ngc 7293
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At an estimated distance of less than 700 light years, NGC 7293 is one of the closest and apparently largest planetary nebulae in the entire sky. The brilliant red annulus is composed mostly of ionized hydrogen appearing to form an overlapping "helical" structure, hence giving the nebula its common name the Helix Nebula. In reality, this shell of energized gas is actually the tenuous remnants of a star that was once much like our very own Sun! The only physical evidence of the progenitor star is its dense core, which has managed to survive and become a type of star known as a white dwarf located near dead center of NGC 7293. With a surface temperature of 117,000 Kelvin (roughly 20 times hotter than our Sun), this tiny blue star releases a large portion of its light in the ultraviolet range of the electromagnetic spectrum thereby ionizing any simple rarified gases in its vicinity. Of course, higher energy levels are located closest to the hot star allowing the ionization of more stable atoms such as helium and oxygen, which aproximate the visual colors of blue and blue-green in the image above. The outer layers of the Helix are ruddy in appearance due to the ionization of the simplest of all the chemical elements, hydrogen. Even at great distances from the central star, the outermost layers of what was once a red giant's atmosphere is illuminated to form a faint "outer halo" which nearly doubles the size of the visible nebula. Also of note are a handful of faint anonymous galaxies throughout the field and the presence of several of the larger cometary "knots" located along the inner edge of the bright red annulus on the high-resoulution version of the above image. Image taken with homemade 8-inch f/5.4 astrograph and SBIG STL-11000M. (L+R)RGB image composed of 100 minutes of luminance and 60 minutes of red, combined with 30 minutes each R,G,B. Be sure to click on the above image for the high resolution version.