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