20min ha ,10min red, 20min blue, 10min green, 50min luminance, exposure with a sxv-h9 and a Takahashi FS60c.
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The Horsehead nebula, also known as Barnard 33, is certainly one of the most appropriately named objects in the entire night sky. Found close to the star Alnitak (the bright white blob dominating the upper left of Jimís photo) at the Eastern end of Orionís belt, it appears as a silhouette against the red backdrop provided by the bright emission nebula IC434. It lies at a distance of around 1,500 light years, according to most sources..
The horse-head like feature is created by what might be described as little more than a wisp of dust-laden gas protruding from the margin of a much, much larger molecular cloud. The rest of the cloud cannot be seen because thereís no luminous background onto which its silhouette can be projected. At least, thereís no continuous bright background, such as that provided by IC434 which enables us to see the region of cloud that forms the Horsehead. However, there will be a continuum of stars lying behind the cloud, the light from which is completely obscured by the cloud. This is demonstrated by the relative paucity of stars in the bottom left of this photograph Ė the only stars we see in that area being foreground stars that lie between us and the molecular cloud. Compare the low density of stars at bottom left with the much higher density at upper right where we are seeing much further into the distance beyond the margin of the obscuring cloud.
At the extreme left of this beautiful photograph is the Flame nebula, also designated NGC2024. This too is reckoned to be up to 1,500 light years away, but some sources put it as low as 900 light years. The emission is probably due to intense radiation from Alnitak. Again there is a wisp of dust-laden gas seen in silhouette against the bright emission of the Flame nebula. It certainly appears that this is a wisp emanating from the same molecular cloud as the wisp that forms the Horsehead, although appearances can be deceptive, especially in astronomical photographs.
Virtually the whole of Orion is pervaded with these molecular clouds, which collectively are known as the Orion Molecular Cloud Complex. These impenetrably dark clouds are so called because the overwhelming majority of the material they contain is in the form of molecules, mostly of hydrogen. However, although something like 95% of all the atoms within molecular clouds are either hydrogen (in molecular form) or helium (which doesnít form molecules of any kind), the remaining few percent is a mixture of almost all the other elements, many of which get together to form a number of interesting and important compound molecules. Thereís currently (December 2010) around 170 molecules of various compounds known to be present within molecular clouds, with perhaps another half a dozen unconfirmed. They vary in complexity from the very simple diatomic hydrogen molecules up to rather more complex assemblages, a pleasantly surprising example of which is alcohol! The author of this article recalls reading many years ago that someone had estimated that the average molecular cloud (whatever "average" means in this context) contains enough alcohol to fill all the oceans on Earth. Mmm, you could have quite a party with that!.
Giant molecular clouds can contain a million or so solar masses of material and generally measure at least some tens of light years across, with some exceeding 100 light years.