Astronomers have assembled the first effective model for both
the shape and evolutionary history of the Owl Nebula, the well-
known planetary nebula in the constellation Ursa Major.

Named for its ghostly similarity to the face of the carnivorous
bird of prey, the Owl Nebula (NGC 3587) has a complex structure
consisting of three concentric shells. The aptly named nebula
boasts a faint outer halo, a circular middle shell, and a roughly
elliptical inner shell. The inner shell houses a bipolar cavity
that forms the owl’s “eyes,” and two areas of enhanced brightness
are seen as the owl’s “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 the first cohesive model for
the appearance and evolution of the Owl Nebula.

Using observations made with the William Herschel Telescope in
La Palma, Spain, and the 0.6-meter Burrell Schmidt telescope at
Kitt Peak National Observatory, the researchers concluded that
the halo of the Owl was formed when the parent star first underwent
significant mass loss after the cessation of fusion in its core.
The resulting instabilities then produced a stellar wind, driven by
a combination of stellar pulsations and radiation pressure.

Evolution of the Owl’s 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,” says Martin A. 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.”

Other planetary nebulae show triple-shell structure similar to the
Owl Nebula and it is likely that they followed this same evolutionary
path, according to 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.”

Despite the name, planetary nebulae are not related to planets.
Sir William Herschel gave these fascinating objects their misleading
name in 1782 because, through his telescope, they resembled the
appearance of Uranus and Neptune. In reality, planetary nebulae
are shells of gas and dust ejected from aging stars. When the mass
loss is finished, the hot core of the star is exposed, causing the
ejected gas to glow.

A newly processed image of the Owl Nebula from this study is available

The Burrell Schmidt telescope is part of the Warner and Swasey Observatory
of Case Western Reserve University, Cleveland, OH. The telescope is located
at Kitt Peak National Observatory near Tucson, AZ, which is part of the
National Optical Astronomy Observatory (NOAO). NOAO is operated by the
Association of Universities for Research in Astronomy (AURA) Inc., under
a cooperative agreement with the National Science Foundation.