Astronomers discover micronovae, a new type of stellar explosion

Micronova discovery joins repertoire of known stellar explosions

With the help of Very Large Telescope (VLT) at the European Southern Observatory (ESO), a team of astronomers has observed a new type of stellar explosion: a micronova. These explosions occur on the surface of certain stars and can burn up around 3,5 billion Great Pyramids of Giza of stellar material in just a few hours.

“We have discovered and identified for the first time something that we are calling a micronova,” explains Simone Scaringi, an astronomer at the University of Durham, UK, who led the study of these explosions published in the journal Nature.

“The phenomenon challenges our understanding of how thermonuclear explosions occur in stars. We thought we already knew that, but this discovery offers us a completely new way for this to happen,” he adds.

Micronovas are extremely powerful events, yet they are also small events on an astronomical scale; they are much less energetic than the stellar explosions known as novae, which astronomers have known for centuries. Both types of explosions occur in white dwarfs, "dead" stars with a mass comparable to our Sun, but as small as Earth in terms of size, meaning they are very dense objects.

A white dwarf in a binary system can "steal" material, essentially hydrogen, from its companion star if both are close enough to each other. As this gas falls onto the very hot surface of the white dwarf star, the hydrogen atoms fuse into helium quite explosively. In novae, these thermonuclear explosions occur across the entire stellar surface.

“Such detonations cause the entire surface of the white dwarf to burn and glow brightly for several weeks,” explains study co-author Nathalie Degenaar, an astronomer at the University of Amsterdam, the Netherlands.

Micronovae are similar explosions, but smaller in scale and faster, lasting only a few hours. They occur in some white dwarfs with strong magnetic fields, where material is routed towards the star's magnetic poles.

“We saw for the first time that hydrogen fusion can also take place in a localized manner. Hydrogen is contained at the base of the magnetic poles of some white dwarfs, such that fusion occurs only at those magnetic poles,” said Paul Groot, co-author of the study and an astronomer at Radbound University, The Netherlands.

“This causes micro-fusion bombs to explode, with about a millionth of the force of a nova explosion; hence the name micronova,” continues Groot. Although the “micro” implies that these events are small, we should not be fooled: just one of these explosions can burn approximately 20 000 000 billion kg of matter, that is, the corresponding to about 3,5 billion Great Pyramids. from Giza.

These new micronovae challenge astronomers' understanding of stellar explosions, and could be more abundant than previously thought. “This shows how dynamic the Universe is. These events can actually be quite common, but as they are extremely fast, they end up being difficult to catch at the moment of the act,” explains Scaringi.

The team initially discovered these mysterious microbursts when they were analyzing data from the TESS satellite (Transiting Exoplanet Satellite Survey) from NASA. “By analyzing the astronomical data collected by TESS, we discovered something unusual: a flash of bright visible light lasting just a few hours. As we investigated the phenomenon more closely, we discovered several other similar signs,” says Degenaar.

The team observed three micronovae with TESS: two in known white dwarfs and a third that required further observations, collected with the X-shooter instrument mounted on the Very Large Telescope (VLT) from ESO, to confirm that it was also a white dwarf.

“With the help of the VLT, we found that all these flashes of visible light were produced by white dwarfs,” says Degenaar. “This observation was crucial for us to interpret our results and for the discovery of micronovae,” adds Scaringi.

The discovery of micronovas joins the repertoire of known stellar explosions. The team now wants to capture more of these elusive events, which requires large-scale screenings and rapid follow-up measurements. “A quick response by telescopes such as the VLT or ESO's New Technology Telescope and their complements of available instruments will allow us to investigate in more detail what these mysterious micronovae really are,” concludes Scaringi.