Hubble's Variable Nebula aka NGC 2261, is a highly variable nebula in Monoceros. Famous for being the first light image of the 200" Palomar Telescope in 1949 taken by Edwin Hubble himself. Movies of the nebula show it varies quite a bit over only a couple days time. Unfortunately, all my attempts to catch this have failed due to horrid weather and my failing to keep trying as my to-do list beckons strongly. So I've settled for once a year. Even then variable seeing and transparency make for a difficult comparison. It was discovered not by Edwin Hubble but by William Herschel on December 26, 1783. It is in the second Herschel 400 observing program. Hubble did discover the nebula itself varied in 1916 though the variability of the illuminating star, R Mon had been known since 1861.
I've included my annual (and twice annual from the winter of 2013-14) images since 2011. The color of the 2011 image is highly suspect. Exposure times vary as does my processing so these aren't usable for scientific comparison. This year conditions were good but cold. When I turned on the camera it was nearly -40. I had to set it for -45C and the fan off to get 3% to 5% cooling power and hope that was enough to keep the temperature regulated -- it was.
Note that not only does the nebula change above the illuminating star but the faint hook-shaped piece of nebulosity south of the star also changes. Being faint some of this is likely conditions. In 2011 it was rather obvious but then it faded. Last few years the part of the hook coming back north is getting stronger but the down-stroke is virtually gone. North of the star the main changes are on the east side though the dark band crossing the lower part of the nebula that was strong a few years ago has vanished in 2015 but seemed to be returning in 2016 only to vanish the last two winters. The current image makes it look a bit fatter than the January 2017 image. I expect there were lots of other changes I missed due to the very long time between images. The color in the 2011 image is somewhat suspect as my attempts at color balance were primitive back then. Likely it is redder than it would have been if processed today. I suppose I should go back and redo it.
R Mon, the variable star at its base, illuminates the nebula. It is a brand new star just exiting its birth cocoon. It is thought dust clouds from this cocoon are still orbiting the star casting various shadows on the nebula causing the variations in its details and color. In animations taken only days apart, it appears illumination of the nebula flows upward from the star hitting more distant parts of the nebula over time. This gives an illusion of material moving but I am quite certain this is more like shining a flashlight beam around on a mostly stationary object. The first animation link is from a University observatory, the second is by amateur Tom Polakis in Tempe, Arizona where clear skies are much more common than here.
The small faint reddish nebulae above Hubble's Variable Nebula is HH 39. It has 6 parts A through F. Sharp-eyed viewers may spot a small galaxy a bit less than halfway between Hubble's Variable Nebula and the left edge. It has a vertical oval envelop around a star-like core. It is 2MASX J06393966+0846004.
Data for my December 26, 2017 image (UT)
14" LX200R @ f/10, L=4x10' RGB=2x10', STL-11000XM, Paramount ME