Astronomers Capture Evidence of a Micronova

Thursday, April 21, 2022

The first known record of an exploding star comes from Chinese astronomers in the second century. A radiant object, bursting with color, appeared suddenly in the night sky and glowed for about eight months before fading away. In the 11th century, the glow of an exploding star hung around for two whole years, appearing brighter than the moon in the beginning. A few centuries have passed since one lit up the darkness above, but astronomers, with their modern-day telescopes, can look for them far beyond Earth, deep in the cosmos.

The non-astronomers among us may not be familiar with the science-y terms that describe massive stars running out of hydrogen, collapsing in on themselves, and destructing in a burst of bright light, but we’re acquainted with the word that describes it all: supernova. The name just sounds mystical and ✨ sparkly ✨, and the pictures are usually beautiful.

Now allow me to introduce you to a new term: micronova.

Astronomers have discovered a new kind of stellar explosion that occurs on the surface of small, dense stars known as white dwarfs. So far, the researchers have spotted these explosions on three white dwarfs, each more than a thousand light-years from Earth. Micronovae, as their name suggests, are far less powerful than supernovae, but they still produce tremendous amounts of energy.

My first reaction to this news, as a very serious science journalist, was Awww. How adorable! A supernova I can take seriously, but a micronova? With a name like that, I just want to squish it. The actual, normal-person takeaway, however, is this: “It just shows how the universe is still full of surprises,” Nathalie Degenaar, an astronomer at the University of Amsterdam and a member of the team that made the discovery, told me. The cosmos is home to an assortment of stars, many of them completely unlike our own, leading their own little star lives. And the celestial objects can still spring new tricks on the astronomers trying to better understand them.

The evidence for micronovae showed up in data from a space-based telescope that Degenaar and her colleagues use to study white dwarfs, particularly the way the stars interact with cosmic material in their surroundings. While analyzing the observations, the scientists noticed mysterious flashes of light. The bursts, bright and unexpected, lasted for several hours. After more follow-up observations, the team realized that “what we were seeing could actually be thermonuclear explosions,” Degenaar said. Astronomers already knew that explosions could occur in two-star systems when a white dwarf steals some hydrogen from its companion star and wraps those molecules around itself. The new coat of hydrogen, heated by the surface of the white dwarf, ignites and burns itself away, producing an explosion that lights up the star’s entire surface for weeks, bursts that astronomers refer to as novae. But they had never seen short-lived flickers like this on a white dwarf before, popping up in small regions.

So they gave the little light shows a new name. Which seems like quite the flex! But such is the nature of astronomy—you find something surprising, you get to pick whatever name you like. “Astronomers will name things all the time,” Raffaella Margutti, an astrophysicist at UC Berkeley who studies stellar explosions, told me. “Some names do not get any traction in the community and do not propagate outside a few papers from some authors; some others get adopted by the community.” About a decade ago, when astronomers detected a new kind of stellar explosion produced when two very dense celestial objects collided, one astronomer suggested calling it a “macronova,” while another suggested “kilonova.” The latter name stuck, Margutti said.

As for the latest discovery, “there is no record of the term micronova having been used previously in astrophysics,” Gregg Wade, an astronomer at the Royal Military College of Canada who studies stars and their properties, told me. But “the phenomenon they describe seems physically plausible, and micronova seems like an apt name for it,” he said.

Micronovae might be common in the universe, but because they are so short-lived, even astronomers’ most sophisticated instruments can miss them. Astronomers now better understand white dwarfs, which is what stars like our sun become after they’ve run out of fuel. (White stars are often described in popular media as “zombie stars” because of their previous lives as sun-like stars. But I like to think they’re the most relatable stars in the universe, because they are, quite literally, burned out.)

There are so many un-sun-like stars out there, and astronomers are eager to understand what makes them tick. Red dwarfs, for example, are cooler and dimmer compared with sun-like stars, and if you lived on a planet revolving around one, the sky would resemble a perpetual sunset. Neutron stars, left behind when a massive star goes supernova, are so dense that a teaspoon of the material inside has the same mass as a small mountain on Earth. Pulsars are absolute weirdos; they spin rapidly and emit luminous, electromagnetic beams like beacons. In 2017, astronomers discovered an unusual star that went supernova, somehow survived, and then exploded again. Astronomers may discover that stellar explosions come in still more sizes—nanonova, perhaps? (Now that’s cute.) The familiar pinpricks of light in our night sky, and the many others beyond our view, are far more quirky than anyone imagined.

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Marina Koren
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