When stars like our Sun use up all their fuel, they shrink to form white dwarfs. Sometimes such dead stars flare back to life in a super hot explosion and produce a fireball of X-ray radiation. A research team led by FAU has now been able to observe such an explosion of X-ray light for the very first time.

"It was to some extent a fortunate coincidence, really," explains Ole Konig from the Astronomical Institute at FAU in the Dr. Karl Remeis observatory in Bamberg, who has published an article about this observation in the reputable journal Nature, together with Prof. Dr. Jorn Wilms and a research team from the Max Planck Institute for Extraterrestrial Physics, the University of Tubingen, the Universitat Politecnica de Catalunya in Barcelona und the Leibniz Institute for Astrophysics Potsdam.

"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," explains the astrophysicist.

The instrument in this case is the eROSITA X-ray telescope, which is currently located one and a half million kilometers from Earth and has been surveying the sky for soft X-rays since 2019. On July 7, 2020 it measured strong X-ray radiation in an area of the sky that had been completely inconspicuous four hours previously.

When the X-ray telescope surveyed the same position in the sky four hours later, the radiation had disappeared. It follows that the X-ray flash that had previously completely overexposed the center of the detector must have lasted less than eight hours.

X-ray explosions such as this were predicted by theoretical research more than 30 years ago, but have never been observed directly until now. These fireballs of X-rays occur on the surface of stars that were originally comparable in size to the Sun before using up most of their fuel made of hydrogen and later helium deep inside their cores. These stellar corpses shrink until "white dwarfs" remain, which are similar to Earth in size but contain a mass that can be similar to that of our Sun.

"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 meters," explains Jorn Wilms.

Stellar corpses resemble gemstones
On the other hand, if you were to shrink an apple to the size of a pin head, this tiny particle would retain the comparatively large weight of the apple. "A teaspoon of matter from the inside of a white dwarf easily has the same mass as a large truck," Jorn Wilms continues. Since these burnt out stars are mainly made up of oxygen and carbon, we can compare them to gigantic diamonds that are the same size as Earth floating around in space. These objects in the form of precious gems are so hot they glow white. However, the radiation is so weak that it is difficult to detect from Earth.

Unless the white dwarf is accompanied by a star that is still burning, that is, and when the enormous gravitational pull of the white dwarf draws hydrogen from the shell of the accompanying star. "In time, this hydrogen can collect to form a layer only a few meters thick on the surface of the white dwarf," explains FAU astrophysicist Jorn Wilms.

In this layer, the huge gravitational pull generates enormous pressure that is so great that it causes the star to reignite. In a chain reaction, it soon comes to a huge explosion during which the layer of hydrogen is blown off. The X-ray radiation of an explosion like this is what hit the detectors of eROSITA on July 7, 2020 producing an overexposed image.

"Using the model calculations we originally drew up while supporting the development of the X-ray instrument, we were able to analyze the overexposed image in more detail during a complex process to gain a behind the scenes view of an explosion of a white dwarf, or nova," explains Jorn Wilms. According to the results, the white dwarf has around the mass of our Sun and is therefore relatively large. The explosion generated a fireball with a temperature of around 327,000 degrees, making it around sixty times hotter than the Sun.

Since these novae run out of fuel quite quickly, they cool rapidly and the X-ray radiation becomes weaker until it eventually becomes visible light, which reached Earth half a day after the eROSITA detection and was observed by optical telescopes.

"A seemingly bright star then appeared, which was actually the visible light from the explosion, and so bright that it could be seen on the night sky by the bare eye," explains Ole Konig.

Seemingly "new stars" such as this one have been observed in the past and were named "nova stella", or "new star" on account of their unexpected appearance. Since these novae are only visible after the X-ray flash, it is very difficult to predict such outbreaks and it is mainly down to chance when they hit the X-ray detectors. "We were really lucky," says Ole Konig.

Research Report:X-ray detection of a nova in the fireball phase

Related Links
University of Erlangen-Nuremberg
Stellar Chemistry, The Universe And All Within It

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