The Hard Heart of a White Dwarf

Physicists have created a strange type of matter thought to exist in the heart of dense stars called white dwarfs. By confining a million beryllium ions in an electromagnetic trap, the team created a crystal made of positive ions, sitting tens of thousands of times farther apart than do the positive and negative ions in a grain of salt. Because like-charged ions naturally repel each other, it normally takes the immense pressure of a star, along with the weird quantum mechanics it brings on, to get them to crystallize. The crystals, described in tomorrow's issue of Science, could be miniature labs for investigating their denser stellar counterparts, or simply for studying matter at a new scale.

After years of trapping ions in their work with atomic clocks, Wayne Itano and colleagues at the National Institute of Standards and Technology in Boulder, Colorado, realized they had just the tools to create conditions analogous to those in a white dwarf. The group took beryllium atoms, slid them into the confining electric and magnetic fields of what's called a Penning trap, and stripped off their outer electron with an electron beam. A laser cooled the atoms to a few thousandths of a degree above absolute zero, about the temperature where computer models predicted that crystallization should occur. The electromagnetic fields of the trap--taking the place of the enormous pressures of the white dwarf--forced the normally repulsive atoms together. Buoyed by the fields and still slightly warm, the atoms spun like a tiny tempest.

To see if they had a crystal, the physicists looked at how the millimeter-sized cloud diffracted laser light. A regular array of dots would indicate the regular atomic architecture of a crystal. But because of the spinning, the pattern formed a series of concentric rings, like a dart board. To take the blur out, the researchers used an electric field to spin the atom cloud at regular rate and took pictures exactly once every revolution--the equivalent of using a strobe light to see the spokes on a spinning bicycle wheel. The resulting picture showed the telltale dots--proof of the crystal.

The images are "pretty dramatic," says Itano. "We were all surprised that it worked that well." "It's great that the Boulder team has been able to demonstrate [the crystals] experimentally," says John Schiffer, a physicist at Argonne National Laboratory in Argonne, Illinois. The million-atom system is so complex, he says, that a computer simulation he's been running for a year has yet to predict how it behaves. "Experiment beat out computer simulations," he concedes.