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.

Astronomers have spotted for the very first time a fiery explosion on a white dwarf, called a nova explosion. Pictured is the researchers’ recreation of the event, which happened in 2020

Overexposed image captured of the nova explosion by the eROSITA X-ray telescope, launched in 2019

Overexposed image captured of the nova explosion by the eROSITA X-ray telescope, launched in 2019

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.

WHAT IS A WHITE DWARF?

A white dwarf is the remnant of a small star that has run out of nuclear fuel.

While large stars – those that exceed ten times the mass of our sun – experience a spectacularly violent climax in the form of a supernova explosion at the end of their lives, small stars are spared such a dramatic fate.

When stars like the sun come to the end of their lives, they exhaust their fuel, grow into red giants, and later expel their outer layers into space.

The hot, very dense core of the old star – a white dwarf – is all that remains.

White dwarfs contain approximately the mass of the sun but have about the radius of the Earth, which means they are incredibly dense.

The surface gravity of a white dwarf is 350,000 times that of Earth’s gravity.

They become so dense because their electrons are crushed together, creating what caused “degenerative matter”.

This means that a more massive white dwarf has a smaller radius than its less massive counterpart.

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.

Pictured is eROSITA, a German-Russian X-ray telescope, ahead of its launch in 2019

Pictured is eROSITA, a German-Russian X-ray telescope, ahead of its launch in 2019

eROSITA is stationed in space about 900,000 miles away at Lagrange Point 2 (L2), a balanced gravity zone between the Sun and Earth (artist's rendering)

eROSITA is stationed in space about 900,000 miles away at Lagrange Point 2 (L2), a balanced gravity zone between the Sun and Earth (artist’s rendering)

THE WHITE DWARF IS SPOTTED “ON AND OFF” IN 30 MINUTES

A white dwarf star has been spotted “turning on and off” in just 30 minutes, researchers reported in 2021.

Using data from NASA’s Transiting Exoplanet Survey Satellite (TESS), the Durham University team witnessed the phenomena in the TW Pictoris star system, 1,400 light-years from Earth.

They found that instead of taking months for the brightness to rise and then fall, it only took about half an hour and was likely due to a fast magnetic field.

Read more: White dwarf star is spotted ‘switching on and off’ in 30 minutes

“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.

White dwarfs are the incredibly dense remnants of sun-sized stars after exhausting their nuclear fuel, shrunk to about the size of Earth (artist's impression)

White dwarfs are the incredibly dense remnants of sun-sized stars after exhausting their nuclear fuel, shrunk to about the size of Earth (artist’s impression)

White dwarfs are so hot that they glow white, but their radiation is so weak that it is difficult to detect them from Earth

White dwarfs are so hot that they glow white, but their radiation is so weak that it is difficult to detect them from Earth

“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.

WHAT WILL HAPPEN TO THE EARTH WHEN THE SUN dies?

In five billion years, the Sun is said to have become a red giant star, more than a hundred times larger than its current size.

Eventually it will eject gas and dust to create an “envelope” of up to half its mass.

The core will become a tiny white dwarf star. It will glow for thousands of years, illuminating the envelope to create a ring-shaped planetary nebula.

In five billion years, the Sun is said to have become a red giant, more than a hundred times larger than its current size.

In five billion years, the Sun is said to have become a red giant, more than a hundred times larger than its current size.

While this metamorphosis will change the solar system, scientists don’t know what will happen to the third rock from the Sun.

We already know that our Sun will be bigger and brighter, so it will likely destroy all life on our planet.

But the survival of the rocky core of the Earth is uncertain.


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