NGC 7293 is a large and well-known 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 and a beautiful example of a remnant of a dying star. It contains a double ring structure, not unlike two coils of a spring; hence the popular name the Helix Nebula.
Although the area of sky surrounding the Helix Nebula is devoid of bright stars, it can be easily located by star-hoping. The nebula lies roughly halfway along an imaginary line connecting Fomalhaut (α PsA - mag. +1.2), the brightest star in the constellation Piscis Austrinus, and ι Aqr (mag. +4.3). Just over a degree east of the Helix is υ Aqr. At magnitude +5.2, this star is faintly visible to the naked eye under dark skies and acts as a good marker.
At magnitude +7.3, the Helix Nebula is the brightest planetary nebula in the sky. Based on this information it would seem an easy target and indeed is visible with binoculars from a dark site. But that's the key, dark skies are required. This is due to its large surface area, which results in a low surface brightness. The main nebula covers 18 arc minutes with the much fainter outer halo spanning some 28 arc minutes - close to the apparent diameter of the full Moon. Charles Messier failed to spot it and the two great astronomers, Sir William and Sir John Herschel, also failed to notice it during their sky searches. It was German astronomer Karl Ludwig Harding who eventually discovered this nebula, sometime before 1824.
When viewed through a pair of 10x50 binoculars, the Helix appears as a faint featureless oval shaped disk. A 100mm (4-inch) telescope reveals finer structural details. Through a 200mm (8-inch) telescope, the nebula appears slightly oval with two thick arcs, gaps and brightness variations across the structure. A nebula filter of the UHC (Ultra High Contrast) or OIII (Oxygen III) variety, especially when combined with averted vision, helps to bring out subtle details. However, it's an illusive object that can be rendered invisible even in medium size amateur telescopes with just a small amount of light pollution. The central star of the Helix Nebula shines at magnitude +13.4 and is therefore only easily seen in large amateur reflectors.
The Helix Nebula is located 695 light-years from Earth and has an actual radius of about 3 light-years. It's marginally brighter than M27, the Dumbbell Nebula. However, it's a much more elusive object due to its low surface brightness. When imaged or photographed it looks spectacular with colourful twists, coils, knots and rings of gas visible. It's a popular object on observing 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|