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FOR RELEASE: January 16, 1996

PHOTO NO.: STScI-PRC96-03 and JPL-P-46536

HUBBLE FINDS SEARCHLIGHT BEAMS AND MULTIPLE ARCS
AROUND A DYING STAR

This image of the Egg Nebula, also known as CRL2688 and located roughly
3,000 light-years from us, was taken in red light with the Wide Field
and Planetary Camera 2 (WFPC2) aboard NASA's Hubble Space Telescope.
The image shows a pair of mysterious "searchlight" beams emerging from
a hidden star, criss-crossed by numerous bright arcs. This image sheds
new light on the poorly understood ejection of stellar matter which
accompanies the slow death of Sun-like stars. The image is shown in
false color. The central star in CRL2688 was a red giant a few hundred
years ago. The nebula is really a large cloud of dust and gas ejected
by the star, expanding at a speed of 20 km/s (115,000 mph). A dense
cocoon of dust (the dark band in the image center) enshrouds the star
and hides it from our view. Starlight escapes more easily in
directions where the cocoon is thinner, and is reflected towards us by
dust particles in the cloud, giving it its overall appearance. Objects
like CRL2688 are rare because they are in an evolutionary phase which
lasts for a very short time (~1,000 to 2,000 years). However, they may
hold the key to our understanding of how red giant stars transform
themselves into planetary nebulae. For the first time, we can see a
10,000 year-old history of mass-ejection in a red giant star in such
exquisite detail. The arcs in CRL2688 represent dense shells of matter
within a smooth cloud, and show that the rate of mass ejection from the
central star has varied on time scales of ~100 to 500 years throughout
its mass-loss history. With Hubble we have detected matter in this
nebula to a radius of 0.6 light-years -- much further out than has been
possible before, giving a better estimate of the amount of matter in
the nebula. Other unexpected results seen in this image are the very
sharply defined edges of the beams and fine spoke-like features which
suggest that, contrary to previous models, the searchlight beams are
formed as a result of starlight escaping from ring-shaped holes in the
cocoon surrounding the star. The spoke- like features result from
shadows cast by blobs of material distributed within the region of the
ring-like holes. Such holes may be carved out by a wobbling,
high-speed stream of matter -- they will play a crucial role in the
shaping of the planetary nebula which will result from CRL2688.
Alternatively, the searchlight beams may result from starlight
reflected off fine jet-like streams of matter being ejected from the
center, and confined to the walls of a conical region around the
symmetry axis. Such fine jets are not unprecedented: they have
recently been observed in Hubble images of a planetary nebula (the
Cat's Eye Nebula). Both the above scenarios require the ejection of
high-speed material in a narrow beam. The presence of such material in
CRL2688 has been inferred from other observations. However, the
mechanism for ejecting high-speed jets or for producing the cocoon are
not understood. But it seems likely that if the central star in such
objects has a faint companion star, the gravitational interaction
between the two stars and/or the outflowing matter from the red giant
star may play an important role in the production of the cocoon and the
jets.

When Sun-like stars get old, they become cooler and redder, increasing
their sizes and energy output tremendously: they are called red giants.
Most of the carbon (the basis of life) and particulate matter (crucial
building blocks of solar systems like ours) in the universe is
manufactured and dispersed by red giant stars. When the red giant star
has ejected all of its outer layers, the ultraviolet radiation from the
exposed hot stellar core makes the surrounding cloud of matter created
during the red giant phase glow: the object becomes a planetary
nebula. A long-standing puzzle is how planetary nebulae acquire their
complex shapes and symmetries, since red giants and the gas/dust clouds
surrounding them are mostly round. Hubble's ability to see very fine
structural details (usually blurred beyond recognition in ground-based
images) enables us to look for clues to this puzzle.

Credit: Raghvendra Sahai and John Trauger (JPL), the WFPC2 science
team, and NASA

Image files in GIF and JPEG format and captions may be accessed
on Internet via anonymous ftp from oposite.stsci.edu in /pubinfo.

GIF JPEG
PRC96-03 Egg Nebula gif/EggNeb.gif jpeg/EggNeb.jpg

Higher resolution digital versions (300dpi JPEG) of the release
photograph will be available temporarily in /pubinfo/hrtemp:
96-03.jpg.

GIF and JPEG images, captions and press release text are available via
World Wide Web at URL http://www.stsci.edu/pubinfo/PR/96/03.html, or
via links in http://www.stsci.edu/pubinfo/Latest.html and
http://www.stsci.edu/pubinfo/Pictures.html.