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:
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