Explosion making white dwarf star 60 times hotter than the sun is seen for first time

Temperatures on the star after hard-to-spot explosion shoot up to around 327,000K (588,000F)

A fiery nova explosion from a white dwarf star that made it at least 60 times hotter than the sun has been observed by scientists who termed the sighting a “fortunate coincidence”.

When stars like the sun run out of fuel, they shrink to form white dwarfs that can sometimes spring back to life in a fiery, super-hot explosion and produce a fireball of X-ray radiation, explained scientists, including those from the University of Erlangen-Nuremberg in Germany.

In a new study, published last week in the journal Nature, researchers reported one such explosion of X-ray light for the very first time from the Reticulum constellation.

In the case of this newly observed explosion, scientists also pointed out that temperatures on the white dwarf could have reached around 327,000K (588,000F), making it around 60 times hotter than the sun.

While the explosion was detected in 2020, it took time to render and process data received by the scientists.

“It was to some extent a fortunate coincidence, really. These X-ray flashes last only a few hours and are almost impossible to predict, but the observational instrument must be pointed directly at the explosion at exactly the right time,” study co-author Ole König said in a statement.

In the research, scientists used the eROSITA X-ray telescope, currently located one and a half million kilometres away from the Earth, that has been surveying the sky for soft X-rays since 2019.

The telescope, on 7 July 2020, had detected strong X-ray radiation in an area of the sky that had been completely inconspicuous.

But when the same position in the sky was surveyed four hours later on that same day, the radiation had disappeared, scientists said.

This flash of X-rays must have lasted “less than eight hours,” they said, adding that such explosions, predicted over 30 years ago, have never been observed directly until now.

These X-ray fireballs, scientists said, occur on the surface of white dwarfs, that are remnants of stars comparable in size to the sun.

As these stars burn up most of their fuel made of hydrogen and later helium deep inside their cores, they shrink to ultra-high densities, reaching sizes similar to that of the Earth, but with a mass like the sun’s.

“One way to picture these proportions is to think of the sun being the same size as an apple, which means Earth would be the same size as a pin-head orbiting around the apple at a distance of 10 metres,” study co-author Jörn Wilms explains.

These contracted stars are so dense that a teaspoon of matter from inside one such white dwarf would have the mass of a large truck, scientists said.

While such fiery X-ray explosions on these collapsed stars happen “all the time”, detecting them during the very first moments when most of the X-ray emission is produced is “really hard”, said researchers.

“Not only the short duration of a flash is a challenge, but also the fact that the spectrum of emitted X-rays is very soft. Soft X-rays are not very energetic and easily absorbed by interstellar medium, so we cannot see very far in this band, which limits the number of observable objects, be it a nova or ordinary star,” said Victor Doroshenko, another study co-author.

“Telescopes are normally designed to be most effective in harder X-rays where absorption is less important, and that’s exactly the reason why they would miss an event like this!” Dr Doroshenko added.

But if the white dwarf is accompanied by a star that is still burning, and when its enormous gravitational pull draws the hydrogen from the shell of the accompanying star, it could cause the collapsed star to reignite, scientists said.

“In time, this hydrogen can collect to form a layer only a few metres thick on the surface of the white dwarf,” Dr Wilms said.

The huge gravitational pull generates enormous pressure which causes the star to reignite.

The white dwarf then “soon comes to a huge explosion” in a chain reaction during which the layer of hydrogen is blown off, researchers explained.

“The physical origin of X-ray emission coming from white dwarf atmospheres is relatively well understood, and we can model their spectra from first principles and in exquisite detail,” said Valery Suleimanov, another co-author.

“Comparison of models with observations allows then to learn basic properties of these objects such as weight, size, or chemical composition,” Dr Suleimanov added.

“I think it illustrates very nicely the importance of collaboration in modern science, and wide range of expertise within the German eROSITA consortium” study co-author Klaus Werner said.