A group of astronomers using ESO’s Very Large Telescope (European Southern Observatory) observed a new type of stellar explosion: a micronova. These explosions occur on the surface of some stars, and each can burn up stellar material equivalent to about 3.5 billion Great Pyramids of Giza in just a few hours.
“We have, for the first time, detected and identified what we call micronovas,” explains Simone Scaringi, an astronomer at Durham University in the UK who led the study of these explosions, published today in Nature. “The phenomenon challenges our understanding of how thermonuclear explosions occur in stars. We thought we knew, but this discovery offers a whole new way to make it happen,” he adds.
The micronovae are very powerful events but small on an astronomical scale; They are far less energetic than stellar explosions known as novae, which astronomers have known for centuries. Both types of explosions occur on white dwarfs, dead stars similar in mass to the Sun but as small as Earth.
A white dwarf in a binary star system (i.e. consisting of two stars) can steal material, mainly hydrogen, from its companion star if the two stars are close enough together. When this gas falls on the white dwarf’s very hot surface, it explosively triggers the fusion of hydrogen atoms into helium. In the Nine, these thermonuclear explosions occur across the entire stellar surface. “Such detonations cause the entire surface of the white dwarf to burn and light up for several weeks,” explains co-author Nathalie Degenaar, an astronomer at the University of Amsterdam in the Netherlands.
Micronovae are similar explosions on a smaller and faster scale, lasting only a few hours. They occur on the surface of some white dwarf stars with strong magnetic fields that direct the material toward the star’s magnetic poles. “For the first time we have seen that hydrogen fusion can also take place locally. Hydrogen can be contained at the base of the magnetic poles of some white dwarfs, so fusion only occurs at those sites,” says Paul Groot, an astronomer at Radboud University in the Netherlands and co-author of the study.
“This results in the detonation of a type of microfusion bomb that has about a millionth the explosive power of a nova, hence the name micronova,” Groot continues. While “micro” suggests these events are small, don’t be fooled: one such explosion alone can incinerate about 20,000,000 trillion kg worth of material, or about 3.5 billion Great Pyramids of Giza.
These new micronovae challenge astronomers’ understanding of stellar explosions and may be more common than previously thought. “That just goes to show how dynamic the universe is. These events can actually be quite common, but because they’re so fast it’s difficult to see them in action,” explains Scaringi.
The team first encountered these mysterious micro-explosions while analyzing data from NASA’s Transiting Exoplanet Survey Satellite (TESS). “When we looked at the astronomical data collected by NASA’s TESS, we spotted something unusual: a bright flash of optical light that lasted several hours. Looking further, we found several similar signals,” says Degenaar.
The team observed three micronovae using TESS: two were from known white dwarfs, but the third required further observations with the X-Shooter instrument installed on ESO’s VLT to confirm that it was a white dwarf.
“Using ESO’s Very Large Telescope, we found that all these flashes of light were produced by white dwarfs,” adds Degenaar. “This observation was fundamental for the interpretation of our result and for the detection of micronovae,” says Scaringi.
The discovery of the micronova expands the repertoire of known stellar explosions. The team now wants to capture more elusive events like these, which will require large-scale investigations and rapid measurements to follow. “The fast response of telescopes like ESO’s VLT or NTT (New Technology Telescope) and the tools available will allow us to unravel more precisely what these mysterious micronovas are,” concludes Scaringi.
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