... Just perfect, simple pinpoints of light that make the Trapezium appear to look almost pentagon shaped.

For what it's worth, the sky was an eight out of ten this morning... good enough to reveal the "Horsehead Nebula" region.

Now, I've never seen the statistics for the Trapezium stars, and after having checked out the magnitudes for the G and H... it's no wonder I've never been able to see more than six!

Cheers...

~the astronomer

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Now there you go again T, Proving that aperture rules! But only when you got an "optically correct" Big 'UN...

Horeshead too! Mighty nice sky, mighty nice.

jeff

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Hi, Guys!

Needless to say, I about fell off the chair when I looked at this thread again!

The picture that Cor inserted at the beginning is sooooo close to "true to eyepiece" that I was amazed!

The one thing the picture did not reveal is the odd scalloped nebula that accompanies the field. Oddly enough, this shows at the observatory... but yet I've never seen it in the dob. (But then, I've never put the power of high magnification on it either...)

The E star is amazingly bright... and red!! This is one easily pulled off at home. The F star reveals itself naturally in the observatory scope as blue... (and here's where it gets really weird, guys...) The "sixth" member I see at home in the dob... is actually H!?! (I just assumed since I could pick it off that it was F! And I'm not joking around about this... ok? That is why I've always maintained that it resembles a pentagon.)

Hey, the dob is good... but magnitude 16 good? Har, har!

Please, for those of you who may have detailed maps of this area... Look in the general region of the H star, and tell me that there is one close enough that I am mistaken!

Cuz' there's just NO WAY the 12.5 could pick that up!

~T

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"T", The moment I read "red" and "blue" I knew you had hit the holy grail. H !!!!! Too cool!

Otto

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Hey, Ottoman!

Are you SURE there are no other stars in the vacinity of H?!

What can be seen through the dob is much tighter in appearance than the above drawing. More like a close series of doubles... The four major stars almost overwhelm the others.

If what I am indeed seeing in the dob is "H"... then it sits clear of the others, and is tiny and white. Is this possible?!

Questions... questions... (sorry! it's how i learn...)

~T

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Why not call up one of the hubble photos of the trapezium and compare its stars to the location of what we thought we saw?

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Hi Otto,

Cor started this whole thread with a Hubble image (at left):

Compare this with T's sketch (as remembered) from a view through the 31 inch Rupp (at right):

Cool huh!

jeff

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Looks like it's "H", to me.

Otto

Hi, Guys...

Well, the night this thread "lit up" again, I realized that I had to go to my own telescope for the answers to my own question... Can the "Trap" be resolved using a 12.5?

And the answer is yes.

The odd "scalloped" appearance to the surrounding nebula is also apparent in my backyard scope. Unusual things happen to nebulae in the dob... It robs them of surface brightness, yet reveals their structure. And it just so happens that pesky "H" star resides in a "dark" portion of space. That is why, at least to my reasoning, the dob is capable of picking it up.

The rest simply requires patience to observe. It is like disparate, or tight doubles... The moment will come when your eye happens to be averted just right... and the sky cooperates... and WOW! There they are...

I am also very surprised that no one comments on the dark nebula that accompanies the area! How can you look at such profusion, and NOT see that total void?!

Go. Look again. And tell me what you see...

~T

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Mak Attack!

Sunday, Nov. 4, 6 am EST, stability of 8 of 10. I was able to see both E and F, with averted vision but definitely there.

otto

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Hi Otto,

Congrats!

Can I suggest something? Next time you see these babies check the FOV around Alnitak for NGC2024 (the Flame Nebula). The one time I caught the E member, the Flame was an astounding sight, showing two dark bars giving the nebulosity a "tripartite" appearance...

Meanwhile of course, Alnitak is a triple and I suspect that it will give a marvelously clean view whenever the Trapezium becomes a Pentazium of Sextazium.

Carpe Noctem,

jeff

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Little Uns Too!

Hi All,

Had a look at this group last night through the 80mm Pup at 120x. On eye movement both E&F were suggested but not definite. (You really do have to know where to look and that's part of the problem - it's very easy to "imagine" stuff on the limits of perception.)

In terms of E, A's variability is a factor. Last night it was quite dim compared to the other three members and this gave the Pup a chance to catch E.

Seeing stability was probably 7+/10 at the time. Transparency in the 5.0+ range (The Flame was easily found near Alnitek.) Alniteks two companions could also be seen (closer A/B on the edge however.) "C" - the 10th magnitude companion - needed aversion at 120X. I suspect C needs to be held direct by any scope (at 120X plus) to have a chance of pinning down E or F.

NOTE TO SELF: Have a look at Alnitak and confirm a clean split of all three with the dim C direct when takling the Trapezium - of course, don't forget to stop and warm yourself in the Flame and fly amongst the undulating clouds of The Great Nebula along the way...

