British and Japanese astronomers using the Very Long Baseline Array radio telescope discovered the magnetic field emanating from a star named W43A, in the constellation Aquila. W43A is about 8,500 light-years from Earth and is in the process of forming a planetary nebula, a shell of brightly-glowing gas lit by the hot ember into which the star will collapse.

During their normal lives, stars similar to the Sun are powered by the nuclear fusion of hydrogen atoms in their cores. As they near the end of their lives they begin to blow off their outer atmospheres and eventually collapse into a white dwarf star about the size of Earth. Intense ultraviolet radiation from the white dwarf causes the gas thrown off earlier to glow, producing a planetary nebula.

In 2002, astronomers discovered the aging star was ejecting twin jets of water molecules. That discovery was a breakthrough in understanding how many planetary nebulae form elongated shapes.

Stars that produce planetary nebulae are spherical, but most nebulae are not. Instead, they show complex and often elongated shapes. The water molecules spewing from W43A are filling space nearly 100 billion miles from the old star, where they are amplifying radio waves at a frequency of 22 gigahertz. Such regions are called masers, because they amplify microwave radiation the same way a laser amplifies light radiation.

The earlier observations showed that the jets are coming out from the star in a corkscrew shape, indicating that whatever is squirting them out is slowly rotating.

"The next question was: What is keeping this outpouring of material confined into narrow jets?" said Wouter Vlemmings of the Jodrell Bank Observatory. "Theoreticians suspected magnetic fields, and we now have found the first direct evidence that a magnetic field is confining such a jet."

Vlemmings said magnetic fields had been detected in jets emitted by quasars and protostars, "but the evidence was not conclusive that the magnetic fields were actually confining the jets. These new VLBA observations now make that direct connection for the very first time."

The VLBA imaged the alignment, or polarization, of radio waves emitted by water molecules in the jets, allowing the astronomers to determine the field's strength and orientation.

"Our observations support recent theoretical models in which magnetically-confined jets produce the sometimes-complex shapes we see in planetary nebulae," said Philip Diamond, also of Jodrell Bank.

Reporting in the March 2 issue of the journal Nature, the astronomers said W43A has reached that transitional phase of its lifespan, which they think will be probably only a few decades old - so W43A offers a rare opportunity to watch the process.

Hiroshi Imai of Kagoshima University in Japan co-authored the Nature paper with Vlemmings and Diamond.

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Very Long Baseline Array radio telescope
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