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Seeing a Star's Final Moments.

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Star's Final Moments.
Star's Final Moments: Image credit: Hubble.

Although stars can burn for billions of years, their final stages can take a relatively short period of time. In many cases, it only takes a few hundred thousand years for dying stars to slough off their outer layers to create the familiar planetary nebula. Since they happen so quickly, they're relatively rare to find, but Astronomers think they've got a candidate with a relatively nearby star called V Hydra. The star is in its final stages, and jets of material have just begun emanating from it.

It takes only a few hundred to a thousand years for a dying Sun-like star, many billions of years old, to transform into a dazzling, glowing cloud called a planetary nebula. This relative blink in a long lifetime means that a Sun-like star's final moments - the crucial phase when its planetary nebula takes shape - have, until now, gone undetected.

In research reported in the Nov. 20 issue of Nature, Astronomers led by Dr. Raghvendra Sahai of NASA's Jet Propulsion Laboratory, Pasadena, Calif., have caught one such dying star in the act. This nearby star, called V Hydrae, has been captured by the Space telescope Imaging Spectrograph onboard NASA's Hubble Space Telescope in the last stages of its demise, just as material has begun to shoot away from it in a high-speed jet outflow.

While previous studies have indicated the role of jet outflows in shaping planetary nebulae, the new findings represent the first time these jets have been directly detected.

"The discovery of a newly launched jet outflow is likely to have a significant impact on our understanding of this short-lived stage of stellar evolution and will open a window onto the ultimate fate of our Sun," said Sahai.

Other institutions contributing to this paper include: University of California, Los Angeles; Princeton University, Princeton, New Jersey; Harvard-Smithsonian Center for Astrophysics, Cambridge, Massachusetts; and Valdosta State University, Valdosta, Georgia.

Low-mass stars like the Sun typically survive around ten billion years before their Hydrogen fuel begins to run out and they start to die. Over the next ten to hundred thousand years, the stars slowly eject nearly half of their mass in expanding, spherical winds. Then - in a poorly understood phase lasting just 100 to 1,000 years - the stars evolve into a stunning array of geometrically shaped glowing clouds called planetary nebulae.

Just how these extraordinary "star-clouds" are shaped has remained unclear, though Sahai, in several previous papers, put forth a new hypothesis. Based on results from a recent Hubble Space Telescope imaging survey of young planetary nebulae, he proposed that two-sided, or bipolar, high-speed jet-like outflows are the primary means of shaping these objects. The latest study will allow Sahai and his colleagues to test this hypothesis with direct data for the first time.

"Now, in the case of V Hydrae, we can observe the evolution of the jet outflow in real-time," said Sahai, who together with his colleagues will study the star with the Hubble Space Telescope for three more years.

The new findings also suggest what may be driving the jet outflows. Past models of dying stars predict that accretion discs - swirling rings of matter encircling stars - may trigger jet outflows. The V Hydrae data support the presence of an accretion disc surrounding, not V Hydrae itself, but a companion object around the star. This companion is likely to be another star or even a giant planet too dim to be detected. The authors have also found evidence for an outlying large dense disc in V Hydrae, which could enable the formation of the accretion disc around the companion.

Further support in favor of a companion-driven jet outflow comes from the scientists' observation that the jet fires in bursts: because the companion orbits the star in a periodic fashion, the accretion disc around it is expected to produce regular spurts of material rather than a steady stream.

The Space telescope Imaging Spectrograph is managed by NASA's Goddard Space Flight Center in Greenbelt, Maryland. The Hubble Space Telescope is a project of international cooperation between NASA and the European Space Agency. The California Institute of Technology, Pasadena manages JPL for NASA.

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