Planetary Nebulae: Shrouds of Mystery

Mercury, July/August 2002 Table of Contents

Courtesy of Bruce Balick, et al., and NASA.

by Sun Kwok

To some, planetary nebulae are the source of pretty pictures. But to the astronomers who study these stellar shrouds, planetary nebulae offer a treasure trove of fascination and mystery.

From a 10-year-old kid looking through his backyard telescope to scientists at the Space Telescope Science Institute using the Hubble Space Telescope, planetary nebulae have been the favorite observing targets of many astronomers. Although we are all fascinated by the magnificent shapes and wonderful colors of planetary nebulae, the journey to understand their origin and inner workings has been a long and difficult one.

The journey began with William Herschel’s unfortunate naming of these objects as "planetary nebulae" for their superficial resemblance to Uranus. Astronomers later associated planetary nebulae with their hot central stars, leading to the erroneous assumption that they are young, because young stars are often hot. In the late 19th century, astronomers found that planetary nebulae spectra are dominated by a pair of bright green lines of unknown origin. This led to the suggestion that these were due to an unearthly element called "nebulium" (see "Lighting the Nebulae," page 17). Astronomers abandoned this idea in the 1920s when the development of quantum physics led to the realization that these lines are due to ionized oxygen atoms radiating in near-vacuum conditions.

This identification helped spark a new discipline called "astrophysics," which involves the application of the knowledge of atomic and nuclear physics to the understanding of celestial phenomena. With these new forms of knowledge at their disposal, astronomers in the 1950s were able to interpret the optical spectra of planetary nebulae, and to derive their temperature and density conditions. These findings led Soviet astronomer Iosef Shklovskii to propose in 1956 that planetary nebulae represent a phase of stellar evolution that old, evolved stars pass through.

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