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32 JULY-AUGUST 2022 ASTRO PUBLISHING lar explosions and may be more abundant than previously thought. “It just goes to show how dynamic the Universe is. These events may ac- tually be quite common, but be- cause they are so fast they are dif- ficult to catch in action,” Scaringi explains. The team first came across these mysterious micro-explosions when analysing data from NASA’s Transit- ing Exoplanet Survey Satellite (TESS). “Looking through astronomical data collected by NASA’s TESS, we discov- ered something unusual: a bright flash of optical light lasting for a few hours. Searching further, we found several similar signals,” says Degenaar. The team observed three micronovae with TESS: two were from known white dwarfs, but the third required further observations with the X-shooter instrument on ESO’s VLT to confirm its white dwarf status. “With help from ESO’s Very Large Telescope, we found that all these optical flashes were produced by white dwarfs,” says Degenaar. “This observation was crucial in interpret- ing our result and for the discovery of micronovae,” Scaringi adds. The discovery of micronovae adds to the repertoire of known stellar explosions. The team now want to capture more of these elusive events, requiring large scale surveys and quick follow-up measurements. “Rapid response from telescopes such as the VLT or ESO’s New Tech- nology Telescope and the suite of available instruments will allow us to unravel in more detail what these mysterious micronovae are,” Scaringi concludes. ! T he illustration on the background shows a two-star system, with a white dwarf (in the foreground) and a companion star (in the background), where micronovae may occur. The white dwarf steals materials from its companion, which is funneled towards its poles. As the material falls on the hot surface of the white dwarf, it triggers a micronova explosion, contained at one of the star’s poles. [Mark Garlick (http://www.markgarlick.com/) ] A stronomers have discovered a new type of explosion occurring on white dwarf stars in two-star systems. This video summarises the discovery. [Mark Garlick (http://www.markgarlick.com/) ] cal scales; they are much less ener- getic than the stellar explosions known as novae, which as- tronomers have known about for centuries. Both types of explosions occur on white dwarfs, dead stars with a mass about that of our Sun, but as small as Earth. A white dwarf in a two-star system can steal material, mostly hydro- gen, from its companion star if they are close enough together. As this gas falls onto the very hot surface of the white dwarf star, it triggers the hydrogen atoms to fuse into he- lium explosively. In novae, these thermonuclear explosions occur over the entire stellar surface. “Such detonations make the entire surface of the white dwarf burn and shine brightly for several weeks,” explains co-author Nathalie Degenaar, an astronomer at the Uni- versity of Amsterdam, the Nether- lands. Micronovae are similar explosions that are smaller in scale and faster, lasting just several hours. They occur on some white dwarfs with strong magnetic fields, which fun- nel material towards the star’s mag- netic poles. “For the first time, we have now seen that hydrogen fu- sion can also happen in a localised way. The hydrogen fuel can be con- tained at the base of the magnetic poles of some white dwarfs, so that fusion only happens at these mag- netic poles,” says Paul Groot, an as- tronomer at Radboud University in the Netherlands and co-author of the study. “This leads to micro-fusion bombs going off, which have about one millionth of the strength of a nova explosion, hence the name mi- cronova,” Groot continues. Although ‘micro’ may imply these events are small, do not be mis- taken: just one of these outbursts can burn through about 20,000,000 trillion kg, or about 3.5 billion Great Pyramids of Giza, of material. These new micronovae challenge astronomers’ understanding of stel-