Despite the large amount of data, this image has some troublesome artefacts, though I have tried to reduce their effect. Firstly there are some arcs of stray light from the bright star Alnilam outside the image. Alnilam is the cental star in Orion's belt, next to Alnitak which is the bright star to the upper left of the image. One arc is blue and there are 3 or 4 green ones (now showing as yellow after combination with the Ha data). There is also some broad, faint, horizontal banding. This prevented me bringing out the low signal areas on the left-hand side.
This image covers a relatively large area and includes several notable objects. Most prominent is the bright star Zeta Orionis, Alnitak, which is the lower left (in the Northern hemisphere) of the 3 stars in Orion's belt.
To the left of Alnitak is the Flame Nebula, NGC 2024. This glowing region of hydrogen gas is about 1000 light years from Earth. Its name comes from the shape resulting from the obscuring dust and gas clouds which produce the characteristic dark lanes. These obscure a very active star-forming region within the hydrogen cloud, which is revealed by IR and X-Ray space telescopes.
Many emission nebulae are dominated by the deep red emission of 'Hydrogen-alpha' light. H-alpha is the Balmer series alpha transition from the third quantum state of hydrogen's single electron to the second (transitions to the first quantum state are in the Lyman series, and are all in the ultraviolet). Three other Balmer series transitions are in the visible spectrum: beta is a cyan colour, while gamma and delta are blue and violet respectively. Their shorter wavelength/higher frequency are due to the higher energy of the transition from successively higher quantum states.
Unlike H-alpha emission nebulae, the Flame Nebula, which is clearly not the characteristic deep red, emits light generated by the recombination of ionised hydrogen (i.e. free protons) and electrons. The ionisation is caused by the UV light from Alnitak: any UV photon with a wavelength shorter than 91 nm has energy exceeding the 13.6 eV needed to completely ionise the atom, creating ionised hydrogen (a proton) and a free electron.
Much of the right-hand side of the image is coloured in reddish tones, and these are the signature of H-alpha emission. This is a large hydrogen cloud. There appears to be a higher density of stars within this area compared to the region to the left. This may be due to star-forming here, or due to more distant stars being obscured by the dust clouds on the left-hand side of the image. Or both! This hydrogen cloud is energised by the nearby bright star sigma Orionis (outside of the image) causing it to emit H-alpha light.
The Horsehead nebula is a dark nebula, mostly composed of thick dust which is obscuring the brighter region behind it. This dark mass is part of the same cloud which obscures the Flame Nebula The Horsehead Nebula is about 1500 light years away and was discovered by Williamina Fleming in 1888, on a photographic plate.
Between the Horsehead and Flame Nebulae is NGC 2023, which is both a reflection and an emission nebula. The bluish light is illuminated by the bright star at the centre, which is also causing fluorescence (in the near infrared) of the molecular hydrogen nearby.
Further over, at centre-left, is IC 435, a reflection nebula.
