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
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