Astronomers have discovered the first direct evidence of white dwarf stars solidifying into giant balls of crystallised elements, a process that was first predicted 50 years ago and defines the ultimate fate of the Sun.

Using a space telescope launched in 2013, astronomers at the University of Warwick in Britain have spotted evidence of the heat expected to be released as stars exhausted of their fuel and rendered into white dwarfs cool and solidify.

White dwarfs are the residual cores of stars whose masses are similar to that of the Sun. Much heavier stars, at the end of their lives, turn into other objects called neutron stars or black holes.

Scientists had predicted five decades ago that as white dwarf stars cool down, they would solidify, or crystallise, but no one knew how much heat would be released in this process or even whether solidification could be proved or observed.

“We could visualise crystallised white dwarfs as giant balls of solid oxygen and carbon, with only tiny amounts of other residual elements,” Pier Emmanuel Tremblay, assistant professor of astronomy at Warwick who led the study, told The Telegraph.

Tremblay and his colleagues selected 15,000 white dwarf stars within 300 light years of Earth and analysed their luminosities and colours — or measures of energy emissions and temperatures — using the European Space Agency’s Gaia telescope.

They found a “pile-up”, or excess, in the numbers of stars at specific sets of luminosities and colours but not corresponding with any single mass or age. They say this “pile-up” coincides with the solidification. Their findings will be published in the journal Nature on Thursday.

Tremblay said their studies suggested that white dwarfs stop their cooling by turning from liquid to nearly 99 per cent solid over about 1.5 billion years. After that they resume cooling.

“The ultimate fate is a fully solid black dwarf, not emitting any light and remaining stable for hundreds of billions of years — a future so distant that it’s a bit difficult to even imagine,” Tremblay said.

He said the Sun was expected to become a white dwarf and experience crystallisation between 8 and 14 billion years from now.

“The Universe is just starting to be old enough that some white dwarfs may have reached a room temperature,” Tremblay said.

“We have not observed any such objects because they would be too faint.”