When stars like our Sun run out of fuel, they shrink to form white dwarfs. Sometimes these dead stars come back to life in a super hot explosion and produce an X-ray fireball. A FAU-led research team has now been able to observe such an X-ray explosion for the very first time.
“It was to some extent a happy coincidence, really,” says Ole König of the FAU Astronomical Institute at Dr. Karl Remeis’ Observatory in Bamberg, who published an article about the sighting in the journal Nature, in collaboration with Professor Jörn Wilms and a research team from the Max Planck Institute for Extraterrestrial Physics, the University of Tübingen, the Universitat Politécnica de Catalunya in Barcelona and the Leibniz Institute for Astrophysics in Potsdam . “These X-ray flashes last only a few hours and are almost impossible to predict, but the observation instrument must be pointed directly at the explosion at 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 monitoring the sky for soft X-rays since 2019. On July 7, 2020, it measured strong x-rays. radiation in an area of the sky that had been completely invisible four hours earlier. When the X-ray telescope probed the same position in the sky four hours later, the radiation was gone. 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 bursts like this were predicted by theoretical research over 30 years ago, but have never been directly observed until now. These X-ray fireballs occur on the surface of stars that were originally comparable in size to the Sun before using up most of their hydrogen and later helium fuel deep within. their core. These stellar corpses shrink until “white dwarfs” remain, which are similar in size to Earth but contain mass that may be similar to that of our Sun. “One way to imagine these proportions is to think that the Sun is the same size as an apple, which means that the Earth would be the same size as a pinhead orbiting the apple at a distance of 10 meters”, explains Jörn Wilms.
Stellar corpses look like gemstones
On the other hand, if you were to shrink an apple to the size of a pinhead, this tiny particle would retain the relatively large weight of the apple. “A teaspoon of material from inside a white dwarf easily has the same mass as a large truck,” continues Jörn Wilms. Since these scorched stars are mostly composed of oxygen and carbon, we can compare them to gigantic Earth-sized diamonds floating in space. These gem-like objects are so hot that 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 still-burning star, which is when the huge gravitational pull of the white dwarf pulls hydrogen from the shell of the accompanying star. “Over time, this hydrogen can accumulate to form a layer only a few meters thick on the surface of the white dwarf,” explains FAU astrophysicist Jörn Wilms. In this layer, the enormous gravitational pull generates enormous pressure which is so great that it causes the star to re-ignite. In a chain reaction, it soon comes to a huge explosion in which the hydrogen layer is blown away. X-ray radiation from an explosion like this is what hit eROSITA’s detectors on July 7, 2020, producing an overexposed image.
“Using calculations from the model we initially developed while supporting the development of the X-ray instrument, we were able to analyze the overexposed image in greater detail in a complex process to get a behind-the-scenes view of an explosion of a white dwarf, or nova,” explains Jörn Wilms. According to the results, the white dwarf has about the mass of our Sun and is therefore relatively large. The explosion generated a fireball with a temperature of about 327,000 degrees, making it about sixty times hotter than the Sun.
Since these novae run out of fuel fairly quickly, they quickly cool and the X-ray radiation weakens until it finally becomes visible light, which reached Earth half a day after the detection of X-rays. eROSITA and has been observed by optical telescopes. “A seemingly bright star then appeared, which was actually the visible light of the explosion, and so bright that it could be seen in the night sky with the naked eye,” says Ole König. Apparently ‘new stars’ like this have been observed in the past and have been named ‘nova stella’ or ‘new star’ due to 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 mostly chance when they hit the X-ray detectors. lucky,” says Ole König.
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Explosion on an observed white dwarf – Astronomy and astrophysics news
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