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Naked White Dwarf Shows its Dead Stellar Engine

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A curiously naked white dwarf star, devoid of any perceptible atmosphere, is giving astronomers their first clear look at the nuclear engines that keep stars burning bright.

About the size of Earth, the dead star is also the hottest white dwarf ever detected by astronomers -- some 30 times the average temperature of the Sun -- leading them to believe it only recently shut down its nuclear reactor within the last 100 years.

"Cosmologically, it is very young," said astronomer Martin Barstow, who led the white dwarf study, during a telephone interview. "It's been doing things within our historical time scale."

Researchers believe the white dwarf, dubbed H1504+65, finished burning its nuclear fuel just a few tens of thousands of years ago, making it the youngest such object on record. The result, they said, is a smoldering stellar relic with a temperature about 200,000 degrees Celsius.

"It's like a little glowing coal," said Barstow, a researcher with the University of Leicester in the United Kingdom.

A white dwarf is the end result of a star that is not massive enough to end its life in a spectacular explosion called a supernova, which redistributes stellar material back into the cosmos. Stars up to eight times as massive as the Sun will end up as white dwarfs - the Sun included - after going through a series of red giant phases, during which they shed most of their outer atmosphere.

But what sets H1504+65 apart is its lack of any hydrogen or helium envelope that normally obscures a white dwarf's nuclear core from astronomers' telescopes. In all other cases to date, at least a thin film of either hydrogen or helium remains to shroud a white dwarf's inner core.

"We're still digesting what some of this means," explained Jeff Kruk, a principal research scientist at Johns Hopkins University in Baltimore, Maryland who also worked on the study. "Usually, that little thin film hides everything."

Cosmic shedding

Because of its lack of atmosphere, H1504+65 allowed researchers to use space-based Chandra X-ray observatory and Far Ultraviolet Spectroscopic Explorer (FUSE) telescope to determine its composition and extrapolate its death throes.

"There's a lot of uncertainty into how long nuclear fusion can carry on transmuting material within stars," Barstow said.

Stars typically consume hydrogen gas through the process of nuclear fusion, which generates helium - an element that can be fused even more into carbon and oxygen. As they run out of fuel, these stars become unstable and periodically shed their outer layers until finally, once they've exhausted all of the available fuel, they collapse into white dwarfs. But if a star is massive enough, it may continue to burn its carbon and oxygen stores as well, converting them into magnesium, which apparently occurred in H1504+65, researchers said, adding that the element appears alongside carbon and oxygen in the cooling star remnant.

"So it may have gone a stage further than other stars," Barstow said. "But we're still also looking at carbon and oxygen, and are able to look at the original nuclear reactor for this star."

It is also possible for magnesium to be produced in stars as a byproduct of helium fusion. Taking that possibility into account, Barstow and his researchers hope to confirm their magnesium find in the next year by using the Hubble Space Telescope to search for sodium within the white dwarf. That element, they said, will clinch the carbon fusion process.

"One of the lessons I come away with is that you can still always be surprised about what you're going to find," Kruk said. "The universe is a strange place and everything you look at is something new."


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