Zeta Orionis and surrounding nebulae
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Zeta is the easternmost of the 3 stars forming Orion's belt. It is the really bright star in these photos, with the long diffraction spikes. My photos are oriented to make the horse head obvious, so North is to the left and East at the bottom.
2010 December 11
Canon EOS5DMkII 254mm Newtonian @ 1200mm 18 x 30s f/4.8 ISO6400 2010-12-11 23:10:27-23:21:37 UT
From Rookhope 54.8
This photo exceeds my wildest expectations of what could be photographed with my amateur equipment. 20 years ago this kind of thing could only be obtained through large telescopes and even then required much longer exposure times on specially sensitised film with accurate guiding.
It is important to note that this was taken with an unmodified Canon DSLR camera. I had previously believed that it was not possible to photograph the Horsehead Nebula without removing the infra red filter from in front of the CMOS detector in a digital SLR camera but this photo proves otherwise. My previous inability to photograph the Horsehead must have been due to light pollution at my suburban observing site. The present photo was taken from the village of Rookhope, high in the North Pennines. No filters have been used here. Clearly the emission nebula behind the Horsehead is reddish, so in other published photos that I have seen the predominant red colour was not just due to using H-alpha filters.
Here is an annotated version of the image:
The Horsehead Nebula is number 33 in the catalogue of dark nebulae compiled by E.E.Barnard in the 1920's. Such nebulae comprise dust particles, rarified but sufficient to block the light of more distant stars because of their huge thickness. It is difficult to determine how far the dark nebulae are from us (they emit no light of course, so spectral lines cannot be measured). It is thought that the Horsehead is about 1,500 light years away. That implies that it would be about 2 light years across.
Further away, because B33 is silhouetted against it, lies IC434. (IC stands for Index Catalogue: 2 supplements added later to the New General Catalogue, NGC, of 1888.) IC434 is a cloud of ionised hydrogen. The composition is known from the emission lines in its spectrum. In particular the H-alpha line has a deep red colour (at the limit of or even beyond the spectral range of digital SLR cameras). This cloud is probably being excited by the light of the star Sigma Orionis, near the top right of this photo.
There are numerous other gaseous nebulae in the field, the biggest being NGC 2024, sometimes called the Flame Nebula, next to Zeta.
2011 January 28
I took a much longer exposure this time: 40.5 minutes. That made the signal to noise ratio much better so considerable detail can be seen, particularly in the neck of the horse.
Canon EOS5DMkII 254mm Newtonian @ 1200mm 81 x 30s f/4.8 ISO6400 2011-01-28 20:52:45-21:42:11 UT
From Rookhope 54.8