Adam Frank could not contain his enthusiasm when we gave him an advance look ata Hubble Space Telescope image, released this morning, of a nebula called Mz3.

"Whoa! Man. Look at that.Wow! That's amazing."

Does Adam Frank enjoy what hedoes?

"Oh, yeah."

And why wouldn't he? One ofFrank's jobs involves trying to figure out what causes the colorful billowingfeatures familiar in more than a Hubble images of planetary nebulae. It's amystery that has been dogging scientists for decades, but particularly sinceHubble's keen eye began revealing structures far more complex and beautifulthan astronomers had imagined.

Death gasp

Planetary nebulae --misnamed because they looked like planets when astronomers first spotted themwith crude telescopes -- are bubbles of gas and dust blown off old stars. Thestars are still hot and bright. In their last gasp before collapse, theyilluminate the gas and make the structures visible.

Many of these planetarynebulae are roughly spherical in shape. But more than half are much morecomplex. Dizzyingly complex figure eights and hourglass shapes, with multipleloops and bubbles frequently shooting off in two opposite directions, leaveastrophysicists dazzled and puzzled.

Beautiful? No question.Understood? Not at all.

Frank, an astrophysicist atthe University of Rochester, has been working with several colleagues on anidea that might explain the features. In the Jan. 25 issue of the journal Nature,the researchers argue that looping magnetic fields, similar to those generatedby our Sun, are behind the beauty.

They say that as matterblows off a dying star, it follows the invisible, twisted magnetic field lines,forming the contorted shapes we see.

But there's a theoreticalproblem with this seemingly simple explanation: Old stars are not supposed tohave strong magnetic fields.

A new picture

A star like our middle-agedSun has a magnetic field because different regions rotate at different rates.Scientists call this "differential rotation." But when stars get old,their overall rotation slows down, and so it has been assumed that there isless potential for differential rotation.

But when Frank and fellownebula expert Eric Blackman put their heads together with a trio of magneticfield experts -- John Thomas, Hugh Van Horn and Andrew Markiel -- a new pictureof old stars emerged.

The team created a computermodel showing how the unseen core of an old star might contract, forcing it tospin more rapidly. The effect is the same as when a ballerina pulls her armsin. Meanwhile, the outer portion of the star dislodges from the core andexpands, slowing the rotation on the surface of the star.

Besides explaining how oldstars might generate a strong magnetic field, the work predicts a new end for astar closer to home.

"That's really the bigsurprise here," Frank said. "When a star like the Sun gets to the endof its lifetime, it will have a very rapid differential rotation -- the insiderotating fast, the outside rotating slow -- in just the right way that you cangenerate a strong magnetic field. And then the magnetic field can produce theexotic shapes you see in these nebulae."