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Planetary nebulae come in an assortment of graceful and intricate forms. For years astronomers have wondered how such irregular objects could be produced by perfectly spherical stars. Now researchers have caught one of these shining suns in the act of nebula formation. Their findings, published online today by the journal Nature, could help explain the unexpected structure of the resulting celestial cloud.
After a dying star consumes its last bit of hydrogen, it begins burning heavier elements at higher temperatures. It then inflates to many times its normal size, becoming an enormous red giant. At a certain point, the now huge body expels its outer gas atmosphere and subsequently collapses into a dense, hot white dwarf. Emitting massive amounts of radiation, this tiny entity causes the previously ejected gas to glow, forming a radiant shell —or planetary nebula— around the dwarf. In the present study, Hiroshi Imai of Japan¿s National Astronomical Laboratory and his colleagues examined a star on the verge of becoming a nebula using the Very Large Baseline Array radio telescopes. They discovered two streams of water molecules spewing from the star at 325,000 miles an hour [see artist's conception above]. "The path of the jets is curved like a corkscrew," observes team member Philip Diamond of the University of Manchester in England. "[It is] as if whatever is squirting them out is slowly rotating, like a child¿s top wobbles just before it falls down."
Scientists believe that such molecular fountains may help create the complex morphologies of planetary nebulae. But the details of how and why this happens remain to be discovered. "Traditional wisdom says that it takes a disk of material closely orbiting the star to produce jets, but we don¿t yet know how such a disk could be produced around such an old star," Diamond comments. Only continued probing will turn up answers to these and other questions. What is certain is that the timing of the new observation was fortuitous. "Our analysis of the water jets indicates that they are only a few decades old," Imai notes. "Once the star collapses of its own gravity into a dense white dwarf, its intense ultraviolet radiation will rip apart the water molecules, making observations such as ours impossible."