Astronomers have discovered a new type of star explosion: a micronova. The stellar explosion occurs on the surfaces of white dwarf stars that over-consume material from a nearby double partner. Accumulation of material on the white dwarf results in a local thermonuclear eruption. The newly discovered type could help us better understand thermonuclear explosions on dead stars.
According to astronomers, the explosions burned tens to hundreds of billions of kilograms of stellar matter in just a matter of hours. The researchers compared this to about one-thousandth the mass of the moon or the size of several billions of the great pyramids at Giza. “We have detected and identified what are known as micronovas for the first time,” said astrophysicist Simon Scaringi of Durham University in the United Kingdom. “This phenomenon challenges our understanding of how thermonuclear explosions occur in stars. We thought we knew, but this discovery represents an entirely new way to get to them.”
A white dwarf is a so-called “dead” star, the remaining core that collapsed after the main sequence star ran out of fuel and expelled its outer matter. Other stars of this type, in different mass classes, are neutron stars and black holes. White dwarfs, whose masses are up to 1.4 times the mass of the Sun, can act as thermonuclear explosion machines in coherent binary systems.
In some rare cases—about 10 have been identified in the Milky Way—the binary systems are so close together that a white dwarf removes material from its companion, resulting in a so-called recurring supernova. As the stars orbit each other, the material – mainly hydrogen – is sucked away from its companion by the smaller, denser and more massive white dwarf. This hydrogen accumulates on the surface of the white dwarf, where it heats up.
Sometimes, the mass becomes so large that the pressure and temperature at the bottom of the layer are high enough to cause a thermonuclear explosion, as the excess material is forcibly pushed into space. This is the story.
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Scaringi and his team found that the micronova resembled a smaller version of this explosion. Researchers have identified a white dwarf emitting a micronova for the first time in data from the TESS Exoplanet Telescope. TESS has been optimized to find very small brightness differences in stars with exoplanets orbiting the solar system. An exoplanet passing in front of the star causes a very small eclipse.
The team detected the micronova in the TESS data when they found a short flash of light from a white dwarf star, not an eclipse. This led to a search for similar repeats in other white dwarfs. In total, they found three eruptions, the third of which led, after follow-up observations, to the discovery of a previously unknown white dwarf star.
The flashes from the TESS data were too small to be nova, are much stronger and last longer. So the team looked for a scenario that could explain the observations and came up with the micronova. “We have now seen for the first time that hydrogen fusion can also occur in a local way,” says astronomer Paul Grote of Radboud University in the Netherlands. “Hydrogen fuel may be at the base of the magnetic poles of some white dwarfs, so fusion only occurs at these magnetic poles. This detonates micronova bombs, which have about a million parts of the force of a nova explosion, hence the name Micronova.”
solve the puzzle?
This discovery could solve a mystery that has persisted for decades. One of the white dwarfs, in the TV Columbae binary star system, has been seen with similar flashes over the past 40 years. Similar outbursts have also been reported in other highly magnetic white dwarfs over the years. This statement can finally tell us why.
The discovery suggests that volcanic eruptions may be very common, but astronomers will need to make more observations to better understand them. “It just shows how dynamic the universe is,” says Scaringi. “These events may be very common, but because they are so fast, it is difficult to see them in action.”
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