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 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 König from the Astronomical Institute at FAU. “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 Jörn 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,” Jörn 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.
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