Astronomers have found a glowing bubble of hot gas and an unexpected X-ray-bright star within the planetary nebula known as the Cat's Eye. They say it could help them understand how stars like our Sun end their lives.

The planetary nebula - the name is a misnomer, it has nothing to do with planets - is the result of a star blowing off its outer gas layers during the last few million years of its life. The Cat's Eye Nebula, also known as NGC 6543, is three thousand light-years distant.

A Hubble Space Telescope (HST) picture, taken at optical wavelengths, has become something of a classic in astronomical imagery.

The new observations, obtained by the orbiting Chandra X-ray telescope, can now be combined with the HST data to see where hotter, X-ray emitting gas appears in relation to cooler material detected at optical wavelengths by Hubble.

Cooling process

"Despite the complex optical appearance of the nebula, the X-ray emission illustrates unambiguously that the hot gas in the central bubble is driving the expansion of the optical nebula," said You-Hua Chu of the University of Illinois.

"The Chandra data will help us to better understand how stars similar to our Sun produce planetary nebulae and evolve into white dwarfs as they grow old."

The Chandra image shows a bright, central star surrounded by a cloud of gas heated to many millions of degrees - and it presents something of a new puzzle. Although incredibly hot, the gas is actually cooler than scientists would have expected.

At first, the researchers thought that a cooler, outer shell of gas might have mixed with the energetic material closer to the central star to create a "lukewarm" area. However, Chu and her colleagues found that the chemical make-up of the hot gas was like that in the wind from the star, and different from the cooler outer material. These results indicate that mixing is not occurring, and that the cooling between the inner and outer shells of material is due to some other process.

Graceful structures

The intensity of the X-rays from the central star was also unexpected. The star itself has a surface temperature of about 60,000 degrees, whereas the X-ray measurement indicates a temperature of a few million degrees.

Martin Guerrero, of the University of Illinois, said: "This is the first time we see such X-ray emission from the central star of a planetary nebula."

Stars like our Sun eventually expand in their old age as they exhaust their nuclear fuel. And near the very end of their lives, they puff off their outer layers to leave small, dense cores known as white dwarfs.

Fast winds emanating from these cores ram into the ejected "atmospheres", pushing them outward and creating the graceful, filamentary structures seen with optical telescopes like the HST.

The new research was presented to a meeting of the American Astronomical Society in San Diego, California, and will be submitted to the Astrophysical Journal.