Abell 39 - Blue Ghost Bubble

This image shows Abell 39, one of the faintest and most delicate planetary nebulae in the sky. What looks like a simple blue sphere is actually the final visible stage of a Sun-like star that shed its outer layers tens of thousands of years ago. The result is an almost perfectly spherical shell of ionized gas, expanding quietly into space and weakly illuminated by the exposed stellar core at its center — now a hot white dwarf.

Abell 39 lies roughly 6,500–7,000 light-years away in the constellation Hercules. Although its integrated brightness is often quoted around magnitude 13–14, this number is misleading. Its surface brightness is extremely low, which is why the nebula behaves more like a much fainter object and demands careful processing and long, clean integration. The smooth, soap-bubble appearance seen here reflects its advanced age: the gas is thin, evenly distributed, and largely free of the chaotic structures seen in younger planetary nebulae.

At the very center of the nebula is the white dwarf remnant, cataloged under the planetary nebula designation PK 047+42.1. This tiny but extremely hot stellar core is all that remains of the original star. It still emits enough ultraviolet radiation to ionize the surrounding shell, though the nebula is clearly approaching the end of its visible life.

What makes this image especially interesting is the depth of the background. Several distant galaxies are visible through and around the nebula, emphasizing just how transparent and faint the shell really is. Among them are PGC 3089352, PGC 1821778, and PGC 2817010, all background galaxies hundreds of millions to over a billion light-years away. One of these, PGC 2817010, appears projected directly within the blue disk of Abell 39, a striking line-of-sight coincidence that highlights the enormous difference in scale between the foreground nebula and the deep extragalactic universe behind it.

Also present in the field is WISEA J162738.11+275451.6, an infrared-selected object from the WISE all-sky survey. Objects like this are typically dusty or active galaxies detected primarily at mid-infrared wavelengths. Its presence here further confirms the depth of the data, spanning not only vast distances but multiple observational catalogs and wavelength regimes.

The entire scene was captured with a Celestron C11 telescope using a focal reducer, totaling 70 exposures of 180 seconds (3.5 hours of integration). For such a faint planetary nebula, this combination of aperture, exposure length, and restrained processing reveals both the subtle structure of Abell 39 and the fragile background galaxies without sacrificing realism or depth.

This image quietly spans an extraordinary range of distance — from a dying star a few thousand light-years away to galaxies over a billion light-years beyond — all recorded in a single, understated frame.