Planetary Nebula SH2-188

All Images
Copyright Steve Cannistra

Please click here or on image for a wide field view.
Please click here for this image in hydrogen alpha light.
Please click here for an inverted version of this image.

Size: Diameter of the brightest arc is about 10'.
Distance: ?
RA: 01h 30m 33s
Dec: 58 degrees 24' 51"

SH2-188 is a planetary nebula in Cassiopeia.  It was first photographed in 1965 and was initially thought to be a supernova remnant, in part due to its asymmetric crescent-like shape.  However, this object is now known to be a planetary nebula, which is consistent with the abundance of carbon, nitrogen, and oxygen in its outer shell.  Unlike SH2-188, however, most planetary nebulae have a symmetric shell that expands outward from its dying star.  Examples include the Ring Nebula, the Helix Nebula, and the Bubble Nebula, to name a few.  When a dying star between 1-5 solar masses finally exhausts helium in its core, it enters into the Asymptotic Giant Branch (AGB) of the HR diagram, where it becomes a red giant again (but now burning helium to carbon in its outer layers).  During this AGB phase, it expels relatively slow moving gas into space.  Eventually the star shrinks, becomes hotter, and expels gas at a much faster rate.  This faster moving solar wind catches up and collides with the slower moving gas from the prior AGB phase, forming a bow shock.  The bow shock region typically forms a symmetric shell of compressed gas that is excited by UV emission from the central star.  This mechanism of PN formation is known as the "Interacting Stellar Winds" model and nicely explains the shape of most PN, except for SH2-188...

So why is SH2-188 so asymmetric?  Another model has been proposed to explain this, in which the rapidly moving PN interacts with the interstellar medium (ISM).  As a result of this interaction, the bow shock is preserved in the direction of forward motion, but the back end of the PN essentially drags behind and eventually dissipates away.   In my image of SH2-188 shown above, the forward crescent shaped arc is easily seen, but there is a fainter portion at the back end that closes the loop, associated with a poorly-defined triangular trail of gas that emits in Ha.  This region is unmistakable when processing this image, and is more easily seen on my Ha image used as luminance.  I have also inverted this image as another means of appreciating this faint region.   My understanding of the processes involved in shaping SH2-188 comes largely from these two articles (Wareing et al 2004, Wareing et al 2005), which I recommend for further reading.

Photographic Details:

Date:  August 30 and 31, 2006
Scope:  Takahashi FS102 at f6, on the G11 Losmandy Mount.
Autoguider:  SBIG STV with e-finder.
Camera:  Maxcam CM10.

Astronomik Ha filter (6 nm bandpass); RGB type II filter set.
Exposures:  Ha:  18 x 10' each.  R and G, 6 x 5' each; B, 6 x 8' each.  All unbinned.  Total exposure: 4.8 hours.
Conditions:  Temperature 60 degrees F; below average transparency due to intermittent high, thin clouds; average seeing; calm.
Post-processing:  This is an Ha(HaR:G:B) image.  Debloomed with Ron Wodaski's Debloomer software, followed by calibration and alignment in Maxim.  Sigma combined using RC Sigma Reject MaximDL, followed by DDP in ImagesPlus (IP).  Further processing in Photoshop CS (16 bit format).  The Ha contribution to the red channel and to luminance was restricted to the planetary nebula region by using a mask in PS.   In this way, I was able to preserve the deep color and stellar profiles throughout the rest of the FOV (i.e., the RGB components), without interference from the Ha luminance channel.

Please note:  Graphics on this website may not be reproduced without author permission.

Back to Nebulae