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article imageAstronomers solve decades old celestial archaeology mystery

By Robert Myles     Mar 26, 2014 in Science
Leicester - New research from an international team of astronomers from the UK's University of Leicester and the University of Arizona sheds light on how stars reaching the end of their useful life and collapsing in on themselves become polluted.
Their conclusions point to the worst-case scenario for Earth. Billions of years hence, our rocky world will be swallowed up the rapidly expanding Sun, before everything be it rocks, oceans, civilizations and all humanity’s material possessions end up as an eternal flotsam in what remains of the Sun, “pollutants.”
Perhaps one day, in the far future, a distant alien civilization will scratch their heads over how our future Sun came to contain the remnants of man-made elements when humanity has long since vanished — Sic transit gloria mundi.
The Leicester and Arizona scientists investigated hot, young, white dwarf stars. These super-dense celestial bodies are the remnants of stars, of a size up to about eight times the size of our Sun, when they run out of fuel in the form of hydrogen.
“Roll on, ye Stars! Exult in youthful prime...Flowers of the sky! Ye too to age must yield”
The Sun’s estimated to be about halfway through its store of hydrogen, which, for the past five billion years, it’s been converting to helium by a process of nuclear fusion. When the Sun has no readily available hydrogen left, it’ll undergo a process of expansion and contraction, first causing it to expand beyond the orbit of Mercury, becoming a Red Giant star in the process.
The Sun will then spend the next one billion years or so as a Red Giant before it contracts, but then undergoes a second expansion causing the envelope of the Sun’s outer atmosphere to reach well beyond Earth’s orbit. At that stage, all that Earth has become will be swallowed up, although long before then Earth will have become uninhabitable. Finally, the Sun will collapse in on itself taking Earth with it, and becoming a white dwarf.
Typically, a white dwarf has about the same mass as the Sun but measures roughly the size of Earth, making white dwarfs extremely dense bodies, their density exceeded only by that of neutron stars and black holes.
For decades, scientists have known that the atmospheres of many white dwarfs, consisting principally of pure hydrogen or pure helium, were contaminated by other elements such as carbon, silicon and iron. Unknown was how these elements, called “metals” in astronomical parlance, got there.
Lead author of the new study, University of Leicester’s professor Martin Barstow, who is also president-elect of the Royal Astronomical Society, said, “The precise origin of the metals has remained a mystery and extreme differences in their abundance between stars could not be explained," adding, “It was believed that this material was "levitated" by the intense radiation from deeper layers in the star."
But during the researchers’ survey of 89 white dwarfs, the scientists discovered that many of the stars showed signs of contamination by rocky material, the left-overs from planetary systems, opening up a new branch of celestial archaeology. As professor Barstow put it, “we are studying the 'ruins' of rocky planets and/or their building blocks, following the demise of the main star.”
Their research, featured in MNRAS- the Monthly Notices of the Royal Astronomical Society, published by Oxford University Press, used data from the NASA/John Hopkins University’s Far Ultraviolet Spectroscopic Explorer telescope to obtain spectra — the dispersal of light by color — which detected “fingerprints” of carbon, silicon, phosphorous and sulfur where these elements were present in the atmosphere of a white dwarf.
Explaining the researchers’ findings, professor Barstow commented, “We found that in stars with polluted atmospheres the ratio of silicon to carbon matched that seen in rocky material, much higher than found in stars or interstellar gas.”
The professor went on to say that their study indicated that around one-third of all hot white dwarfs were contaminated in this way, with the debris most likely in the form of rocky minor planet analogues.
Artist s Impression of Debris around a White Dwarf Star - what our solar system might look like bill...
Artist's Impression of Debris around a White Dwarf Star - what our solar system might look like billions of years hence after the Sun has exhausted its store of hydrogen and become a white dwarf.
NASA, ESA, STScI, and G. Bacon (STScI)
These statistics are important as the search for exoplanets — planets outwith our solar system — gathers pace, with projects like the European Space Agency’s Plato planet-hunter telescope scheduled as part of the ESA’s Cosmic Vision 2015-25 Program.
Professor Barstow said his team’s study implied that a one-third proportion of stars like our Sun, as well as stars that are a little more massive like Vega and Fomalhaut, build planetary systems containing terrestrial planets. Fomalhaut’s solar system has the distinction of containing the first exoplanet or extra-solar planet visible to the eye in photographic images.
Summarizing, professor Barstow added, “The mystery of the composition of these stars is a problem we have been trying to solve for more than 20 years. It’s exciting to realize that they are swallowing up the left-overs from planetary systems, perhaps like our own, with the prospect that more detailed follow-up work will be able to tell us about the composition of rocky planets orbiting other stars."
So now it’s confirmed. One day humanity will reach the stars, or at least one of them. Except ending up as pollutants in the swirling morass of a future white dwarf Sun might not be quite how most visualize reaching the final frontier.
More about Astronomy, Astrophysics, white dwarfs, life of stars, celestial archaeology
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