A newly discovered type of stellar explosion could help us better understand thermonuclear outbursts on dead stars.
The new phenomena are called micronovae, and they take place on the surface of white dwarf stars that are actively slurping down material from a close binary companion. The accumulation of material onto the white dwarf results in a localized thermonuclear burst: the micronova.
These explosions have been seen burning through tens to hundreds of quintillions of kilograms of stellar material in hours, astronomers say.
If that's hard to imagine, this is in the ballpark of several billion Great Pyramids of Giza, according to the researchers. Or, if you prefer another comparison, around a thousandth of the Moon's mass.
"We have discovered and identified for the first time what we are calling a micronova," says astrophysicist Simone Scaringi of Durham University in the UK.
"The phenomenon challenges our understanding of how thermonuclear explosions in stars occur. We thought we knew this, but this discovery proposes a totally new way to achieve them."
White dwarfs in close binary systems can function as thermonuclear explosion machines. A white dwarf is what is known as a 'dead' star – the remaining collapsed core after a main sequence star has run out of fuel and ejected its outer material. Other stars of this kind, in different mass classes, include neutron stars and black holes.
This collapsed core is very dense. White dwarf stars have a mass up to 1.4 times the mass of the Sun, packed in a sphere the size of Earth. Many of them can be found in binary systems.
In some rare cases – about 10 have been identified in the Milky Way – the binary systems are close enough that the white dwarf strips material from the companion, resulting in what is known as a recurrent nova.
As the stars two whirl around each other, material – primarily hydrogen – is siphoned off the companion by the smaller, denser and more massive white dwarf. This hydrogen accumulates on the white dwarf's surface, where it heats up.
Periodically, the mass becomes so great that the pressure and temperature at the bottom of the layer is sufficient to trigger a thermonuclear explosion, violently expelling the excess material into space. That's the nova.
A micronova, Scaringi and his team found, is like a smaller version of this explosion.
The researchers first identified a white dwarf emitting a micronova in data from the TESS exoplanet-hunting telescope. TESS is optimized for finding very small brightness variations in stars with orbiting exoplanets; the exoplanet passing in front of the star causes a very small dimming.
In TESS data, the team discovered micronovae when they found a brief flash of light from a white dwarf star, rather than a dimming. This prompted a search for similar events in other white dwarfs. In total, they found three bursts – the third of which, after follow-up observations, led to the discovery of a previously unknown white dwarf star.
But the flashes were too small to be a nova, which are much more powerful and longer-lasting. So the team set about finding a scenario that could explain the observations. They found that the most likely explanation was micronovae.
When a white dwarf with a powerful magnetic field is in a close binary, it can siphon material from its companion. The magnetic field channels this material to the white dwarf's poles, where it accumulates to eventually cause an outburst, similar to (but smaller in scale) than a typical white dwarf nova.
"For the first time, we have now seen that hydrogen fusion can also happen in a localized way," says astronomer Paul Groot of Radboud University in the Netherlands.
"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. 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 finding could solve a decades-long mystery. One of the white dwarfs, in the binary system TV Columbae, has been observed exhibiting similar flashes over the last 40 years or so. Similar bursts have also been reported on other highly magnetized white dwarfs over the years. This explanation could finally tell us why.
The finding suggests that the bursts may be quite common, but astronomers are going to need to collect more observations in order to understand them in greater depth.
"It just goes to show how dynamic the Universe is," Scaringi says. "These events may actually be quite common, but because they are so fast they are difficult to catch in action."
The research has been published in Nature.