Description and Basic Characteristics
The Helix Nebula (NGC 7293), also referred to as the 'Eye of God', is a planetary nebula in the Aquarius constellation that was discovered in the early 1800's by Karl Ludwig Harding. It lies at a distance of 700 light years, a fact that makes it one of the nearest planetary nebulae to Earth. The name 'Helix' denotes the strange helix-like shape of the nebula gases, a rather complicated structure that we are lucky to observe from our Earth's perspective.
The proximity of the Helix, and the fact that it covers an area of the sky of about a quarter of the full Moon, has allowed a more detailed analysis of its complicated structure. The nebula has a shape of prolate spheroid with the largest concentrations of material lying across an equatorial disk whose major axis is inclined about 21° to 37° from our vantage point. The simpler approach to describe its structure, suggests that it consists of an outer ring expanding at a rate of 40 km/s, and an inner disc with an expansion rate of about 32 km/s. Color enhanced images are used to depict the various details of the formation and make them visible to the human eye.
A rather important feature is the presence of small and highly symmetrical knots of gas and dust, described as comets with their tails extending away from the core. Each knot has the size of our solar system and given the fact that the nebula contains more than 20,000 of them, we can only imagine its enormous size. Although they have been extensively studied, their exact nature is still puzzling to the scientists. Some basic characteristics of the nebula are listed below:
Age: ~ 10,600 years, based solely upon its expansion rate of 31 km/s.
Distance: 700 light-years
Apparent magnitude: +13.5
Diameter of the inner disk: 8×19 arcmin (0.52 pc)
Diameter of the outer torus: 12×22 arcmin (0.77 pc)
Diameter of the outer-most ring: 25 arcmin (1.76 pc)