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Space can be a terrifying place: there are huge explosions impossible to imagine. The queens are supernovae, very massive starbursts that can shine more than 100,000 stars combined. There are also novae, in which a white dwarf in a binary system ‘swallows’ its companion. Now, a team of astronomers using the European Southern Observatory’s Very Large Telescope (ESO’s VLT) has found a new starburst that also occurs in binary systems: micronovae. The results have just been published in the journal ‘Nature’.
Although their name may suggest otherwise, they are actually powerful stellar explosions that can consume 20,000 trillion kilograms in just a few hours.
Or its equivalent: 3,500 million Great Pyramids of Giza razed after its explosion.
“We have discovered and identified for the first time what we call a micronova,” he explains. Simone Scaringi, an astronomer at the University of Durham (United Kingdom) who led the study. “The phenomenon challenges our understanding of how thermonuclear explosions occur in stars. We thought we knew, but this discovery proposes a totally new way », he adds.
The thief stars
A white dwarf (dead star with a mass close to that of our Sun, but as small as Earth) in a two-star system can steal material, mainly hydrogen, from its companion star if they are close enough. As this gas falls onto the very hot surface of the white dwarf star, it triggers hydrogen atoms to explosively fuse into helium. 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 for several weeks,” explains the co-author, Nathalie Degenaarastronomer at the University of Amsterdam (The Netherlands).
Micronovae are similar explosions, smaller in scale and faster, lasting only several hours. They occur in some white dwarfs with strong magnetic fields, which funnel material toward the star’s magnetic poles. “For the first time, we have now seen that hydrogen fusion can also occur in a localized way. Hydrogen fuel may be contained at the base of the magnetic poles of some white dwarfs, so fusion only occurs at these magnetic poles,” he says. Paul Groot, astronomer at Radboud University (The Netherlands) and co-author of the study. “This causes microfusion bombs to explode, which have about a millionth of the force of a nova explosion, hence the name micronova.”
These new micronovae challenge astronomers’ understanding of exploding stars and may be more abundant than previously thought. “It just shows how dynamic the Universe is. These events can actually be quite common, but because they are so fast, they are difficult to detect in action,” explains Scaringi.
three micronovae
The team first found these mysterious microbursts by analyzing data from NASA’s Transiting Exoplanet Survey Satellite (TESS). “Looking at astronomical data collected by NASA’s TESS satellite, we discovered something unusual: a bright flash of optical light that lasts a few hours. Looking further, we found several similar signals,” confirms 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 the help of ESO’s Very Large Telescope, we found that all these optical flares were produced by white dwarfs,” says Degenaar. “This observation was crucial for interpreting our result and for the discovery of micronovae,” adds Scaringi.
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