The image is a stack of 25 x 300 second exposures, ISO 1600, 25% crop, processed with 30 dark and 30 bias frames. Limiting magnitude on the original is 19. Software used was PHD2, DSS, XnView, StarNet++, and StarTools.

Abell 39 Hercules, Planetary Nebula

This low surface brightness planetary nebula in Hercules was discovered around 1955 by Abell and Wilson during the National Geographic Society Palomar Observatory Sky Survey. Planetary nebulae are formed when red giant stars run out of nuclear fuel in their cores, gravitationally collapse into white dwarfs, and eject their outer layers into an expanding shell of gas. Illuminated by the central white dwarf, the shell remains visible in blue-green light for tens of thousands of years before diffusing into space.

Abell 39 is about 3 arcmin in angular diameter, 13.7 in apparent magnitude, and 2.5 LY in radius, lying at the distance of 6,800 LY. The central star is a 15.5 magnitude subdwarf of spectral class O, indicating surface temperature between 40,000 and 100,000 K, about ten times hotter than the Sun. The nebula is almost perfectly spherical because it is undeformed by movement through the interstellar medium. The E rim is obviously denser than the W, and the central star is slightly displaced toward the W probably because the gas ejection was asymmetrical. Based on the expansion speed of around 35 km/sec, the nebula is approximately 22,000 years old.

As the annotated image shows, the field is strewn with small, faint galaxies. The most remote one is PGC 1821778 with a redshift of 0.13163, which indicates the lookback time distance of 1.67 billion LY, and apparent recession speed of 36,900 km/sec. Professional images show that the two "knots" in the nebula marked as GAL are actually faraway background galaxies whose light is passing through the gaseous shell.

Annotated image link
https://www.cloudynights.com/gallery/image/108241-abell-39-hercules-planetary-nebula-tsapo100q-annotated/