User:WilliamKF/Abell 39

Abell 39 is a low surface brightness planetary nebula (PN) in the constellation of Hercules. It is the 39th entry in George Abell's 1966 catalog (and 27th in his 1955 catalog) of 86 old planetary nebulae which either Abell or Albert George Wilson discovered before August 1955 as part of the National Geographic Society - Palomar Observatory Sky Survey. It is estimated to be about 6,800 light-years from earth and 4,600 light-years above the Galactic plane. It is almost perfectly spherical and also one of the largest known spheres with a radius of about 2.5 ly.

Central Star
Its central star (CS) is slightly west of center by about 2&Prime; or 0.1 light-years. This offset does not appear to be due to interaction with the interstellar medium, but instead, it is hypothesized that a small asymmetric mass ejection has accelerated the CS. The mass of the CS is estimated to be about 0.61 M☉ with the PN being an additional 0.6 M☉.

This PN has a nearly a uniform spherical shell. However, the eastern limb of the nebula is 50% more luminous than the western limb. Additionally, irregularities in the surface brightness are seen across the face of the shell. The source of the east-west asymmetry is not known but it could be related to the offset of the central star.

Structure
The PN's bright rim has an average thickness of about 10&Prime;.1 or about 0.34 light-years. There is a faint halo that extends about 18&Prime; beyond the bright rim giving a complete diameter of around 190&Prime; under the assumption that this emission is uniform around the PN.

Assuming an expansion velocity between 32 and 37 km/sec and a 0.78 parsec radius, the expansion age of this PN is estimated to be around 22,100$$ years.

Observations reveal only about half the oxygen levels as found in our sun.