Astronomers discover a new type of starburst: a micronova

A team of astronomers has observed a new type of starburst, a micronova, using the Very Large Telescope (VLT) at the European Southern Observatory (ESO). These volcanic eruptions occur on the surfaces of some stars, and can burn about 3.5 billion Great Pyramids of stellar material in a matter of hours (temper nature20 April).

Micronovae are very violent events, but they don’t live up to much by astronomical standards. They are much less powerful than the supernovae that astronomers have known for centuries. Both types of explosions occur in white dwarfs: depleted stars that have about the same mass as our sun, but are as small as Earth.

A white dwarf that is part of a binary star system can extract material, especially hydrogen, from its companion, if it is close enough. When this gas falls on the superheated surface of the white dwarf star, the hydrogen atoms fuse very quickly to form helium. The result: a thermonuclear explosion extending over the entire surface of the star. “Such an explosion causes the entire surface of the white dwarf to ignite and shine brightly for weeks,” said Natalie Degenar, an astronomer at the University of Amsterdam who is involved in the research team.

Micro blasts

Micronovae are similar in nature, but less dense and shorter in duration. They occur in white dwarfs with strong magnetic fields, which direct material toward the star’s magnetic poles. This also results in a thermonuclear explosion, but locally and of limited size. However, in such a micronova explosion, about 20,000,000 trillion kilograms of hydrogen still turned into helium.

Astronomers tracked these “small explosions” when analyzing data from NASA’s Transiting Exoplanet Survey Satellite (TESS). Looking at the data, they detected a bright flash of light that lasted a few hours. After that, two more were discovered. Two of the three explosions occurred on already known white dwarf stars, and the third was later identified as a white dwarf.

The new microstructure challenges current ideas about stellar eruptions and may be more numerous than previously thought. But how many there are won’t be clear until more of these short-lived eruptions are observed.

Photo: Micronova impression. The blue disk surrounding the bright white dwarf is made of hydrogen extracted from the other star. Gas accumulates at the poles of the dwarf star and explodes. (ESO/M. Kornmesser, L. Calçada)