Abell 39
A nearly perfect spherical planetary nebula in Hercules.
Abell 39, also cataloged as PN A66 39, is a planetary nebula in Hercules with a low surface brightness. It appears as a nearly perfect sphere and ranks among the largest known spherical nebulae, with a radius of roughly 1.4 light-years. This object is the 39th entry in George Abell’s 1966 catalog of planetary nebulae (and the 27th in his 1955 catalog), which lists 86 old planetary nebulae discovered by Abell or Albert George Wilson before August 1955 during the National Geographic Society–Palomar Observatory Sky Survey. The nebula lies about 3,800 light-years from Earth and sits about 2,600 light-years above the Galactic plane.
Its central star sits slightly west of the nebula’s center, offset by about 2 arcseconds, or 0.1 light-years. This displacement is not thought to result from interactions with the interstellar medium; instead, it may be caused by a small, asymmetric mass ejection that accelerated the star. The star’s mass is estimated at 0.61 solar masses, while the nebular material adds another 0.6 solar masses. The nebula has a nearly uniform spherical shell, but its eastern limb is 50% brighter than the western limb, and there are small brightness irregularities across the shell’s face. The reason for this east–west asymmetry is unknown, though it might be linked to the star’s offset. The central star is classified as a subdwarf O star with a surface temperature of 8,900 K.
The bright rim of the nebula has an average thickness of about 10.1 arcseconds, or 0.19 light-years. A faint halo extends roughly 18 arcseconds beyond this rim, suggesting a total diameter of around 190 arcseconds if the emission is uniform all around. Based on an assumed expansion velocity between 32 and 37 km/s and a radius of 0.4 parsecs, the nebula has been expanding for an estimated 11,000 years. Background galaxies are visible near and even through the nebula. Oxygen in the nebula is only about half as abundant as in the Sun.
- Catalog entries
- 39th in the 1966 Abell Catalog of Planetary Nebulae, 27th in the 1955 catalog
- Distance from earth
- about 3,800 light-years
- Distance above galactic plane
- 2,600 light-years
- Radius
- about 1.4 light-years
- Central star mass
- about 0.61 M☉
- Nebula mass
- about 0.6 M☉
- Expansion age
- estimated 11,000 years
Lore & Background
Abell 39 was discovered as part of the National Geographic Society – Palomar Observatory Sky Survey, either by George Abell or Albert George Wilson before August 1955. It is one of 86 old planetary nebulae cataloged by Abell. The nebula is estimated to be about 3,800 light-years from Earth and 2,600 light-years above the Galactic plane.
The central star is slightly west of center by about 2″ or 0.1 light-years. This offset is not attributed to interaction with the interstellar medium; instead, it is hypothesized that a small asymmetric mass ejection accelerated the central star. The central star is classified as a subdwarf O star with a surface temperature of 8900 K.
The nebula has a nearly uniform spherical shell, but the eastern limb is 50% more luminous than the western limb, and irregularities in surface brightness are seen across the shell. The source of this east–west asymmetry is not known but could be related to the offset of the central star. The bright rim has an average thickness of about 10.1″ (0.19 light-years), and a faint halo extends about 18″ beyond it, giving a complete diameter of around 190″.
Reader's Guide
Abell 39 is significant as one of the largest known spherical planetary nebulae, with a radius of about 1.4 light-years. Its nearly perfect spherical shape makes it a valuable object for studying the dynamics of planetary nebula formation and evolution. The offset of the central star by about 0.1 light-years, hypothesized to result from an asymmetric mass ejection, provides insight into possible asymmetries in the progenitor star's mass loss. The east–west brightness asymmetry, with the eastern limb 50% more luminous, remains unexplained but may relate to the star's offset. The nebula's expansion age of about 11,000 years, based on an assumed expansion velocity between 32 and 37 km/s, offers a timescale for its evolution. Background galaxies visible near and through the nebula allow for studies of interstellar extinction. The oxygen abundance in the nebula is only about half that of the Sun, indicating chemical enrichment patterns. George Jacoby et al. created a spectacular image of Abell 39 in 1997, and it was featured as NASA's Astronomy Picture of the Day on 28 July 2005.
Did You Know?
- Abell 39 is almost perfectly spherical and one of the largest known spheres with a radius of about 1.4 light-years.
- Its central star is offset from center by about 2″ or 0.1 light-years, possibly due to an asymmetric mass ejection.
- The eastern limb of the nebula is 50% more luminous than the western limb.
- Oxygen is only about half as abundant in the nebula as in the Sun.
More in Planetary Nebulae, Part 2 1-24
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