Astronomers have detected extremely violent explosions on exhausted stars

It’s an entirely new type of starburst that scientists have dubbed a “micronova.”

We know that things are often violent in the universe. Supernovae, in which the outermost layers are thrown into the universe and which release an incredible amount of energy, are certainly not rare. Meanwhile, we know of many stellar explosions, including regular novas and supernovae. But we can now add a new type of starburst to this list, the micronova.

Contrary to the name, micronovas are very violent outbursts on the surface of white dwarf stars (see box). However, a micronova is less powerful than a regular nova; The type of stellar explosion that astronomers have known for centuries. In addition, micronovas are smaller in size and only last for a few hours. Finally, a micronova consumes about one in a millionth the amount of ordinary supernova fuel. Hence the term “micro”; one million.

More about white dwarfs
As the star ages, hydrogen fusion in the star’s core stops and it swells. Then a red giant is created. As the star then blows its outer layers of gas and dust, the core shrinks, leaving a white dwarf. The star has reached the end of his life. Such a white dwarf is roughly the size of the Earth, but has a much greater density (a teaspoon of white dwarf matter weighs the same as an elephant on Earth!). The dying process also has a major impact on planets orbiting such a star. Most planets gravitate toward and swallow the star, and our sun will end up becoming a compact white dwarf in five billion years. In fact, about 97% of stars turn white dwarfs.

The team first encountered these mysterious micro-eruptions when analyzing data from planet hunter TESS. “When we reviewed that data, we discovered something unusual: a bright flash of light that lasted for a few hours,” researcher Natalie DeGeneres said. “As we searched further, we found more of these signals.”

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Three white dwarfs
Eventually, astronomers discovered Micronova on three different white dwarf stars. All three have a companion star that “steals” hydrogen from. In addition, they have a strong magnetic field. Researchers believe that hydrogen gas collects at the poles due to the magnetic field. The idea is that the temperature and pressure at the poles will slowly rise until hydrogen fuses into helium, resulting in a thermonuclear explosion.

This artistic impression shows a binary star system, consisting of a white dwarf (in the foreground) and a companion star (in the background), in which a micronova can occur. The white dwarf extracts the substance from its companion, which is directed to its electrodes. When the material falls on the hot surface of the white dwarf, it causes a micronova explosion confined to one of the white dwarf’s poles. Photo: Mark Garlick

The results show that the universe still has surprises in store for us. “We thought we knew how explosions happen on stars,” study leader Simon Scaringi says. “But this finding suggests that they can also start in completely different ways.” The results suggest that hydrogen fusion may also be a very local phenomenon. Hydrogen can accumulate at the magnetic poles in some white dwarfs, limiting fusion at that spot.

Make no mistake! Although the prefix “micro” indicates that these are trivial events, nothing could be further from the truth: one such volcanic eruption consumes approximately 20,000,000 trillion kilograms, or approximately 3.5 billion Great Pyramids of Giza, of fuel!

red verb
Why are astronomers only now discovering the existence of a micronova? “This is partly due to short stature,” researcher Paul Grote explains. “It’s hard to catch them red-handed. This phenomenon is very similar to Type I explosions in neutron star systems, but only now have we been able to establish a correlation with the data from TESS.”

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The newly discovered micronova challenges current ideas about star explosions. The search for Micronova continues. The team plans to capture more such extremely violent outbursts on depleted stars in the near future using a combination of long-range observation campaigns and rapid follow-up measurements. Perhaps more are waiting to be discovered.

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