Abell Catalog of Planetary Nebulae

Lore & Background

The catalog was composed of 86 entries thought to be planetary nebulae. About half of the objects were discovered by Albert George Wilson, with the remainder discovered by Abell, Robert George Harrington, and Rudolph Minkowski. All discoveries were made before August 1955 on photographic plates from the 48-inch Samuel Oschin telescope at Mount Palomar, as part of the National Geographic Society – Palomar Observatory Sky Survey.

Reader's Guide

The Abell Catalog of Planetary Nebulae remains a significant reference for astronomers studying these objects. Four of its entries were already known from previous catalogs: Abell 50 (NGC 6742), Abell 75 (NGC 7076), Abell 37 (IC 972), and Abell 81 (IC 1454). Four others were later rejected as not being planetaries: Abell 11 (reflection nebula), Abell 32 (red plate flaw), Abell 76 (ring galaxy PGC 85185), and Abell 85 (supernova remnant CTB 1, noted as possibly such in Abell's 1966 paper). Three more were not included in the Strasbourg-ESO Catalogue of Galactic Planetary Nebulae: Abell 9, Abell 17 (red plate flaw), and Abell 64. The planetaries in the list are best viewed with a large aperture telescope (e.g., 18-inch) and an OIII filter.

Did You Know?

• The catalog was created in 1966 by George O. Abell and contained 86 entries.
• About half the objects were discovered by Albert George Wilson.
• Four entries were later rejected as not being planetary nebulae, including Abell 76, which is a ring galaxy.
• Abell 85 was noted in Abell's 1966 paper as possibly a supernova remnant.
• Only Abell 17 was omitted from the Strasbourg-ESO Catalogue of Galactic Planetary Nebulae; Abell 9 and Abell 64 are included.

The Misleading Name and Early Encounters

The name 'planetary nebula' is one of astronomy's most persistent misnomers. These objects have absolutely nothing to do with planets; they are expanding, glowing shells of ionized gas flung outward by red giant stars near the end of their existence. The label stuck because early telescopic observers saw small, round, planet-like disks against the starfield. In January 1779, the French astronomer Antoine Darquier de Pellepoix sketched the Ring Nebula and noted it was 'as large as Jupiter and resembles a fading planet.' A few years later, William Herschel, who had just discovered Uranus, encountered similar round objects and compared them to planets 'of the starry kind.' He eventually catalogued seventy-eight such objects in his Class IV list, though most turned out to be distant galaxies. Charles Messier had spotted the Dumbbell Nebula in Vulpecula back in 1764, long before anyone had a name for the class. Despite the inaccuracy, the term 'planetary nebula' has remained firmly embedded in astronomical vocabulary ever since, a small fossil of eighteenth-century visual impression surviving well into the modern era.

Cracking the Spectral Code

For nearly a century after the first telescopic sketches, astronomers had no real idea what planetary nebulae actually were. That changed in the mid-nineteenth century when spectroscopy entered the field. On August 29, 1864, William Huggins pointed his prism at the Cat's Eye Nebula and recorded something startling: instead of the smooth continuum with dark absorption lines he saw in stars and in the Andromeda Nebula, this object blazed with discrete emission lines. The brightest sat at 500.7 nanometres, a wavelength matching no known element. The natural assumption was a new element, and the working name 'nebulium' was coined, echoing how helium had been identified in the solar spectrum a few years earlier. Unlike helium, however, nebulium was never isolated on Earth. In the early 1900s, Henry Norris Russell offered a more economical explanation: the line came from a familiar element under conditions never encountered in a laboratory. By the 1920s, physicists confirmed that in extremely low-density gas, electrons can linger in metastable energy levels that collisions would normally quench. Transitions from those levels in nitrogen and oxygen ions produce the 500.7-nanometre line and its companions, now called forbidden lines. The key revelation was that nebulae are composed of extraordinarily rarefied gas.

A Brief, Brilliant Final Act

Every planetary nebula is the final, spectacular exhalation of a star whose mass falls between roughly one and eight times that of the Sun. When such a star exhausts its nuclear fuel, it swells into a red giant and gradually sheds its outer atmosphere into space. Once that envelope has fully dissipated, the hot, dense core that remains—termed the planetary nebula nucleus, or PNN—bathes the surrounding ejected gas in intense ultraviolet radiation. That ultraviolet light ionizes the expanding shell, and as the ionized atoms recombine they emit visible photons, painting the nebula in vivid colors. The entire spectacle is remarkably brief on cosmic timescales, lasting perhaps only a few tens of thousands of years, a fleeting interval compared with the billions of years a star spends in earlier evolutionary phases. Our own Sun is expected to follow this same path, producing a planetary nebula at the very end of its life. The central stars of these objects are extraordinarily hot, and it is precisely their ultraviolet output that powers the glow we observe.

Shapes, Secrets, and Stellar Recycling

Before the Hubble Space Telescope began operations in the 1990s, many astronomers assumed planetary nebulae were roughly spherical, symmetric shells. Hubble's sharp images shattered that assumption, revealing an astonishing diversity of shapes and structural features. Only about one-fifth of known planetary nebulae are close to spherical; the vast majority display complex, non-symmetric structures. The physical mechanisms responsible for this variety remain only partially understood, though candidate drivers include binary central-star systems, stellar winds, and magnetic fields that may play a role in shaping the outflow. Beyond their visual beauty, planetary nebulae serve as vital agents of galactic chemical evolution. By expelling the elements forged in their progenitor stars into the surrounding interstellar medium, they enrich the raw material from which future stars and planets will form. Astronomers also detect planetary nebulae in more distant galaxies, and the spectral data from those remote objects provide valuable constraints on the chemical abundances of those systems, offering a window into how element distributions have evolved across cosmic time.

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