The socalled "Ring-nebula" aka as NGC 6720 is probably one of the most popular planetary nebulas, located in the constellation "Lyra" in a distance of 2300 light years. When Antoine Darquierand observed a comet in 1779 he just by chance came across this very bright PN that shines at 9m7. Ever since it is a very spectacular object for both, visual and photografic observation. Even in a standard telescope the ring-shape which donates the name can easily be seen. The very well visible central star was discovered by Friedrich Hahn 21 years later.

The apparent size in such telescopic view measures some 1.2 arc minutes in its longitudinal axis. However, very deep astroimages like this one above reveal its true size, which is more than twice that, currently 3.4 arc minutes. This so called outer shell is expanding hydrogen gas, that is expelling from the central hot star (~100.000K) at a velocity of approximately 19 km per second, which equals the beginning of this expulsion some 20.000 years ago when a gian red star commenced its dying-process. Ever since the nebula keeps on spreading into outer space and nowadays the absolute diameter of the entire objects is said to hold 1.3 light years.

The greenish interior of the ring mostly comes from ionized oxygen and nitrogen gas, that is heated by the radiation of the central star, which - like in almost all planetary nebulae - happens to be a white dwarf, not bigger than earth. In the enlarged crop one can also see some dark spots within the inner ring-structure which represent huge amounts of dust and dirt, standing out as remnants of the dying central star.