Astronomers have learned an almost unfathomable amount about stars in recent decades, but the heavens still manage to surprise us on occasion. A team of scientists using the European Southern Observatory’s Very Large Telescope have discovered a completely new type of stellar explosion, which has been labeled a “micronova.” As the name implies, a micronova is not as violent as a nova, but we’re still talking about a nuclear explosion that lasts for hours.
The novas scientists were previously familiar with can last weeks or months. They were first identified as an astronomical event hundreds of years ago, when they appeared in the sky as “new” stars. In reality, these events result from white dwarfs interacting with other types of stars. A white dwarf is the collapsed core of a star that has expended its nuclear fuel but does not have enough mass to become a neutron star or a black hole. These objects are still extremely dense, composed of so-called “electron-degenerate matter.”
Novas occur when a white dwarf in a binary system pulls matter like hydrogen away from its companion star. The surface of a white dwarf is still extremely hot, causing the hydrogen to fuse into helium, producing a thermonuclear explosion so bright that we can see it from many light years away.
The international team became aware of a new stellar event when analyzing data from NASA’s Transiting Exoplanet Survey Satellite (TESS), which tracks the luminance of stars in order to spot possible exoplanets. It looks for small, repeated dips caused by a transiting exoplanet, but the team noted several bright flashes that didn’t match known astronomical phenomena. The flashes appeared to be coming from white dwarfs, but they were too short-lived and localized to be novas. Thus, micronovas.
A new paper lays out a mechanism that explains the micronovas. Instead of matter igniting in a fusion fireball all over the star, it is funneled toward the polar regions by the white dwarf’s magnetic field. A particularly strong magnetic field could also confine the hydrogen to those areas, triggering localized thermonuclear explosions. See above for an animation of this model.
To confirm this hypothesis, astronomers are going to need to identify more micronovas. The team suspect these events could be plentiful throughout the universe, but their shorter duration and lower brightness will make them harder to detect than a traditional nova.
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