The Helix Nebula (NGC 7293) is a large and famous planetary nebula located in the faint zodiac constellation of Aquarius. Also known as Caldwell 63 it's one of the nearest objects of its type; a beautiful remnant of a dying star containing a double ring structure not unlike two coils of a spring, hence the popular name "The Helix Nebula".
Although the immediate area surrounding the Helix Nebula is devoid of any particular bright stars, the region can be easily located by star-hoping. NGC 7293 lies roughly halfway along an imaginary line connecting Fomalhaut (α PsA - mag. +1.2) the brightest star in the southerly constellation of Piscis Austrinus and ι Aqr (mag. +4.3). Just over one degree to the east of the Helix Nebula is υ Aqr. At magnitude +5.2 this star is visible to the naked eye under dark skies, simplifying the search for the Helix Nebula.
At magnitude +7.3 the Helix Nebula is the brightest planetary nebula in the sky. Based just on this information it would seem an easy target and indeed it is visible with just binoculars at a dark site on a moonless evening. But that is the key, dark skies are required. This is due to its large surface area resulting in a low surface brightness. At its widest point the main nebula covers 18 arc minutes with the much fainter outer halo spanning some 28 arc minutes – close to the diameter of the full Moon. The low surface brightness meant that Charles Messier failed to spot it. Even the two great astronomers, Sir William and Sir John Herschel, failed to notice it during their sky searches until German astronomer Karl Ludwig Harding eventually discovered it sometime before 1824.
When viewed through a pair of 10x50 binoculars the Helix Nebula appears as a faint featureless oval shaped disk. A 100mm (4-inch) telescope reveals finer structural details. Through a 200mm (8-inch) telescope the Helix Nebula appears as a slightly large oval ring with two thick arcs, gaps and variations of brightness visible across the structure. A Nebula filter of the UHC (Ultra High Contrast) or OIII (Oxygen III) variety especially when combined with averted vision helps bring out more subtle details. However it's an illusive object that can be rendered invisible even in medium sized amateur telescopes with just a small amount of light pollution. The central star of the Helix Nebula is of magnitude +13.4 and therefore only easily visible in the largest of amateur telescopes.
The Helix Nebula is located only 695 light-years from Earth and has an actual radius of about 3 light-years. It's the brightest planetary nebula and marginally brighter than M27 the famous Dumbbell Nebula. However, it's a much more elusive target due to its low surface brightness. When imaged or photographed the Helix Nebula looks spectacular, beautifully colourful with twists, coils, knots and rings of gas. It's a popular object on astronomer's observational lists.
NGC 7293 Data Table
|Object Type||Planetary Nebula|
|RA (J2000)||22h 29m 38s|
|DEC (J2000)||-20d 50m 14s|
|Apparent Size (arc mins)||18 x 18|