Sh2-313 (Abell 35) has long been considered a large planetary nebula located in the constellation Hydra. Jacoby, (Astrophys. J., v 244, 902-911, March 15, 1981), studied the nebula and the bright central star (HIP 62905) and estimated the distance to be approximately 1200 light years from the Earth, however, recent research indicates the distance to the central binary star system is approximately 160 parsecs (520 light years). Jacoby also deduced the central star is a binary system with a very hot white dwarf (BD 22-3467) in a close orbit. This has been subsequently verified with the temperature of the companion white dwarf in excess of 150,000 K. As shown in the image above, there is a prominent bow shock feature to the right of the central star. This feature stands out strongly in OIII filtered images. It is also weakly visible in long exposure Ha images. This bow shock indicates the central binary star system is plowing through the interstellar medium at supersonic speeds. Hollis, et al (Astrophys. J. v 456, 644-650, January 10, 1996) supported these conclusions. This feature, and other recent studies, indicates the nebula is not a classic planetary but is a region of the interstellar medium that is being ionized by intense radiation from the hot white dwarf forming a Stromgren sphere.
The image above is known as a mapped, or false color, image and was acquired using narrowband filters. It was assembled using the standard Hubble Palette with SII mapped to Red, Ha mapped to Green and OIII mapped to Blue. The stars were overlaid with data from a Bi-Color (Ha - OIII - OIII) image to give them a more natural appearance. A near true color (Ha,OIII,OIII) image of Sh2-284 may be viewed here.