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article imageRemarkable discovery of 'Earth-sized diamond' white dwarf star

By Robert Myles     Jun 24, 2014 in Science
Charlottesville - A team of US-based astronomers has identified an ancient stellar remnant, a white dwarf star so cool that it’s caused carbon to crystallize effectively producing an Earth-sized diamond in space.
Their “gem” of a discovery, what is possibly the coldest, faintest white dwarf star ever detected, was made using the National Radio Astronomy Observatory's (NRAO) Green Bank Telescope, in West Virginia and Very Long Baseline Array (VLBA), a system of 10 radio telescopes, operated remotely from their Array Operations Center in Socorro, New Mexico, along with other observatories.
Team member, David Kaplan, assistant professor in the Department of Physics at the University of Wisconsin-Milwaukee described the find as “a really remarkable object," adding, “These things should be out there, but because they are so dim they are very hard to find."
White dwarf stars are the Methuselahs of our galaxy, ancient objects once similar to our Sun but which have collapsed under their own gravity forming extremely dense objects composed mainly of carbon and oxygen.
Over billions of years, white dwarfs cool and fade. The likelihood is that this latest white dwarf discovered is about the same age as our Milky Way galaxy, at around 11 billion years old.
The newly discovered white dwarf is a companion star to a pulsar star catalogued as PSR J2222-0137. Pulsars are neutron stars, the super-dense end-result of stars, many times more massive than our Sun, which exploded as supernovas.
As it spins on its axis, a neutron star emits a beam of radio waves but due to a neutron star’s extremely powerful magnetic field, these radio beams are focused and emitted from their parent star in much the same way as a beam of light from a lighthouse sweeps across the sea. When one of these pulsar-originating radio beams sweeps, lighthouse-like, across Earth, radio telescopes can capture the pulse of radio waves.
The white dwarf’s pulsar companion was the first object found in this star system picked out by Jason Boyles, then a graduate student at West Virginia University in Morgantown, using the Green Bank Telescope (GBT).
Initial observations of the pulsar revealed it was spinning at more than 30 times per second. The pulsar appeared bound gravitationally to a companion star. Initially the candidates for this assumed companion were another neutron star or, more probably, a white dwarf. Calculations showed the two objects were orbiting each other every 2.45 days.
After these findings, a long period of observation followed with the pulsar being tracked for two years by Adam Deller, an astronomer at the Netherlands Institute for Radio Astronomy (ASTRON) using the VLBA.
Deller’s detective work pinpointed the pulsar’s location and distance from Earth — approximately 900 light-years away in the direction of the constellation Aquarius — information that was essential to refine the model used to time the pulses reaching Earth using the GBT.
Next, around a century after Einstein first postulated his theory of relativity, the researchers used that very same theory to study how the, as yet, unidentified companion to the pulsar's gravity warped space.
That warping of space caused a delay in the radio signal from the pulsar reaching Earth as he pulsar passed behind its stellar partner. Measuring these delays helped astronomers determine the orientation of the orbits and the individual masses of the two stars. They found the pulsar has a mass 1.2 times that of our Sun with the companion star, the white dwarf, weighing in at 1.05 times the Sun’s mass.
The data pointed strongly to the pulsar’s companion star not being a second neutron star since the orbits of the two bodies were too orderly for a second supernova to have taken place.
Having worked out the white dwarf's location to such high degree of accuracy, the astronomers knew what the white dwarf's brightness should be at that distance. They ought to have been able to observe the white dwarf in optical and infrared light, or so they thought. But neither the Southern Astrophysical Research (SOAR) telescope in Chile nor the 10-meter Keck telescope in Hawaii, among the most extremely sensitive telescopes ever constructed, was able to detect it.
"Our final image should show us a companion 100 times fainter than any other white dwarf orbiting a neutron star and about 10 times fainter than any known white dwarf, but we don't see a thing," commented Bart Dunlap, one of the team of astronomers and a graduate student at the University of North Carolina at Chapel Hill. "If there's a white dwarf there, and there almost certainly is, it must be extremely cold."
The researchers calculated that the white dwarf would be no more than a comparatively cool 3,000 degrees Kelvin (2,700 degrees Celsius). By comparison, NASA estimates that the temperature at the centre of our Sun is 1.571 x 10⁷ degrees Kelvin. Or, put another way, that’s 15,710,000 K, more than 5,000 times hotter than the white dwarf!
Although other stars similar to the white dwarf have been identified, and theoretically, at least, that type of star shouldn’t be too rare, because their relative coolness gives them a low intrinsic brightness, they hard to track down.
Astronomers believe that the constituency of such cool collapsed stars would largely be composed of crystallized carbon, not unlike a diamond.
Elusive in the sky but diamond, as The Beatles might have trilled.
More about White dwarf, white dwarf stars, pulsar, diamond star, diamonds in space
 
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