Biggest known diamond in the universe

Confirming what the Beatles always knew, astronomers have actually found a diamond in the sky - directly above Australia. It is the biggest known diamond in the universe, in fact.

According to American astronomers at the Harvard-Smithsonian Centre for Astrophysics, a white dwarf star in the constellation of Centaurus, next to the Southern Cross, has been found to have a 3000-kilometre-wide core of crystallised carbon, or diamond.

It weighs 2.27 thousand trillion trillion tonnes - that's 10 billion trillion trillion carats, or a 1 followed by 34 zeroes. The biggest earthly jewel is one of the British crown jewels, the 530-carat Star of Africa.

However, this cosmic jewel is hidden beneath a layer of hydrogen and helium gases, with the diamond core making up between 50 and 90 per cent of its mass. "It's the mother of all diamonds," said astronomer Travis Metcalfe, who led the team of researchers that studied the star.

"Some people refer to it as Lucy, in a tribute to the Beatles song Lucy in the Sky with Diamonds."

Known officially as BPM 37093, the star confirms a theory, first raised in the early 1960s, that cool white dwarfs should have a diamond core.

A white dwarf is what small stars, those up to about the size of the sun, turn into when they run out of nuclear fuel and die.

The intense pressures at the heart of such dead stars compress the carbon into diamond.

But confirming this theory has only been possible recently.

Lucy "pulsates", which means its light fluctuates at regular intervals. "By measuring these pulsations, we were able to study the hidden interior of the white dwarf, just like seismograph measurements of earthquakes allow geologists to study the interior of the Earth," Dr Metcalfe said.

"We figured that the carbon interior of this white dwarf has solidified to form the galaxy's largest diamond."

This means that other white dwarfs must also have diamond cores. Our own sun will become a white dwarf when it dies in 5 billion years. Two billion years after that, its ember core will crystallise as well, leaving a giant diamond in the centre of our solar system.

Vince Ford, a research officer at Mount Stromlo Observatory near Canberra, said astronomers, including Australians, had observed the star for more than eight years.

The star is about 50 light years away (500 trillion kilometres) - a fair distance as far as stars go. This means it is about 400 times too faint to see with the naked eye.