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Over the Christmas 2011 period, these three deep-sky images were extracted from the light-polluted skies of Edinburgh. M45 is an open cluster about 130 pc away. Light from these stars is reflected or scattered off dust near the cluster, seen as the whispy blue "nebula".
The region south of Orion's Belt includes the famous Orion nebula (M42), here toward the bottom right. It is at a distance of about 400 pc. The light of this diffuse nebula or HII region is emitted by hydrogen gas that has been ionised by light from young stars that are forming from this nebula complex.
An area near the top left has been given an extra brightness boost in the image processing, to bring out the Flame nebula and the Horsehead nebula. The Flame is just left of the bright star at the top edge. The Horsehead nebula is a small dark cloud of dust and gas silhouetted against a "bright" background nebula. The background nebula is quite faint, making this difficult to make out in the image. These two bright nebulae are also HII regions, i.e. radiate due to hydrogen ionisation. Their emission is mainly in the deeply red Hα spectral line, which is not well detected by consumer-grade digital cameras.
M31 is the famous Andromeda galaxy, the nearest "full-size" galaxy outside our own Milky Way. (The southern sky has the two Magellanic Clouds, which are physically smaller but closer companions of the Milky Way.) The galaxy has a bright central bulge of stars, and a larger circular disc of stars, gas and dust. The disc is inclined to our line of sight and appears elliptical. Note the dark lanes of dust and gas that obstruct the otherwise smooth distribution of starlight from the disc. There are two small companion galaxies. NGC205 is somewhat top-right from M31. Less obvious, and difficult to distinguish from a star, is M32 a similar distance below M31. These galaxies are at a distance of 800,000 pc.
M45, M42 and M31 can be seen with the naked eye. M31 is the furthest you can see with the naked eye.
Canon EOS 400D, f = 135 mm, f/4, 800 ISO, stack of ~80 raw-format exposures of 30 s each. After stacking, a curved background was fitted and subtracted to remove light pollution.
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