‘Micronova’ explosions spotted on distant zombie stars

Image: Mark Garlick

There are different types of star explosions, from massive supernovae to plain old novae. Scientists think they have found a way for a star's surface to explode.

A white dwarf is a type of zombie star. The leftover cores of dead stars, remnants of our Sun that have used up all their fuel and blown most of their materials out into space, are what these odd objects are. White dwarfs are small but dense and have the same mass as the Sun. Micronovae may occur on their surface if the right conditions are present.

It is a type of phenomenon that scientists didn't know existed until now. Simone Scaringi, an astronomer at Durham University, is the lead author of the Nature.

It was by chance that Scaringi and his team stumbled upon this strange phenomenon. They were working with NASA's TESS, a space-based telescope that is designed to look for planets outside our Solar System. The team wasn't looking for exoplanets, they were using the telescope to study the variations in brightness of hundreds of stars.

White dwarfs have neighboring stars that are close by, so Scaringi is interested in studying them. Most stars in the Universe are in pairs. A white dwarf is around a star like our Sun. The super dense white dwarf will act like a vacuum when this happens.

The surface of the white dwarf will eventually be covered in hydrogen. At some point, the pressure of that layer gets so high that it causes a thermonuclear explosion. The white dwarf is still there after the event, but the hydrogen it stole is gone. Scientists have known about a nova for hundreds of years.

Image: ESO/M. Kornmesser, L. Calçada

When they watched the white dwarf systems, they saw something different. The team noticed that one white dwarf was bright for a short time.

The team noticed that there were two other white dwarfs in the same system. They began to put the pieces together. The white dwarfs had very strong magnetic fields. The team wondered if the hydrogen that the white dwarfs were pulling off of their neighboring stars was getting funneled onto the stars.

This process can be compared to the Aurora on Earth. Our planet has a magnetic field that is powered by the movement of Earth's liquid iron core. Our planet's northern and southern magnetic poles are where charged particles from our Sun are trapped. The charged particles from the Sun clashing with our atmosphere are what causes the Northern or Southern Lights.

The team thinks that the white dwarfs are getting similar effects. The white dwarfs have magnetic fields that direct the material to small areas near the poles. As the material builds up in these spots, it eventually causes a thermonuclear explosion, but they are much smaller than a regular nova and much more centralized. The researchers think that these events are about 1 million times less bright than a regular nova, but they still burn through a lot of material.

None of the explanations the researchers considered fit with their observations. When it comes to a new discovery in science, nothing is certain. The exact mechanism that would cause a micronova explosion is still unknown. The researchers think they could be happening more often than we think.