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05/01/93

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THE MYSTERY OF THE FASTEST MOVING STAR STILL PUZZLING

STANFORD - Scientists who announced several weeks ago the discovery of the fastest moving star ever seen are now pondering the inevitable question: how can you explain something that large moving that fast?

How do you accelerate 2.7 octillion tons (27 followed by 26 zeros) from a standstill to over 1,800 kilometers per second, about one- half of one percent of the speed of light? That could be as fast as 4 million miles per hour.

"For a star to be moving this fast, the energies released are really enormous," said Roger W. Romani, a Stanford astrophysicist. "There is something as heavy as our sun, suddenly kicked up to such an enormous speed. That costs a lot of energy, a stupendous amount of energy."

Romani, a member of the team that initially discovered the speeding star, believes an asymmetric explosion may have created a "neutrino rocket" to start the star on its journey.

Several weeks ago, a joint team of researchers from Stanford University and Cornell University discovered the pulsar in the constellation Cepheus.

Cornell astronomer James M. Cordes and doctoral student Scott C. Lundgren worked with Romani on the program that discovered this speeding star.

"We're studying neutron stars, the collapsed remnants of very heavy stars," Romani said. "This particular project was trying to find what happened when these neutron stars dump lots of energy into the interstellar medium."

The Cornell researchers were looking for fast moving neutron stars and pulsars (a pulsar is a spinning neutron star, emitting radio signals like a lighthouse). Initially, they monitored the star using the giant radio telescope at Arecibo, Puerto Rico. After deciding it was a likely candidate for further study, they called in Romani to lend his expertise in astrophysical theory and visual astronomy.

"Jim Cordes and his grad student Scott Lundgren had spent some time working on the radio physics aspect, so it was a happy marriage with my expertise here," said Romani.

Using the 200-inch Hale telescope atop Mount Palomar, Calif., Romani and the Cornell scientists photographed the star.

Trailing behind the racing object is a sort of wake. Named the "Guitar Nebula" for its distinctive shape, this wake is the result of the star's bow shock through the interstellar medium.

A second observation revealed that the star had moved an astonishing distance, allowing the estimation of its speed.

"Our conservative estimate puts it at about 1,000 kilometers (about 600 miles) a second," Romani said. "Our best estimate is more like 1,800 kilometers per second (about 1,100 mph)."

That's the equivalent of going coast to coast in five seconds, Romani said.

The question now facing astronomers is what forces could accelerate a neutron star, weighing perhaps one and a half times as much as our own sun, up to such an enormous velocity.

As a massive star dies, it explodes in a titanic cosmic detonation, a supernova. During this blast, the star flings off much of its outer gaseous atmosphere, leaving behind the tiny and super-dense neutron star. Tremendous energies are released during the supernova blast, but that alone does not answer the question.

"We know that enough energy is available. . . . We need to find some way of tapping that energy to give the star its kick," Romani said.

Several theories have been suggested, but Romani said that one seems to carry the most credence: As a supernova explodes, it releases a vast number of tiny particles called neutrinos.

"Over the period of a second or two, a flood of neutrinos was emitted from the core of this star," he said. "It seems possible that some of them were squirted off in one direction."

An asymmetric release of neutrinos would act like a "neutrino rocket" to push the star's remains off on their journey in the opposite direction.

Romani plans to continue observing the speeding pulsar, observing its interaction with the interstellar gas and using it to unlock other cosmic mysteries.

Pulsars possess extremely strong magnetic fields. Combining this magnetic field with the pulsar's rotation produces what amounts to a giant particle accelerator with several times the energy of Stanford's Linear Accelerator Center.

The interactions of particles at these energies are of great interest to physicists.

"It's like a ready-made experiment," Romani said. "The only difficulty is observing the results."

Eventually, he hopes to study the pulsar with a German satellite-mounted X-ray telescope, as well as the Hubble Space Telescope, once it receives its corrective optics.

The public attention garnered by this object pleases Romani.

"It's serendipitous that the object is so attractive and the science is pretty significant as well," he said.

This story was written by R. Nicholas Strauss, a science writing intern at the Stanford News Service.

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