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EarthSky // Science Wire, Space Release Date: Jun 24, 2014

Earth-sized diamond in space is coolest white dwarf star

The coldest, dimmest white dwarf star ever detected is so cool that its carbon has crystallized, forming – in effect – an Earth-sized diamond in space.

Artist impression of a white dwarf star in orbit with pulsar PSR J2222-0137. It may be the coolest and dimmest white dwarf ever identified. Image credit: B. Saxton (NRAO/AUI/NSF)

Artist impression of a white dwarf star in orbit with pulsar PSR J2222-0137. It may be the coolest and dimmest white dwarf ever identified. Image credit: B. Saxton (NRAO/AUI/NSF)

A team of astronomers has identified possibly the coldest, faintest white dwarf star ever detected. This ancient stellar remnant is so cool that its carbon has crystallized, forming — in effect — an Earth-size diamond in space.

White dwarfs are the extremely dense end-states of stars like our sun that have collapsed to form an object approximately the size of Earth. Composed mostly of carbon and oxygen, white dwarfs slowly cool and fade over billions of years. The object in this new study is likely the same age as the Milky Way, approximately 11 billion years old.

Pulsars are rapidly spinning neutron stars, the superdense remains of massive stars that have exploded as supernovas. As neutron stars spin, lighthouse-like beams of radio waves, streaming from the poles of its powerful magnetic field, sweep through space. When one of these beams sweeps across Earth, radio telescopes can capture the pulse of radio waves.

The pulsar companion to this white dwarf, dubbed PSR J2222-0137, was the first object in this system to be detected. These first observations revealed that the pulsar was spinning more than 30 times each second and was gravitationally bound to a companion star, which was initially identified as either another neutron star or, more likely, an uncommonly cool white dwarf. The two were calculated to orbit each other once every 2.45 days.

The pulsar was then observed over a two-year period. These observations pinpointed its location and distance from Earth — approximately 900 light-years away in the direction of the constellation Aquarius.

Bart Dunlap is a graduate student at the University of North Carolina at Chapel Hill and one of the team members. He said:

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. 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). Our sun at its center is about 5,000 times hotter.

Astronomers believe that such a cool, collapsed star would be largely crystallized carbon, not unlike a diamond. Other such stars have been identified and they are theoretically not that rare, but with a low intrinsic brightness, they can be deucedly difficult to detect.

A paper describing these results is published in the Astrophysical Journal.

Read more from the National Radio Astronomy Observatory