Astronomers in Germany have spotted a fiery ‘nova explosion’ from a white dwarf for the very first time.
Researchers observed the event, thanks to data from the joint German-Russian eROSITA X-ray telescope, which is stationed in space about 900,000 miles away.
The X-ray flash – dubbed YZ Reticuli – completely overexposed the centre of eROSITA’s detector, which records emitted photons.
White dwarfs are the incredibly dense remains of sun-sized stars after they exhaust their nuclear fuel, shrunk down to roughly the size of Earth.
Sometimes such dead stars flare back to life in a super hot explosion and produce a fireball of X-ray radiation.
These nova explosions occur from white dwarfs in a binary system – a system that consists of two stars that are gravitationally bound.
Astronomers have spotted a fiery explosion on a white dwarf, called a nova explosion, for the very first time. Pictured is the researchers’ recreation of the event, which occurred in 2020
Overexposed image picked up of the nova explosion event by the eROSITA X-ray telescope, which launched in 2019
The researchers have now been able to observe such an explosion of X-ray light for the very first time, which came from a white dwarf in the constellation Reticulum.
Although the observation was made by eROSITA back in July 2020, it has only just been detailed in a new study, led by astronomers at Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU) in Erlangen, Germany.
‘It was to some extent a fortunate coincidence, really,’ said study author Ole König at FAU. ‘We were really lucky.
‘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.’
eROSITA is floating in space at Lagrange Point 2 (L2), an area of balanced gravity between the Sun and Earth about 900,000 miles (1.5 million km) away.
eROSITA has been surveying the sky for soft X-rays since 2019, although due to the breakdown of cooperation between Germany and Russia after the invasion of Ukraine, the instrument stopped collecting data on February 26, 2022.
Less than a year after it started operations, on July 7, 2020, eROSITA measured strong X-ray radiation in an area of the sky that had been completely inconspicuous only four hours prior.
When the X-ray telescope surveyed the same position in the sky four hours later, the radiation had disappeared. Therefore, the the X-ray flash must have lasted less than eight hours.
X-ray explosions such as this were predicted by theoretical research back in a 1990 study but have never been observed directly until now.
These fireballs of X-rays occur on the surface of white dwarfs – 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 and shrinking down.
White dwarfs, which are mainly made up of oxygen and carbon, are similar to Earth in size but contain a mass that can be similar to that of our sun.
Pictured here is eROSITA, a joint German-Russian X-ray telescope, prior to its launch in 2019
eROSITA is stationed in space about 900,000 miles away at Lagrange Point 2 (L2), an area of balanced gravity between the Sun and Earth (artist’s depiction)
‘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,’ said Professor Jörn Wilms, also at FAU.
Trying to explain a white dwarf, the researchers said you should imagine shrinking an apple to the size of a pin head. This tiny particle would retain the comparatively large weight of the apple.
Just a teaspoon of matter from the inside of a white dwarf easily has the same mass as a large truck.
White dwarfs are so hot they glow white, but the radiation from them is so weak that it is difficult to detect them from Earth.
In a binary star system (a solar system with two stars), white dwarfs can be accompanied by another star that is still burning.
In this case, 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.
In this layer, the huge gravitational pull generates enormous pressure that is so great that it causes the star to reignite, leading 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.
White dwarfs are the incredibly dense remains of sun-sized stars after they exhaust their nuclear fuel, shrunk down to roughly the size of Earth (artist’s impression)
White dwarfs are so hot they glow white, but the radiation from them is so weak that it is difficult to detect them from Earth
‘Using the model calculations we originally drew up while supporting the development of the X-ray instrument, we were able to analyse 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,’ said Professor Wilms.
The explosion generated a fireball with a temperature of around 327,000 degrees Kelvin, 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.
This visible light 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,’ said König.
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.
The new study has been published in the journal Nature.