ROCHESTER, N.Y. -- An intriguing new snapshot from the orbiting Chandra X-ray Observatory functions something like a coroner's photograph, showing scientists the violent details of how stars die. The image is the first ever to show the "hot bubble" that scientists have predicted to exist for a brief time as sun-like stars go through their death convulsions.
The image was released here at the meeting of the American Astronomical Society. It details a region in the planetary nebula BD+30 3639 that lies in our galaxy somewhere between 5,000 and 8,000 light-years from the sun. The nebula is what remains of a dying sun-like star. It is an expanding cloud of hot gas with a hot, compressed core at the center -- a dense star called a white dwarf. The nebula is similar to what may remain of the sun some 5 billion years from now. (The misnomer "planetary nebula" was applied to round gas clouds with sharp edges by early astronomers because they resembled dim planets.)
Scientists believe the bubble results from a high-energy collision between the star's ejected atmosphere and a new, 2 million-m.p.h. (3.2 million-kilometer-per-hour) wind that's shooting away from the white dwarf at its smoldering core.
"Theorists have told us for a long time now that when sun-like stars are in the last dying stages, such stars should be emitting X-rays from a hot bubble of gas that's formed in the collision of gas ejected as these stars die," said Joel Kastner, an astronomer at Rochester Institute of Technology and leader of the science team that announced the discovery. "We believe that this image is definitive proof of the existence of a hot bubble inside a planetary nebula."
In theory, stars like the sun turn into red giants at the end of their lives. They swell up, and the bulk of their outer layers is expelled out into space. The hot, central core that remains eventually collapses into a white dwarf. When the white dwarf is formed, it begins to emit an intense stellar wind, streams of charged particles that travel outward at more than 2 million m.p.h.
When this energetic wind rams into the expanding shell of stellar atmosphere that was shed long before, the collision produces shock waves and heats the impact area to temperatures of millions of degrees. It is this shock front that is revealed in the new Chandra image.
What is surprising to many researchers is the lopsided shape of the bubble. "This suggests that the red giant atmosphere was not ejected symmetrically," Kastner said in a prepared statement. The asymmetry may indicate that the star in its demise orbited with an unseen companion star.
Until now, scientists finding X-ray sources in regions like this planetary nebula were not certain where the X-rays were coming from. The white dwarf at the center should not be hot enough to produce X-rays. The theory of a hot bubble would explain the X-rays, but until now, that has only been theory. Previous observations have hinted that X-rays might be originating from broader regions, but this observation gives conclusive proof, the researchers said.
Chandra's data also reveal the composition of the super-shocked gas that is emitting the powerful X-rays. Kastner and his colleagues have detected a large abundance of neon gas there, which they say is evidence that the ejected atmosphere came from the deepest layers of the central star. Such deep layers are where nuclear fusion creates neon and other heavy elements.
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