These are a whole new kind of star explosions that scientists have dubbed ‘micronovas’.

We know that things are often violent in the universe. Exploding stars, in which the outer layers are thrown into the universe and which release an incredible amount of energy, are certainly not rare. Meanwhile, we know of the existence of several stellar explosions, including ordinary novas and supernovas. But now we can add a new kind of star explosion to this list, namely the micronovas.

Micronova

Contrary to the name, micronovas are extremely violent outbursts on the surface of white dwarf stars (see box). However, micronovas are less powerful than regular novas; the type of star explosion astronomers have known for centuries. In addition, a micronova is smaller in size and lasts only a few hours. Finally, a micronova consumes about one-millionth the amount of fuel of a regular nova. Hence the term ‘micro’; one millionth.

More about white dwarfs
As a star ages, the hydrogen fusion in the star’s core stops and it swells. Then a red giant is created. When the star then blows away its outer layers of gas and dust, the core contracts, leaving a white dwarf. The star has come to the end of its life. Such a white dwarf is about the same size as Earth, but has a much greater density (a teaspoon of white dwarf matter weighs about as much as an elephant on Earth!). The dying process also has a major influence on the planets that orbit such a star. Most planets will be drawn towards the star and swallowed. Our sun will also end up as a compact white dwarf in five billion years. In fact, about 97 percent of all stars turn into white dwarfs.

The team first encountered these mysterious micro-explosions when analyzing data from planet-hunter TESS. “When we went through that data, we discovered something unusual: a bright flash of light that lasted for a few hours,” said researcher Nathalie Degenaar. “As we searched further, we found more such signals.”

Three white dwarfs

Ultimately, the astronomers spot micronovas on three different white dwarf stars. All three have a companion star from which they ‘steal’ hydrogen. In addition, they possess a strong magnetic field. The researchers suspect that the hydrogen gas collects at the poles due to the magnetic field. The idea is that the temperature and pressure at the poles will slowly rise until the hydrogen fuses into helium, creating a thermonuclear explosion.

This artistic impression shows a binary star system, consisting of a white dwarf (in the foreground) and a companion star (in the background), in which micronovas can occur. The white dwarf extracts matter from its companion, which is directed to its poles. When the material falls on the hot surface of the white dwarf, it causes a micronova explosion that is confined to one of the white dwarf’s poles. Image: Mark Garlick

The findings show that the universe still has surprises in store for us. “We thought we knew how explosions occur on stars,” says study leader Simone Scaringi. “But this discovery suggests that they could also get underway in completely different ways.” The results suggest that hydrogen fusion may also be a very local phenomenon. The hydrogen can accumulate at magnetic poles in some white dwarfs, limiting the fusion to that spot.

Make no mistake! Although the prefix ‘micro’ suggests that these are insignificant events, nothing could be further from the truth: One such eruption consumes approximately 20,000,000 trillion kilograms, or approximately 3.5 billion Great Pyramids of Giza, of fuel!

red act

Why are astronomers only now discovering the existence of micronovas? “That is partly because they are so short,” explains researcher Paul Groot. “They are hard to catch in the act. The phenomenon is very similar to the Type-I outbursts in neutron star systems, but only now were we able to establish the link with the data from TESS.”

The newly discovered micronova challenges existing ideas about star explosions. The hunt for micronovas continues. The team plans to capture more such extremely violent explosions on depleted stars in the near future using a combination of long-term observation campaigns and rapid follow-up measurements. And possibly many more are waiting to be discovered.