A new study of a familiar face in space suggests how some dying stars make the shapes they do.
The object, called the Owl Nebula, is a planetary nebula in the constellation Ursa Major. Planetary nebulae have nothing to do with planets but looked like the fuzzy disks of planets in early telescopes.
Named for its obvious similarity to the face of the carnivorous bird of prey, the Owl Nebula, officially called NGC 3587, has a complex structure consisting of three concentric shells. A faint outer halo surrounds a circular middle shell and a roughly elliptical inner shell. The inner shell houses a bipolar cavity that forms the owls eyes, and two areas of enhanced brightness are seen as the owls forehead and beak.
In an article published in the June 2003 Astronomical Journal, researchers from the University of Illinois at Urbana-Champaign, the Instituto de Astrofisica de Canarias in Spain, and Williams College in Williamstown, MA, present what they say is the first cohesive model for the appearance and evolution of the Owl Nebula.
The model may help explain the shapes of other, similar planetary nebulae.
The researchers concluded that the halo of the Owl was formed when the parent star first underwent significant mass loss after fusion at its core stopped. Instabilities then produced a stellar wind, driven by a combination of stellar pulsations and radiation pressure.
Evolution of the Owls parent star caused the stellar wind to intensify to a "superwind," driving even more gas and dust outward to form the middle shell. A subsequent faster stellar wind compressed the superwind to form the inner shell and bipolar cavity, but that wind has since ceased.
The cavity is currently being back-filled with nebular material in the absence of the fast stellar wind, much as air flows back out of a balloon if you stop blowing into it.
"Different evolutionary models can produce the same structure for the nebula, but until now none has been able to also account for its motion," said Martin Guerrero of the University of Illinois, the lead author of the recent study. "There are many investigations of physical structures of planetary nebulae, but most studies only look at one piece of data and tend to ignore the bigger picture."
Observations were made with the William Herschel Telescope in La Palma, Spain, and the 0.6-meter Burrell Schmidt telescope at Kitt Peak National Observatory.
Other planetary nebulae show triple-shell structure similar to the Owl Nebula and it is likely that they followed this same evolutionary path, said co-author Karen Kwitter of Williams College. "These nebulae form an illuminating sample to study, and the Owl Nebula is the nearest one, only about 2,000 light-years from Earth."
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