A new set of extremely high resolution images from the Hubble Space Telescope show that the last gas of a dying star explodes into space in two surprisingly narrow jets.

Taken with the Near Infrared Camera Multi-Object Spectrometer (NICMOS) on March 28, 1998, these best-ever snapshots are challenging astronomer's understanding of a rare moment near the end of a star's life.

As stars like the prosaically named OH231.8+4.2 seen in the images age, they expand from a ball roughly the size of our sun into red giants.

Nuclear fusion inside the red giant inflates the star and powers a 700,000 kilometer-per-hour wind that eventually carries off most of the mass of the star, leaving behind only the 200,000 degree core of the original star.

Although astronomers routinely find the glowing bubbles of gas -- called planetary nebulae -- left behind by this process, they rarely catch stars in the act of ejecting gas.

"The transition time is only a few thousand years," says University of Arizona astronomer John Bieging, which is only a fraction of a percent of a typical star's multi-million-year lifetime.

The new images make OH231.8+4.2 -- nicknamed the Rotten Egg nebula because it contains high concentrations of hydrogen sulfide -- by far the best studied of the few known examples of stars evolving from red giants to planetary nebulae. 

Even the tiny ember-like core of the star shine through the dust ejected from the red giant.

"Since we can see the central star, we can tell exactly how the jets emerge," says Caltech astronomer William Latter.

And "now that the geometry is understood, we have a clear idea of how much mass leaves the star, and at what velocity," adds Rochester Institute of Technology astronomer Joel Kastner.

In addition, the images pose a difficult question: How does a spherical star form such decidedly non-spherical things as the amazingly narrow jets in the Rotten Egg nebula?

"It's a real mystery," says Bieging, who notes that theorists are already busy constructing detailed computer models of the star in hopes of explaining the new pictures.

By Mark Sincell, Discovery News Brief

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