Astronomers have discovered highly localized thermonuclear blasts coming from the surface of three white dwarf stars—unusually short-lived events they have dubbed “micronovae.” They’re similar to novas, except these blasts can burn through a tremendous amount of material in just a few hours, roughly equivalent to 3.5 billion Great Pyramids of Giza. According to the authors of a new paper published in the journal Nature, micronovae could be common in the Universe; they’re just difficult to detect because they don’t last very long.
“The phenomenon challenges our understanding of how thermonuclear explosions in stars occur,’ 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 goes to show how dynamic the Universe is.”
Astronomers have known about novas for centuries. The 16th-century astronomer Tycho Brahe coined the term after witnessing a supernova in 1572, describing it in his treatise De nova stella (“concerning the new star”). The terms were used interchangeably until the 1930s when scientists began distinguishing between the events, since their causes and energies seemed quite different. Novas typically are the result, not of new stars, as the name implies, but the remnants of ancient stars known as white dwarfs.
The process begins with a binary system, in which one of the two stars turns into a red giant, leaving just a white dwarf remnant core still in orbit with the other star in the system. A white dwarf is small and incredibly dense because it collapses so tightly that its electrons are smashed together, forming “electron-degenerate matter.” Eventually the electrons provide enough of an outward-pressing force to halt the star’s collapse.
One of the first white dwarf stars discovered, dubbed 40 Eridani B, had a density over 25,000 times that of the Sun, packed into a much smaller volume (roughly the size of Earth)—an observational deduction that astronomers initially deemed impossible. A second white dwarf, Sirius B (orbiting 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 which their light brings to us. The message of the companion of Sirius when it was decoded ran: “I am composed of material 3,000 times denser than anything you have ever come across; a ton of my material would be a little nugget that you could put in a matchbox.” What reply can one make to such a message? The reply which most of us made in 1914 was—”Shut up. Don’t talk nonsense.”
Of course, it wasn’t nonsense at all, as scientists eventually confirmed. And it’s the unique properties of white dwarf stars that give rise to novas. If a white dwarf is close enough to its companion star, it begins siphoning off matter (usually hydrogen) from its companion star’s outer atmosphere. The hydrogen falls onto the white dwarf’s very hot surface, and its atoms fuse into helium in a thermonuclear explosion. For a nova, this occurs across the entire surface of the star, producing an intense, bright light that can be observed for several weeks.
So Scaringi and his fellow astronomers were surprised to find bright flashes of light, akin to a nova, that only lasted for just a few hours while analyzing data from NASA’s Transiting Exoplanet Survey Satellite (TESS). Launched in 2018, TESS’s mission is to hunt for planets outside our Solar System by looking for periodic dips in light from stars—evidence that an exoplanet might be orbiting such a star.
Further investigation revealed two more similar events, which the astronomers dubbed micronovae. Two of those events were observed on stars already known to be white dwarfs. The team relied on additional observations from the European Southern Observatory’s Very Large Telescope to confirm the third was also a white dwarf.
But why were these thermonuclear explosions so strangely 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 on an accreting white dwarf. The star’s powerful magnetic fields funnel matter toward the magnetic poles, triggering a thermonuclear explosion confined by those same magnetic fields.
“For the first time, we have now seen that hydrogen fusion can also happen in a localized way. The hydrogen fuel can be contained at the base of the magnetic poles of some white dwarfs, so that fusion only happens at these magnetic poles,” said co-author Paul Groot, an astronomer at Radboud University in the Netherlands. “This leads to micro-fusion bombs going off, which have about one-millionth of the strength of a nova explosion, hence the name micronova.”
The next step is to identify even more micronova events to verify this hypothesis. “Supposedly they are plentiful; they’re just really hard to find,” said Scaringi. “Having found more micronova, hopefully we can try and develop our theories onto how thermonuclear explosions can actually occur when material is magnetically confined on a white dwarf.”
DOI: Nature, 2022. 10.1038/s41586-022-04495-6 (About DOIs).
Listing image by ESO/M. Kornmesser, L. Calçada