Meet the micronova: astronomers discovered a new type of stellar explosion

Artistic animation of a micronova, a new type of stellar explosion. Credit: European Southern Observatory.

Astronomers have discovered highly localized thermonuclear explosions coming from the surface of three white dwarf stars, unusually short-lived events that they have dubbed “micronovae.” They are similar to novae, except these explosions can burn up a huge amount of material in just a few hours, roughly the equivalent of 3.5 billion Great Pyramids of Giza. According to the authors of a new article published in the journal Nature, micronovae could be common in the Universe; they’re just hard to spot because they don’t last very long.

“The phenomenon challenges our understanding of how thermonuclear explosions occur in stars,” said co-author Simone Scaringi, an astronomer at Durham University in the UK. “We thought we knew this, but this discovery proposes a totally new way to achieve them. It just shows how dynamic the Universe is.”

Astronomers have known about novae for centuries. The 16th-century astronomer Tycho Brahe coined the term after witnessing a supernova in 1572, describing it in his treatise. of new wake (“about the new star”). The terms were used interchangeably until the 1930s, when scientists began to distinguish between events as their causes and energies seemed quite different. Novas are usually the result, not of new stars, as their name implies, but of the remnants of old stars known as white dwarfs.

Enlarge / Astronomers made the discovery while analyzing data from NASA’s Transiting Exoplanet Survey Satellite (TESS).

POT

The process begins with a binary system, in which one of the two stars becomes a red giant, leaving only a remnant white dwarf core still in orbit with the other star in the system. A white dwarf is small and incredibly dense because it collapses with such force that its electrons crush each other, forming “electron degenerate matter.” Eventually, the electrons provide enough outward pressure force to stop the star from collapsing.

One of the first white dwarf stars discovered, dubbed 40 Eridani B, had a density more than 25,000 times that of the Sun, packed into a much smaller volume (about the size of Earth), an observational deduction that astronomers initially believed impossible. A second white dwarf, Sirius B (in orbit around the star Sirius), was discovered soon after and appeared incredibly dense. As astronomer Arthur Eddington put it in 1927:

We learn about the stars by receiving and interpreting the messages that their light brings us. Sirius’ companion’s message when decoded read: “I am composed of material 3,000 times denser than anything you have ever found; a ton of my material would be a little nugget you could put in a matchbox.” What response can be given to such a message? The answer most of us gave in 1914 was: “Shut up. Don’t talk nonsense.”

Of course, it was not nonsense at all, as scientists finally confirmed. And it is the unique properties of white dwarf stars that give rise to novae. If a white dwarf is close enough to its companion star, it begins to extract matter (usually hydrogen) from the outer atmosphere of its companion star. Hydrogen falls onto the very hot surface of the white dwarf and its atoms fuse into helium in a thermonuclear explosion. For a nova, this occurs across the entire surface of the star, producing bright, intense light that can be observed for several weeks.

Durham University astronomer Simone Scaringi is part of the team that discovered three micronovae.
Enlarge / Durham University astronomer Simone Scaringi is part of the team that discovered three micronovae.

University of Durham

So Scaringi and his fellow astronomers were surprised to find bright, nova-like flashes of light that only lasted a few hours while analyzing data from NASA’s Transiting Exoplanet Survey Satellite (TESS). Launched in 2018, TESS’s mission is to search for planets outside our Solar System by looking for periodic dips in starlight, evidence that an exoplanet could be orbiting such a star.

Further investigation revealed two other similar events, which astronomers called micronovae. Two of those events were observed in stars that are already known to be white dwarfs. The team relied on additional observations from the European Southern Observatory’s Very Large Telescope to confirm that the third was also a white dwarf.

But why were these thermonuclear explosions so oddly localized? A follow-up paper published in the Monthly Notices of the Royal Astronomical Society proposes that micronovae could be triggered by magnetic confinement of material in a growing white dwarf. The star’s powerful magnetic fields funnel matter toward the magnetic poles, causing a thermonuclear explosion confined by those same magnetic fields.

“For the first time, we have seen that hydrogen fusion can also occur in a localized manner. Hydrogen fuel may be contained at the base of the magnetic poles of some white dwarfs, so fusion only occurs at these magnetic poles,” said co-author Paul Groot, an astronomer at Radboud University in the Netherlands. “This leads to the explosion of microfusion bombs, which are about a millionth of the force of a nova explosion, hence the name micronova.”

The next step is to identify even more micronova events to verify this hypothesis. “They’re supposed to be plentiful; they’re really hard to find,” Scaringi said. “Having found more micronovae, we can hopefully try to develop our theories about how thermonuclear explosions can actually occur when material is magnetically confined to a white dwarf.”

DOI: Nature, 2022. 10.1038/s41586-022-04495-6 (About DOIs).

Image of listing by ESO/M. Kornmesser, L. Calçada

Add Comment