Reborn Star Surprises Astronomers

An old star that's been reborn has surprised researchers by flying through the process 100 times faster than predicted.

The star is a white dwarf, an aged Sun-like star that has used up its nuclear fuel and collapsed. A teaspoon of its material would weigh about 10 tons here on Earth.

The star, named V4334 Sgr, in the constellation Sagittarius. It is better known as "Sakurai's Object," after Japanese amateur astronomer Yukio Sakurai, who discovered it on Feb. 20, 1996, when it became suddenly bright. It's the first such outburst observed in modern times.

Theory predicts the star's nuclear furnace had re-ignited for one last blast.

"We've now produced a new theoretical model of how this process works," said Albert Zijlstra, of the University of Manchester in the United Kingdom. New observations from the National Science Foundation's Very Large Array (VLA) radio telescope support the model, Zijlstra said.

The findings are detailed in the April 8 issue of the journal Science.

Astronomers think a white dwarf's final eruption involves a burst of fusion in a shell of helium that surrounds a core of heavier nuclei such as carbon and oxygen.

Computer simulations indicated that heat-spurred convection would bring hydrogen from the star's outer envelope down into the helium shell, driving a brief flash of new nuclear fusion. This would cause a sudden increase in brightness. The original computer models suggested a sequence of observable events that would occur over a few hundred years.

"Sakurai's object went through the first phases of this sequence in just a few years -- 100 times faster than we expected," Zijlstra said. "So we had to revise our models."

The new scheme predicted the star should rapidly reheat and begin to ionize gases in its surrounding region. "This is what we now see in our latest VLA observations," Zijlstra said.

Our Sun will become a white dwarf in a few billion years, after first swelling into a red giant phase that will engulf the orbit of Earth, vaporizing anything in its way.

"It's important to understand this process," Zijlstra said. "Sakurai's Object has ejected a large amount of the carbon from its inner core into space, both in the form of gas and dust grains. These will find their way into regions of space where new stars form, and the dust grains may become incorporated in new planets. Some carbon grains found in a meteorite show isotope ratios identical to those found in Sakurai's Object, and we think they may have come from such an event. Our results suggest this source for cosmic carbon may be far more important than we suspected before."

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