Pulsar Creation Theory Remodeled

A three-dimensional computer model that shows how pulsars obtain their spin could lead to a greater understanding of what happens when stars die.

John Blondin, professor of physics in North Carolina State University's College of Physical and Mathematical Sciences, with colleague Anthony Mezzacappa at the Oak Ridge National Laboratory, used the CRAY X1E supercomputer to develop a 3-D model of a pulsar’s creation and in the process discovered that conventional wisdom concerning the formation of these celestial objects wasn’t correct. Their findings are published in the Jan. 4 edition of the journal Nature.

Volume rendering of 3-D simulation of a pulsar's formation.

Pulsars are rapidly rotating neutron stars formed in supernova explosions, which occur when a massive star reaches the end of its life and explodes. The remaining matter is compressed into a dense, rapidly spinning mass -- a neutron star, or pulsar -- so-called because scientists first discovered them due to their regularly timed radio emissions.

“Picture something about the mass of the sun being pushed down to the size of a small American city, like Raleigh,” Blondin said. “That’s what happens when a neutron star is formed. We’ve known about pulsars since the 1960s. We can determine how fast they’re spinning by how rapidly they pulse. It’s like a searchlight on a lighthouse -- each time the pulsar spins, and emits a radio pulse directed toward earth, we pick up on it. The period between the pulses tells us how fast it’s spinning.”

Pulsars spin very rapidly -- 20 or more times per second. Scientists have assumed that the spin was caused by the conservation of angular momentum from a star that was spinning before it exploded.

“Think about figure skaters,” Blondin said. “They start a spin with their arms and legs farther out from the body, and increase their rotation speed when they pull their limbs in more tightly. That’s what the conservation of angular momentum is -- the idea that if you take a large object with a slight rotation and compress it down, the rotation speed will increase.”

However, scientists had no idea if the stars that were producing the pulsars were even spinning to begin with. Blondin and his colleague decided to create a computer model of a supernova explosion using the new Cray X1E supercomputer at the National Center for Computational Sciences, the only computer with enough processing power to accomplish the task. The resultant model demonstrated that a pulsar’s spin doesn’t have anything to do with whether or not the star that created it was spinning; instead, the spin is created by the explosion itself.

“We modeled the shock wave, which starts deep inside the core of the star and then moves outward,” Blondin said. “We discovered that as the shock wave gains both the momentum and the energy needed to blow outward and create the explosion, it starts spiraling all on its own, which starts the neutron star at the center of the star spinning in the opposite direction. None of the previous two-dimensional modeling of supernova explosions had picked up on this phenomena.”

Blondin said he hopes this new information about the creation of pulsars will lead to a greater understanding of supernova explosions.

“Supernova explosions produce many of the heavy elements found on the periodic chart, like gold,” he said. “Understanding these explosions can help us to better understand our own planet and solar system.”

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