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X-Ray Observations Reveal Gleam in Cat's Eye Nebula
Since the discovery in 1994 of the exotic Cat's Eye Nebulae, astronomers have puzzled over its eerie feline appearance. Planetary nebulae are clouds of gases cast into space by a star as it nears the end of its life. Most look like cocoons, which is why early astronomers incorrectly dubbed them as young planets. The Cat's Eye Nebulae, though, looks like a giant eye peering out from space. Its glittering "cornea" is composed of intricate structures in the shape of knots, bubbles, and filaments.
Now LAS astronomers believe they know how the eye got its gleam. X-ray images taken by You-Hua Chu and James Kaler using NASA's Chandra X-ray Observatory show "jets of hot gases" responsible for the nebulae's intriguing structure. The Chandra observations also revealed a previously unknown X-ray source at the central star.
"Optical images reveal an inner elliptical shell surrounded by an envelope with multiple, interlocking lobes bounded by sharp, filamentary structures. Despite the complex appearance of the Cat's Eye, the X-ray emission shows that hot gas is driving the expansion," says Chu.
A powerful stellar wind emanating from the star's hot core slams into the ejected gas, pushing it outward into the graceful knots, bubbles, and filaments seen in optical telescopes.
"In the Cat's Eye, the stellar wind is traveling at about 4 million miles per hour," Chu says. "When this fast wind encounters gas that was previously expelled, it is shock-heated to hundreds of millions of degrees, generating the X-rays we saw with Chandra."
"The diffuse X-ray emission showed excellent correspondence with some of the optical features in the denser and cooler outer shell," Chu says. "The thermal pressure of the hot gas is about twice as high as the pressure in the cool nebular shell. Clearly, the hot gas is playing an essential role in the structure and evolution of the Cat's Eye Nebula."
The Cat's Eye Nebula, also known as NGC 6543, is 3,000 light-years from Earth and was formed about 1,000 years ago. Scientists believe that our own sun will be in a similar stage in five to six million years.
Summer 2001