Astronomers see a nova explosion on a white dwarf for the first time

Astronomers in Germany have spotted a fiery “nova explosion” of a white dwarf for the first time ever.

Researchers observed the event, using 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 center of eROSITA’s detector, which records the emitted photons.

White dwarfs are the incredibly dense remnants of sun-sized stars after they have exhausted their nuclear fuel, shrunk to about the size of Earth.

Sometimes these dead stars come back to life in a super hot explosion and produce a fireball of X-rays.

These nova explosions occur from white dwarfs in a binary system – a system made up of two gravitationally bound stars.

Researchers were able to observe such an X-ray burst for the very first time, which came from a white dwarf in the constellation Reticulum.

Although the observation was made by eROSITA in July 2020, it has only just been detailed in a new study, conducted by astronomers from the Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU) in Erlangen, Germany.

“It was to some extent a happy coincidence, really,” said study author Ole König of the FAU. ‘We were really lucky.

“These X-ray flashes only last a few hours and are almost impossible to predict, but the observing instrument must be pointed directly at the explosion at exactly the right time.”

eROSITA floats in space at Lagrange point 2 (L2), a zone of balanced gravity between the Sun and the Earth around 900,000 miles (1.5 million km).

eROSITA has been surveying the skies 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 its commissioning, the On July 7, 2020, eROSITA measured strong X-ray radiation in an area of ​​the sky that was completely invisible only four hours before.

When the X-ray telescope probed the same position in the sky four hours later, the radiation was gone. Therefore, the X-ray flash must have lasted less than eight hours.

X-ray bursts like this were predicted by theoretical research in a 1990 study, but have never been directly observed until now.

These x-ray fireballs occur on the surface of white dwarfs – stars that were originally comparable in size to the sun before using up most of their hydrogen and later helium fuel in the deeper in their core and shrink.

White dwarfs, composed mostly of oxygen and carbon, 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,” said Professor Jörn Wilms, also at FAU.

When trying to explain a white dwarf, researchers said you should imagine reduce an apple to the size of a pinhead. This tiny particle would retain the relatively large weight of the apple.

Just a teaspoon of matter from inside a white dwarf easily has the same mass as a big truck.

White dwarfs are so hot that they glow white, but their radiation 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 may be accompanied by another star that is still burning.

In this case, the enormous 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 a few meters thick on the surface of the white dwarf.

In this layer, the enormous gravitational pull generates enormous pressure which is so great that it causes the star to re-ignite, resulting in a huge explosion during 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 the model calculations we originally made while supporting the development of the X-ray instrument, we were able to analyze the overexposed image in more detail in a complex process to get a view behind the scenes of a white dwarf, or nova, explosion,” Professor Wilms said.

The explosion generated a fireball with a temperature of about 327,000 degrees Kelvin, making it about sixty times hotter than the Sun.

Since these novae run out of fuel fairly quickly, they quickly cool and the X-radiation weakens until it eventually becomes visible light.

This visible light reached Earth half a day after the detection of eROSITA and was observed by optical telescopes.

“A seemingly bright star then appeared, which was actually visible light from the explosion, and so bright that it could be seen in the night sky with the naked eye,” König said.

Apparently, “new stars” such as this have been observed in the past and have been named “nova stella”, or “new star” because 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 mostly chance when they hit the X-ray detectors.

The new study was published in the journal Nature.