|Designation|| NGC 7293|
|Right ascension||22h 29m 38.55s|
|Declination||-20° 50′ 13.6″|
|Type of object||Planetary nebula|
|Dimensions|| 8'x19' (core)|
|Magnitude|| Apparent Mag: +7.6|
Absolute Mag: +6.58
|Distance from Earth||695 ly|
|Radial velocity||-15±10 km/s|
|Proper motion|| RA: 32 mas/yr|
Dec.: -5 mas/yr
The Helix nebula (NGC 7293, Cadwell 63) is a planetary nebula in the constellation of Aquarius. The brightest planetary nebula in the night sky, it is a popular target for astronomers. However its large area means it has a low surface brightness and so dark skies are needed to see it properly through binoculars. The nebula's dazzling appearance has lead some to call it the "Eye of God." Despite being a large nebula, none of Charles Messier, William Herschel or John Herschel, who are credited with so many discoveries, discovered the Helix nebula. The nebula was first observed in the early 19th century.
Properties and Structure
The outer part of the nebula has an apparent size of 12x22 arcseconds, an apparent size around half that of the Moon and a physical size of some 5.87 light years. Its inner ring has a size of 8x19 arcseconds. Like most planetary nebulae, the nebula is expanding. Interestingly, the inner and outer regions of the Helix nebula are expanding at different rates, with the inner disk expanding at 32 km/s and the outer ring expanding at roughly 40 km/s. The star in the centre of the nebula has an apparent magnitude of +13.4.
The nebula was the first nebula discovered to contain "cometary knots" and is believed to have on the order of 20,000 such knots. These knots comprise of a bright globules of material with tails that extend radially out from the nebula (point away from the nebula). These knots, which have masses similar to that of Earth but have radii several times the distance between Pluto and the Sun (some 100 AU), have been observed in other planetary nebulae such as the Ring nebula, the Dumbbell nebula and the Eskimo nebula. It has been suggested they are formed by a strong stellar wind produced by the nebula's central star.