It is the 39th entry in George Abell's 1966 Abell Catalog of Planetary Nebulae.
With a diameter of 2.5 light years Abell 39 is a planetary nebula 4600 light years away from Earth, in the constellation Hercules.
Unlike many other planetary nebulae, Abell 39 is an extremely simple structure, with an almost perfectly spherical shape.
The only aspect that ruins the almost perfect symmetry of this nebula is the position of the star inside it, which appears slightly offset from the center. This may be due to several reasons, such as the interaction with the interstellar medium, but observations suggest it is due to an asymmetric mass ejection occurred in the past, which has slightly accelerated the star to its current position.
Due to the fact that Abell 39 is so large it is very thin, which makes it an object of low surface brightness which is consequently very difficult to observe.
Credit: ESO.
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