A Rose of a Different Color: NGC 2237-2239 in SII, Ha, and OIII

Rosette Nebula
All Images Copyright Steve Cannistra

Please click here for a medium size (30%)
Please click here for a larger size (50%)
Please click here for a central crop (70%)

Magnitude:  4.8 (for NGC 2244 star cluster in center)
Distance: 5500 light years
RA: 6h 32m 33s
Dec: 4 degrees 57' 05"
Position Angle:  0 degrees

The Rosette Nebula is a winter showpiece for astrophotographers, as well as for visual observers at low power.  This large emission nebula is located in Monoceros and can be seen in dark locations with the naked eye, especially if viewed through a UHC filter held up to the sky.  The nebula actually consists of several sections (NGC 2237-2239), with a central ladder like star formation known as NGC 2244.  The central stars are young and newly-formed, and their radiation is responsible for excitation of hydrogen atoms within the nebula itself, which in turn emit light in the red at 656.3 nm wavelength.  Like most emission nebulae, however, the central young stars emit high intensity radiation that also excites other types of elements within the cloud to emit at their own characteristic wavelengths.  For instance, sulfur (SII) emits at 673.0 nm, and OIII emits at 500.7 nm (most intense) and, to a lesser extent, at 495.9 nm.  By using filters that capture SII, Ha, and OIII emission, it is possible to map these colors into the more conventional R, G, and B color palette, respectively.  Assigning these narrowband images in this order (highest to lowest wavelengths) to R, G, and B defines the Hubble Palette, which is how the above image was constructed.

Spend some time on the larger images and notice where each element overlaps, and where each element has a distinct, non-overlapping location.  For instance, the central portion of the nebula is more intensely blue (OIII), whereas the peripheral "skirt" is a mixture of reddish-orange (dominant SII with a touch of Ha) and yellow (equal parts of SII and Ha).  You can also see regions that are "apple green" in color, representing an area that dominates in Ha (at 7 o'clock, for instance).  The colors intermingle in various places throughout the image, creating a fascinating and pleasing color portrait that conveys important information about the relative signals that are emitted from these locations.  Rob Gendler's description of the Rosette Nebula may be found here.

Photographic Details:
Date:  November 29 (Ha), December 11 (OIII), December 12 (SII), 2009
Scope:  Takahashi FSQ106 at f5 on the Takahashi NJP Mount
Autoguider:  SBIG ST-402 with 60mm guidescope, focal length 227mm
Camera:  Apogee U16M at -20C, with 7 position 50mm square filter wheel (Apogee FW50-7S)
Baader narrowband filters, 50mm square
Exposures (20 minute subs):  Ha, 240 minutes; OIII, 240 minutes, SII, 300 minutes, all unbinned.  Total exposure 13 hours
Post-processing:  Calibrated in Maxim, aligned and stacked using DeepSkyStacker, followed by DDP in ImagesPlus (IP).  Further processing in Photoshop CS (16 bit format) using the clipped layer mask method originally described by Travis Rector

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