White dwarf star
Researchers have discovered a "diamond-like" white dwarf starNasa, ESA, STScI, and G. Bacon (STScI)

A team of astronomers have identified the coldest, faintest white dwarf star estimated to be around 11-billion-years-old.

Using the National Radio Astronomy Observatory's Green Bank Telescope and Very Long Baseline Array, researchers at the University of Wisconsin-Milwaukee have identified an ancient stellar remnant believed to be so cold that its carbon has crystallised, forming an earth-sized diamond in space.

"It's a really remarkable object," study leader David Kaplan, told Science World Report. "These things should be out there, but because they are so dim they are very hard to find."

Consisting of carbon and oxygen, white dwarves are considered to be the remnants of a star like our Sun that have gradually diminished over billions of years.

Researchers discovered the new star by observing a pulsar companion to the white dwarf dubbed PSR J2222-0137. Pulsars are highly magnetised, rotating neutron stars.

Two years ago, according to Nature World News, the astronomer Adam Deller observed a pulsar that was 900 light years away in the constellation Aquarius.

Using Einstein's theory of relativity, the researchers examined how the gravity of the companion warped space, causing delays in the radio signal as the pulsar passed behind it.

These delays helped the team 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, while the companion has a mass 1.05 times larger.

The data indicated that the pulsar companion could not have been a second neutron star, as the orbits were too orderly for a second supernova to have taken place.

Knowing its location with such high precision and how bright a white dwarf should appear at that distance, the astronomers believed they should have been able to observe it in optical and infrared light.

Neither the Southern Astrophysical Research telescope in Chile nor the Keck telescope in Hawaii were 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," said team member Bart Dunlap, as reported by Phys.org. "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 a relatively cool 3,000 degrees Kelvin, approximately 2,700 degrees Celsius.

The study was published in the Astrophysical Journal.