Small explosions called micronovae discovered on dead stars

Micronovae are about 1 million times less bright than a classical nova and last just half a day, but they release as much energy as the sun would in a day


20 April 2022

Artist’s impression of a two-star system where micronovas may occur

ESO/M. Kornmesser, L. Calçada

A new type of explosion has been discovered on dead stars that is much smaller but more frequent than other stellar explosions.

When stars like our sun reach the end of their lives, they can expel their outer layers, leaving behind their dense cores as a white dwarf. Thousands of white dwarfs are known to exist in our galaxy as pairs with larger stars, where the white dwarf can suck in – or accrete – material from its companion.

Over thousands of years, this can lead to powerful explosions known as novae or even supernovae, where the entire star is obliterated. Simone Scaringi at Durham University, UK, and his colleagues used NASA’s Transiting Exoplanet Survey Satellite (TESS) to discover that smaller explosions were also taking place, dubbed micronovae.

Micronovae are about 1 million times less bright than a classical nova, says Scaringi, lasting just half a day, compared with several weeks for novae.

The brevity of the events meant they had previously been missed, but TESS was able to spot them during its around-the-clock observations of the galaxy in search of exoplanets. Three were seen up to 5000 light years from Earth, with the white dwarfs brightening temporarily before dimming again.

The exact mechanism behind the explosions isn’t clear, but it is thought they may be caused by hydrogen accumulating at the poles of the star – perhaps as much as the mass of an asteroid in just 100 days. Eventually, the hydrogen reaches sufficient temperatures and pressures to ignite fusion and cause a localised thermonuclear explosion that releases as much energy as the sun would in a day.

Only very magnetic white dwarfs may be able to accumulate hydrogen at their poles in this way, meaning not all should experience micronovae. Finding and studying more of them could reveal these processes, and perhaps explain how white dwarfs are able to accrete enough mass to explode as supernovae.

“It shows how dynamic our universe is,” says Scaringi. “If you’re not looking at the right time, you might miss these things.”

Journal reference: Nature, DOI: 10.1038/s41586-022-04495-6

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