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Astronomers have identified a white dwarf star that’s so cold, its carbon core has crystallised to form one big diamond.
Image: B. Saxton (NRAO/AUI/NSF)
White dwarf stars are formed when a red giant - like our Sun - runs out of fuel. With no more hydrogen to keep it burning, the red giant will collapse on itself, shrink down to about one hundredth of its original size, and gradually get colder and dimmer over billions of years. Called white dwarves, these ancient star remnants have become tightly packed nuggets of carbon and oxygen, with a mass comparable to that of our Sun squished into the volume of the Earth.
All this cooling and compacting crystallises the white dwarf’s carbon core, and what does crystallised carbon make? Diamonds. In this case, a diamond the size of the Earth.
A new white dwarf star has been detected by a team of astronomers from observatories around the world, including the National Radio Astronomy Observatory’s Green Bank Telescope and the Very Long Baseline Array in the US and the Netherlands Institute for Radio Astronomy, and it's colder than any other white dwarf star ever found. Sitting around 900 light-years away from Earth, it’s about 11 billion years old - the same age as the Milky Way - and is 10 times fainter than any white dwarf ever discovered. This makes it super-hard to spot.
“It’s a really remarkable object," one of the astronomers, David Kaplan from the University of Wisconsin-Milwaukee in the US, told Astronomy.com. “These things should be out there, but because they are so dim, they are very hard to find."
The only reason they found it was because it had a companion - a pulsar, named PSR J2222-0137. This pulsar spins around the white dwarf at a rate of 30 times per second. Pulsars are remnants of exploded stars, and unlike white dwarves, are easy to spot, thanks to the powerful radio waves that continuously burst from them into space.
According to Astronomy.com, Einstein himself had a hand in connecting the pulsar to its companion white dwarf:
By applying Einstein’s theory of relativity, the researchers studied how the gravity of the companion warped space, causing delays in the radio signal as the pulsar passed behind it. These delayed travel times helped the researchers determine the orientation of their orbit and the individual masses of the two stars. The pulsar has a mass 1.2 times that of the Sun and the companion a mass of 1.05 times that of the Sun.
The newly discovered white dwarf star is the coolest white dwarf ever found, the researchers calculating its temperature to be around 2700 degrees Celsius (4900 degrees Fahrenheit). When you consider that some white dwarfs can reach temperatures of 200,000 degrees Celsius, our new diamond in the sky is pretty chilled. | 
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