Документ взят из кэша поисковой машины. Адрес оригинального документа : http://www.arcetri.astro.it/~palla/ANDREAIMMAGINI/ISM/StarTear.txt Дата изменения: Tue Jan 30 12:59:32 2001 Дата индексирования: Sat Dec 22 08:11:15 2007 Кодировка: Поисковые слова: cygnus

EMBARGOED UNTIL: 1:00 a.m. (EDT) July 13, 2000

PHOTO NO.: STScI-PRC00-23


HUBBLE WATCHES STAR TEAR APART ITS NEIGHBORHOOD

NASA's Hubble Space Telescope has snapped a view of a stellar demolition
zone in our Milky Way Galaxy: a massive star, nearing the end of its
life, tearing apart the shell of surrounding material it blew off
250,000 years ago with its strong stellar wind. The shell of material,
dubbed the Crescent Nebula (NGC 6888), surrounds the "hefty," aging star
WR 136, an extremely rare and short-lived class of super-hot star called
a Wolf-Rayet. Hubble's multicolored picture reveals with unprecedented
clarity that the shell of matter is a network of filaments and dense
knots, all enshrouded in a thin "skin" of gas [seen in blue]. The whole
structure looks like oatmeal trapped inside a balloon. The skin is
glowing because it is being blasted by ultraviolet light from WR 136.

Hubble's view covers a small region at the northeast tip of the
structure, which is roughly three light-years across. A picture taken by
a ground-based telescope [lower right] shows almost the entire nebula.
The whole structure is about 16 light-years wide and 25 light-years
long. The bright dot near the center of NGC 6888 is WR 136. The white
outline in the upper left-hand corner represents Hubble's view.

Hubble's sharp vision is allowing scientists to probe the intricate
details of this complex system, which is crucial to understanding the
life cycle of stars and their impact on the evolution of our galaxy. The
results of this study appear in the June issue of the Astronomical
Journal.

WR 136 created this web of luminous material during the late stages of
its life. As a bloated, red super-giant, WR 136 gently puffed away some
of its bulk, which settled around it. When the star passed from a
super-giant to a Wolf-Rayet, it developed a fierce stellar wind - a
stream of charged particles released from its surface - and began
expelling mass at a furious rate. The star began ejecting material at a
speed of 3.8 million mph (6.1 million kilometers per hour), losing
matter equal to that of our Sun's every 10,000 years. Then the stellar
wind collided with the material around the star and swept it up into a
thin shell. That shell broke apart into the network of bright clumps
seen in the image. The present-day strong wind of the Wolf-Rayet star
has only now caught up with the outer edge of the shell, and is
stripping away matter as it flows past [the tongue-shaped material in
the upper right of the Hubble image].

The stellar wind continues moving outside the shell, slamming into more
material and creating a shock wave. This powerful force produces an
extremely hot, glowing skin [seen in blue], which envelops the bright
nebula. A shock wave is analogous to the sonic boom produced by a jet
plane that exceeds the speed of sound; in a cosmic setting, this boom is
seen rather than heard. The outer material is too thin to see in the
image until the shock wave hits it. The cosmic collision and subsequent
shock wave implies that a large amount of matter resides outside the
visible shell. The discovery of this material may explain the
discrepancy between the mass of the entire shell (four solar masses) and
the amount of matter the star lost when it was a red super-giant (15
solar masses).

The nebula's short-term fate is less spectacular. As the stellar wind
muscles past the clumps of material, the pressure around them drops. A
decrease in pressure means that the clumps expand, leading to a steady
decline in brightness and fading perhaps to invisibility. Later, the
shell may be compressed and begin glowing again, this time as the
powerful blast wave of the Wolf-Rayet star completely destroys itself in
a powerful supernova explosion.

The nebula resides in the constellation Cygnus, 4,700 light-years from
Earth. If the nebula were visible to the naked eye, it would appear in
the sky as an ellipse one-quarter the size of the full moon. The
observations were taken in June 1995 with the Wide Field and Planetary
Camera 2. Scientists selected the colors in this composite image to
correspond with the ionization (the process of stripping electrons from
atoms) state of the gases, with blue representing the highest and red
the lowest observed ionization.

Credits: NASA, Brian D. Moore, Jeff Hester, Paul Scowen (Arizona State
University), Reginald Dufour (Rice University)

EDITORS NOTE: Electronic image files are available on the Internet at
http://hubble.stsci.edu/go/news
http://oposite.stsci.edu/pubinfo/pr/2000/23 and via links in
http://oposite.stsci.edu/pubinfo/latest.html and
http://oposite.stsci.edu/pubinfo/pictures.html