Science

Meet the micronova: Astronomers discover a strange new stellar explosion

These bursts almost always happen when a white dwarf strips material from a companion star.

Astronomers have discovered a new way for stars to explode. In a paper published Wednesday in Nature, a team of astronomers describes small, fast stellar explosions, which they’ve named micronovae. A micronova can flare up in minutes and fade in just a few hours.

The study’s authors suggest that micronovae happen when a white dwarf (a small, dense remnant of a Sun-like star that burned up all its fuel) pulls in material from a nearby star. If white dwarf’s magnetic field is strong enough, it channels that incoming material to a small area around its magnetic poles, where it triggers a relatively small, contained thermonuclear reaction. The resulting explosion is about a millionth as powerful as a typical nova.

What’s new — Durham University astrophysicist Simone Scaringi and his colleagues were combing through data from NASA’s Transiting Exoplanet Survey Satellite (TESS), looking for images of white dwarfs. The team wanted to learn more about how these super-dense stars capture, or accrete, material from their partners in binary star systems (systems where two stars orbit a shared center of gravity). But a series of bright, fast bursts of light from three white dwarfs caught their attention.

TV Columbae, a white dwarf, suddenly flared twice as bright as normal within about half an hour. Within another 12 hours, the sudden brightness had faded again. Blink and you’ll miss it, in astronomical terms. Three days later, it happened again — and repeated at least one more time.

Another white dwarf star, El Ursae Majoris, did something very similar. So did a recently-discovered object called ASASSN-19bh; an x-ray instrument on the European Southern Observatory (ESO)’s evocatively-named Very Large Telescope (VLT) confirmed that it, too, was a white dwarf. Clearly, some white dwarfs were up to something very strange.

“The bright and fast bursts really stood out in the data, and it took over a year for us to make the connection that what we were seeing could actually be thermonuclear bursts,” Scaringi tells Inverse.

Based on how quickly the stars brightened, how slowly they faded, and the amount of energy being released in each burst, Scaringi and his colleagues suspect that giant thermonuclear explosions are happening in the stars’ upper layers. The astrophysicists suggest that these reactions are triggered by gas pulled in from the companion piling onto the area around the white dwarf’s magnetic poles, compressing atoms together under enough pressure to trigger nuclear fusion — but only in the area around the poles. That’s different from a full-fledged nova, in which accreted gas triggers thermonuclear reactions that engulf the entire white dwarf.

Dubbed a “micronova,” one of these stellar explosions releases about a millionth of the energy of a typical nova. But a micronova is still a cosmic force to be reckoned with. The ones Scaringi and his colleagues observed each burned up an average of 20 million trillion kilograms of stellar gas in a matter of minutes or hours (for comparison, Earth’s mass is about 6 trillion trillion kilograms).

This is what astronomers think is happening within a micronova system. ESO/M. Kornmesser, L. Calçada

Here’s the background — This is how a typical nova happens: When a white dwarf and its companion star orbit each other too closely, the white dwarf’s gravity can actually pull gas away from the other star. Astronomers call this accretion, and when that accreted gas lands on the surface of the already super-dense white dwarf, the heat and pressure can squish hydrogen atoms until they start fusing together into helium atoms.

One thermonuclear reaction sets off a series of others, so the whole surface of the star is engulfed in what’s called a thermonuclear runaway. The whole star burns brighter for several days or weeks, and then — once all the extra nuclear fuel is used up in those runaway reactions — it settles back to its usual luminosity as if nothing ever happened.

Because the bursts of optical and UV light Scaringi and his colleagues witnessed were so brief, compared to the several days or weeks of a proper nova, they suspect that micronovae involve a much smaller amount of material and a much smaller area.

The most likely explanation, Scaringi and his colleagues claim, is that some white dwarfs, like TV Columbae and ASASSN-19bh, have especially strong magnetic fields, which funnel all the incoming gas straight to the star’s magnetic poles and hold it there. So when that extra mass triggers a thermonuclear runaway, it’s confined to the poles instead of swallowing up the whole star’s surface.

If they’re right, that could also explain why micronovae seem to happen in clusters. In the TESS data Scaringi and his colleagues examined, the white dwarf TV Columbae flared up three times, each about three days apart. El Ursae Marjoris flared up twice, with a day in between. The astrophysicists suggest that as gas flows in from the companion star, stars like TV Columbae and El Ursae Majoris may be burning up just part of that flowing stream of material in each short burst.

ASASSN-19bh, on the other hand, seems to gobble up its whole accretion column at once, in a single big burst with about twice the energy of TV Columbae’s whole cluster of micronovae combined.

Why it matters — Scaringi and his colleagues may have just discovered a new kind of stellar explosion. They may also have explained the strange flashes of light astronomers have noticed from some white dwarf stars for the last 40 years. reports of brief optical and UV flareups from TV Columbae in particular date back to at least the 1980s. And astronomers have reported seeing similar flareups from a handful of other white dwarf stars, such V1223 Sagittarri and DW Cancri, both of which fit the description of a white dwarf that might be prone to micronovae: They’re both accreting mass from their partners, and they’re both likely to have very strong magnetic fields.

“It may be that localized thermonuclear runaways on accreting white dwarfs are more common than previously thought,” wrote Scaringi and his colleagues in their recent paper.

Micronovae may also shed some light on the strange goings-on of neutron stars (the small, incredibly dense ex-cores of collapsed supergiant stars). Astronomers watching neutron stars with X-ray telescopes have noticed short, bright bursts of X-ray radiation that behave a lot like the short, bright bursts of optical and UV light from white dwarfs — only faster, and at much shorter wavelengths.

Scaringi suggests that the two kinds of stellar explosion may share a similar cause. “Because neutron starts are so small, the even smaller ignition area will burn through material faster than what we see during micronovae on the much larger white dwarf cousins,” he says.

What’s next — Scaringi and his colleagues plan to search for more micronovae, and hopefully observe them as they happen with spectrographs (instruments that break light into its individual wavelengths, which correspond to the chemical elements that make up the light’s source) and even space-based X-ray telescopes. Studying the properties of the X-ray radiation from a micronova could help confirm that it is, in fact, a thermonuclear reaction and not something else.

To catch a micronova in action, however, astronomers will have to search a huge swath of the sky, then react quickly to get a closer look with the right instruments when they see a white dwarf star suddenly brighten.

“This is very challenging because micronovae are fast, transient, events,” Scaringi tells Inverse. “Knowing where to look, and when to look, with such short notice is going to be very difficult, but not impossible.” The best bet for catching a micronova in action will be the New Technology Telescope and the Very Large Telescope, both perched on the edge of Chile’s Atacama Desert as part of ESO’s La Silla Observatory. Both telescopes “can have response times as fast as a few hours or less,” according to Scaringi.

In the meantime, Scaringi and his colleagues are also working on a detailed model to explain the physics that trigger a micronova, which they’ll eventually be able to compare with new data.

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