Astronomers have discovered the coldest white dwarf around 900 light-years away from Earth. The white dwarf star, about 11 billion years old, and is believed to be composed mostly of crystallized carbon, making it a diamond in the sky.

Astronomers from the University of Wisconsin-Milwaukee, led by David Kaplan, used the National Radio Astronomy Observatory’s (NRAO) Green Bank Telescope and Very Long Baseline Array to discover the coolest and faintest white dwarf star to date. The research was published in the Astrophysical Journal.

Stars, up to eight times the size of the sun, at the end of their life, collapse and become white dwarfs. These incredibly dense stars are made up largely of carbon and oxygen and may have as much mass as the sun but are the size of Earth, according to the NRAO press release

The astronomers used the NRAO's Green Bank Telescope to observe a pulsar, a rapidly spinning neutron star, 900 light-years away. Pulsars "blink" in a consistent fashion and astronomers can use these stars to find nearby objects and to detect gravitational waves. The Green Bank Telescope was used to observe radio waves emitted by the pulsar, named PSR J2222-0137, and discovered the pulsar was part of a system and gravitationally bound to a companion star, NRAO notes.

After two years of observing the pulsar, astronomers determined how the companion altered the pulsar blinks in order to determine the orbit orientation and mass of the stars -- the pulsar has a mass 1.2 times that of the sun while the companion star has a mass 1.05 times that of the sun. Neutron stars are created by the supernova of a star and because of the regular, circular nature of the system's orbit the team eliminated a second neutron star as the companion star.

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While the astronomers believed they would be able to observe the white dwarf star, they were unable to, which led to the conclusion that the star is extremely cold, about 2,700 degrees Celsius at its core. For a comparison, the temperature of the sun at its core is around 15 million degrees Celsius.

"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," team member Bart Dunlap, a graduate student from the University of North Carolina at Chapel Hill, said in a statement.

As for the space diamond, ancient white dwarfs aren't uncommon in space, but they're incredibly difficult to find. "We expect a large number of old white dwarfs to be around," Kaplan, the lead astronomer, said. "They are just hard to see, and if we don’t know where to look, they are basically impossible to pick out."