| At the end of
the sun's lifetime, in about 5 billion years, the hydrogen that fuels its
thermonuclear reactions will be spent. Our star will expand into a red
giant and begin shooting its mass into space.
After a brief period of several
thousand years, most of its mass will have been thrown off, and a dying
ember will remain of what was once the sun's core. This process of mass-loss
occurs so quickly that astronomers rarely witness it in other parts of
the universe.
Now, a team of astronomers
using the Hubble Space Telescope's near-infrared camera has captured detailed
images of a star in the violent throes of its own death. The images of
the rotten-egg nebula (visible at the right) show the object spitting its
mass into space in two different directions at speeds up to 450,000 mph
(700,000 kilometers per hour).
The nebula was given its
unflattering name because the cloud contains sulfur compounds in abundance.
What makes these images stand
out, apart from the remarkable speed of the mass-ejection, is the nebula's
unique shape, said John Bieging, an astronomer at the University of Arizona
who helped produce the Hubble images.
"All stars in this type of
(red) giant stage do eject mass, but their shape tends to be more-or-less
spherical. That is, it tends to come out uniformly in all directions,"
Bieging said. "However, when stars get close to the end of this stage of
evolution they very often produce these double-lobed bipolar kinds of ejections,"
he said.
The physics that drives this
sort of column-shaped ejection is a great mystery, Bieging said. Some theoretical
models explain that the jet-like flows are caused by magnetic field interactions,
especially in certain double-star systems.
"If you have two stars opening
each other then you can get all sorts of funny effects," he said. "So it
is likely that this is actually a double star, a binary which is very close
in separation that may be the mechanism that generates the columniation."
The star (or stars) at the
nebula's center is not visible because it is obscured in its own dirty
cloud.
The rate at which the star
is ejecting mass is so high, it likely blew off the equivalent of about
one percent of the sun's mass every 100 years at its peak. At this rate,
Bieging said he expects the star will have lost almost all its mass within
a couple thousand years.
"This mass-loss is a very
critical process because it keeps most stars from blowing up. They just
more-gently give off a large part of their original material," Bieging
said.
When it has lost most its
mass, the star at the center of the rotten-egg nebula will be only a glowing
cinder of a star in the middle of a bow tie-shaped "planetary nebula."
Such clouds actually have nothing to do with planets. The name is the legacy
of an 18th century astronomer who thought these nebula to resemble planetary
disks.
This is the end of the line
for the so-called main-sequence stars -- stars that have a mass between
about half a solar mass and eight times that of the sun. Stars more massive
than that will explode as supernovae. The giant stars evolve much faster
and aren't able to shed their mass fast enough to avoid collapsing and
rebounding in enormous explosions.
Astronomers find the mass-loss
process as seen in the rotten-egg nebula extremely important in the overall
process of stellar and galactic evolution. The material ejected by dying
stars is eventually incorporated into clouds of dust and gas that collapse
to form new stars or other celestial bodies.
"Some of the building blocks
of the terrestrial planets, including the Earth, must have come from stars
of this sort," Bieging said. In fact, recent analysis of certain meteorites
and stratospheric
dust found on Earth appear to be the same type of material that is
spewing out of the rotten egg nebula and other dying stars, he said.
Others who worked with Bieging
to produce these latest Hubble images include William Latter of the California
Institute of Technology and Casey Meakin, a graduate student at the University
of Arizona. |
