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White dwarf stars observed slowly crystallising and turning into solid spheres

White dwarf stars observed slowly crystallising and turning into solid spheres

Illustration shows a white dwarf star in the process of solidifying. Image: University of Warwick/Mark Garlick For the first time, astronomers have observed the white dwarf stars slowly crystallising and turning into solid spheres. White dwarfs are the remnants of dying stars of a size similar to the Sun. When such stars expire after consuming all their fuel,

White dwarf stars observed slowly crystallising and turning into solid spheres

Illustration shows a white dwarf star in the process of solidifying. Image: University of Warwick/Mark Garlick

For the first time, astronomers have observed the white dwarf stars slowly crystallising and turning into solid spheres.

White dwarfs are the remnants of dying stars of a size similar to the Sun. When such stars expire after consuming all their fuel, they shed their outer layers and become dense white dwarf stars. As a white dwarf, they continue to cool down for billions of years.

Scientists estimate that up to 97 per cent of stars in the Milky Way Galaxy will end up as white dwarfs, while other massive stars will become black holes or neutron stars.

About 50 years ago, scientists predicted that the gaseous cores of white dwarfs should crystallise and become solid with the cooling of the hot gases inside them.

Scientists have now been able to verify the theory, thanks to observations recorded by the European Space Agency’s Gaia satellite. It was able to provide observations of thousands of such objects in the universe all exhibiting the same pattern, thereby confirming the process.

“Previously, we had distances for only a few hundreds of white dwarfs and many of them were in clusters, where they all have the same age,” said Pier-Emmanuel Tremblay from the University of Warwick, and the lead author of the study.

Tremblay said that the Gaia satellite provided the team with significant data about the distance, colour and brightness of thousands of white dwarfs sitting in the outer disc of the Milky Way, and covering a range of masses and different ages.

In the current study, researchers investigated more than 15,000 stellar remnant candidates in Gaia observation data. These white dwarfs were located within 300 light years of Earth.

According to the team, they were able to find a large number of white dwarfs of specific colours and luminosities. These stars were not related to each other in terms of evolution (that is, having a similar age). In fact, they all were actually found to be turning into solid spheres through a process of crystallisation.

Researchers reveal that this crystallisation process is similar to the process we commonly see on Earth where water vapours first turn into liquid and then change into a solid the colder they get.

The temperature at which the solidification of the white dwarf’s core occurs is extremely high – approximately 10 million degrees Celsius.

Scientists further reveal that this crystallisation lasts for several billion years, and the heat released during the process slows down the evolution of the white dwarfs. The star stops dimming and appears much younger (up to two billion years) than its age.

“White dwarfs are traditionally used for age-dating of stellar populations such as clusters of stars, the outer disc, and the halo in our Milky Way,” said Pier-Emmanuel.

“We will now have to develop better crystallisation models to get more accurate estimates of the ages of these systems.”

The findings of the study are published in journal Nature.

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