M42 Trapezium (Sept. 22, 2012)

The lovely little cluster called the Trapezium lies at the heart of M42, the Great Nebula in Orion, and is named after the shape made by its brightest stars.  M42 and the Trapezium are one of the grandest sights in the sky and are subjects for serious study.

The cluster's stars are close together, literally as well as apparently.  They appear so close that they were once thought to be one star, Theta Orionis.  As astronomers over time split the star into more and more components, they named the components with letters.  The brightest star in the cluster is Theta Orionis C.  It is very bright, very hot, and generates most of the ultraviolet radiation that makes the gas around it glow.  In the image, the star at the top of the Trapezium is D.  The two brighter other stars that make up the trapezoid, going from left to right, are A and B.  Here is a close crop of the cluster:

As more components were discovered, more letters were used.  The semi-bright star between A and B (and just to the right of A) is E.  The star just above C is F (which looks elongated; actually there are two stars there).  The star just below D is G, and directly below G is I.  Moving to the lower left of I is H, which is just below C and to the left of A.  Next to H, to H's immediate left, is another star, and I have no idea what it's name is.  Nor do I know the names of the other, nearby stars.  Actually, I was surprised to see so many here.  But this is not anywhere close to all the stars in the cluster.  Many of them shine in wavelengths other than those allowed by the two filters that I used to take this image.  I should explain.

One of the cool things about this nebula is that stars in it are still forming.  There is still a lot of dust obscuring stars that have formed, and the stars do not all glow with the same intensity at all wavelengths.  When we take pictures, we often use different filters that cut out all but a few wavelengths.  This allows us to focus only on the gases that light up the nebula, to the exclusion of almost all other light (including streetlights in front of my house).  For example, this image records light with a wavelength of 656.28 nm.  At that wavelength we find light emitted by ionized hydrogen.  Very near it we also find the light of ionized nitrogen, and my filter (called an h-alpha filter because it focuses on the hydrogen emission) just happens to pass light from a band 12 nm wide.  It is wide enough that it also picks up the nitrogen signal.  This image also records light taken through another filter that allows the wavelengths of 495.9 nm and 500.9 nm, and this light is emitted by ionized oxygen.  This filter is often called an OIII filter.  The h-alpha line is in the red part of the spectrum, and the OIII lines are green, but I've mapped OIII to blue here, also, because blue and red make a better picture.  So here you see the stars as they shine at those wavelengths.

But look at this next image.  I also took an image through an SII filter, which allows in light of ionized sulfur, at wavelengths of 671.6 and 673.1 nm.  The image is quite different:

More stars appear, and they are brighter.  Why is that so?  It may be because these stars radiate more light from ionized sulfur than from hydrogen, nitrogen, or oxygen.  I suspect that is the case.  Notice that stars G, H, and I are dimmer in this SII image, and that F is not elongated.  

The best way to see this cluster, though, is at infrared wavelengths.  Not until wavelengths of 750 nm do we reach the infrared.  Infrared light cuts through the dust, so an infrared image shows hundreds more stars in the area.  An example from the Hubble Space Telescope is here.  How cool is that?  On the other hand, when the Hubble records in h-alpha, nitrogen, and OIII, as I did, it picks up the same stars I do in the first image above (and a few more with Hubble's better resolution and bigger aperture, of course, but nothing like the infrared), as here.

Anyway, I took these images of the Trapezium from my backyard, in about an hour's time.  The stars are so bright that only short exposures would keep the stars small enough to remain separate.

Telescope: Orion 254mm f/4.7 Newtonian and Astro-Tech Coma Corrector (eff. at f/5.17)
Camera and Exposure: SXVF-H9 (H-alpha: 11x30"; OIII: 15x30"; SII: 15x30"), T-shirt flats
Filter: Astronomik 12nm Ha, OIII, and SII
Guiding: SX Lodestar and SX OAG
Mount: Takahashi NJP
Software: Nebulosity, Maxim DL, Registar, Photoshop CS3
Location: The Woodlands, TX

Trapezium Stars E & F, AR6, and Atik 16

The Trapezium of Orion! These six stars (you must click on the picture to see the larger image) are gravitationally bound. This is a "multiple star," not just a double or triple star. There are several others in the same tiny group. In fact, you can see in this image a hint of a few others---perhaps G, H, and I---if you compare this image with the one near the bottom of this link. There may even be other stars shown in this image. Look at the blobby shape of the lowest star. It turns out that faint stars exist just to the lower left and right of this star. My little AR6 with this makeshift imaging system can't resolve them, but perhaps light from these stars is deforming this component of cluster. (Thanks to Anjal Sharma of the Huntsville Amateur Astronomy Society for pointing out the deformation and the possibility of other stars there.)

Anyone who reads this blog much knows that I check the Trapezium almost every time it is up and I am out. I take great delight in seeing the four stars hanging together in the sky. I am even more delighted when I see the E and F components. I've described them here and in three earlier posts. But they are so easy to see in the AR6! Moreover, they are bright stars. So I figured: The focal length of the AR6 is 1219mm. If I can attach the Atik 16 to the AR6 and use an Ha filter, I can take many very short exposures and stack them to show the E and F components. This is the result. This is unguided on the LXD75, and it was pretty shaky. In fact, this is only 28 out of the 100 sub-exposures I took, but they were only 1- or 2-second subs, so I didn't lose much time. Shakiness and "seeing" took a toll. Also, the thing took some time to set up. I was afraid Orion would move behind my neighbors' tree before I was finished. In the end, I had 30 minutes to spare. The Ha filter is an Astronomik 13nm bandwidth. One key to the process was Nebulosity's Drizzle function, which yielded a better resolution than mere stacking or combining.

Here are two great images of the Trapezium by Samir Kharusi and Roland Christen. Both make me want to try again soon.