NGC 2261 Hubble's variable nebula
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There are only a handfull of nebulae which vary in brightness over shorter periods than hundreds of years. From my experience so far, Hubble's is much the easiest to photograph. It is in the constellation of Monoceros.
2012 Mar 18
Canon EOS 5D MkII ISO6400 48x32s
254mm Newtonian f=1200mm f/4.8
HEQ5 equatorial mount, driven but not guided
2012 Mar 18 22:19:28-22:48:11 UT
From Rookhope 54.8
2012 Jan 22
For comparison with 5 days ago (see below):
Canon EOS 5D MkII ISO6400 53x32s
254mm Newtonian f=1200mm f/4.8
HEQ5 equatorial mount, driven but not guided
2012 Jan 22 22:26:34-22:57:23 UT
From Rookhope 54.8
The star at the apex of this nebula is denoted R Monocerotis. That means it was the first known variable star in the constellation of Monoceros. It varies between about magnitudes 10 and 12. A possibility is that the brightness of the nebula follows the brightness of the star, after some time delay. I believe that connection has not been proved however.
2012 Jan 17
Canon EOS 5D MkII ISO6400 39x32s
254mm Newtonian f=1200mm f/4.8
HEQ5 equatorial mount, driven but not guided
2012 Jan 17 00:02:59-00:26:35 UT
From Rookhope 54.8