Carpe Noctem,

jeff

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Hi All,

There have been several occasions now where I have caught Trap E on the limits of perception. Last night for instance the seeing wavered between 7 and 8/10. (I viewed Jupiter through both the Pup and Argo. Jupiter was stable through the Pup but "tremulous" through Argo.) Under best stability I could just hold Trap E with the 80mm with moderate aversion. Then seeing would slightly degrade and it could only be caught on eye movement. Meanwhile transparency was in the 5.0+ range. It's clear that a three inch scope CAN reveal E under very good conditions. BUT 120x plus was required to give the necessary magnitudinal reach. (Through Argo E is straightforward at 70x under similar conditions.)

Now the quest is on to determine if E can be "Trapped" at 67mm!

Boy is this fun!

jeff

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Jeff Takes All the "Fun" Out of It!

Hi All

Last night through 150mm Argo I did a "thorough" characterization of the seeing needed to reveal this group as a "Sextazium". Of course, all this predicates that one use a scope of "modest" aperture. So a paralleling characterization through the 12.5 would be really helpful to get the "Full Bore" perspective. Setting this question aside, let me lay it on you...

Views of the Gas Giants smacked of "7/10" stability in the skies middle third. Main belts were visible on Jupiter along with barges in the NEB. Cassini less than "ribbonlike" around Saturn - but still very present. Edge focus was very good on both planets but low contrast details were "soft". Dimmer (6th magnitude stars) showed airy disks with a single "walking" diffraction ring. Brighter stars (magnitudes 3&4) showed ugly, flaring first diffraction rings surrounding visible airy disks. First magnitude stars would not reveal an airy at all -despite use of the highest possible magnifications (540x).

Once again all this says "7/10".

My next stop was Rigel. At 120x, beautiful radiant blue primary with pale blue seventh magnitude secondary roughly 10 arc seconds to the south. At this distance well outside the "starburst" image of the first magnitude primary. (The secondary smacked of the tiniest of airy disks.) Had seeing been say 5/10 the starburst would have consumed the secondary. 6/10, it would have been seen as a moth "dancing around the primary's tongues of flame".

Then Alnitak: 10th magnitude greyish come held easily direct one arc minute north of the 2nd magnitude blue primary. The 4th magnitude blue gray secondary embroiled in the primaries roiling starburst image which surpassed its first diffraction ring to the south and slightly trailing. What I might call an "ugly-split". At 120x the very brightest lobe of the Flame Nebula could be vaguely detected northeast.

On to the Great Nebulae! At 52x splendid. Eagle Obscuration well defined. The Eagle's southeast wing showed a faint tinge of "pink". While the bright nebulae was all blue-gray. Faintly glowing nebulosity - much more concentrated at M43 - extended along the eagle' eastern frontier. The Eagle's beak pointed toward my ultimate destination - the Trapezium. (BTW T: You call the Eagle's beak the "Fishmouth".)

The Trapezium lies in the very midst of the Great Nebula's brightest region of condensation (as one might expect). I'd estimate the region's average surface brightness to be about magnitude 7 (in round arc-minute increments). Over "round" arc-seconds this is probably closer to magnitude 16. But you'd really have to run up the magnification to completely dilute it to this point!) Thus all the stars in the budding Trapezium region have to show themselves against a relatively bright background. While the dimmer ones are easily overmatched by their brighter conferes. (In fact the dimmest 8th mag member "B" can easily be missed in two inch scopes without aversion or in larger scopes under poor seeing and/or adjustment.)

Now "E" as we all know forms a triangle with A and B. (Note: designators A- D are based on right ascension and not magnitude. A is the westernmost member of the group. The remaining designators E and so on are based on order of discovery.) Under the conditions of the night "E" was not much of a challenge. One thing I noticed was that E is perceptibly closer to brighter A than much dimmer B.

But 7/10 seeing overhead did not support a view of "F". For as we know F lies closest to the brightest member of the group "C". And C was not in the mood for "seeing" F. It flashed, it roiled and otherwise made a pest of itself. Using all my tricks, (aversion, eye movement, magnification etc.) I couldn't even imagine seeing dim "F" in the neighborhood of all that commotion.

On the occasions when F has been visible in the past (through Argo) all four Trapezium members showed fine airy disks with gently walking first diffraction rings (8/10 seeing). Meanwhile the Gas Giants were giving superbly detailed views overhead and the night sky was reasonably transparent (~5.0 ZULM).

So what's my conclusion? A well-collimated, optically correct six inch scope capable of "8/10" seeing should be able to reveal both E and F at between 1-2 mms of exit pupil (75 - 150x) on any night of fair transparency and very good stability.

Meanwhile, experience has shown that a deeper sky (5.5 ZULM) is needed to reveal E plainly through a three and change inch scope under similar 8/10 stability conditions.

I sure hope all this analysis hasn't taken the FUN out of observing this region of the sky!

Oh yes T, I've also seen a brightening in the region of "H". (In fact when I first saw this I thought I was seeing "E"!) I suspect we are not seeing "H" but some kind of unresolved "hot spot" caused by the small group of stars in the region and the nebulae itself...

Clear and Steady,

jeff

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