NGC 7662 is aplanetary nebula located in the northernconstellationAndromeda. It is known as theBlue Snowball Nebula,Snowball Nebula, andCaldwell 22. This nebula was discovered October 6, 1784 by the German-born English astronomerWilliam Herschel. In theNew General Catalogue it is described as a "magnificent planetary or annular nebula, very bright, pretty small inangular size, round, blue, variable nucleus". The object has anapparent visual magnitude of 8.3 and spans an angular size of32″ × 28″.[2]Parallax measurements give a distance estimate of 5,730 ± 340 ly (1,757 ± 103 pc).[1]
NGC 7662 is a popular planetary nebula for casual observers. A small telescope will reveal a star-like object with slight nebulosity. A 6" telescope with amagnification around 100x will reveal a slightly bluish disk, while telescopes with aprimary mirror at least 16" in diameter may reveal slight color and brightness variations in the interior.[4]
This nebula has an elliptical shape with a triple-shell structure. The brightest is the main shell, which spans12″ × 18″. This is surrounded by a fainter outer shell, which has an elliptical form. Both shells are enclosed by a faint, circular halo,[6] some134″ in diameter. The two shells can be modeled asprolate spheroids, with the inner shell having the greater elongation, amajor axis tilt of 50° to the line of sight, and a hull thickness of~2.5″.[7] Several knots and a jet-like structure are visible, which displayemission lines and low ionization. Based on the expansion rate, the estimated age of the nebula is 3,080 years.[6]
Media related toNGC 7662 at Wikimedia Commons
