Since ancient times, people have witnessed the sudden appearance of bright objects in the sky that can shine for weeks or even months.
But now astronomers have found that the same process that causes stellar explosions like some types of supernova can also happen on a much smaller scale.
"We have discovered and identified for the first time what we are calling a micronova," said Simone Scaringi, an astronomer at Durham University in the UK.
"What's incredible is that these bursts are very fast; they only last 10 hours to half a day and then are gone," said Dr Scaringi, who led the discovery.
These fleeting flashes appear to just happen at the poles of some types of magnetised stars, rather than right across the star.
Dr Scaringi said the discovery, reported in the journal Nature, challenged our understanding of how thermonuclear explosions happened in some stars.
White dwarfs and stellar explosions
Sudden explosions in the sky were called "novae" by 17th century astronomer Tycho Brahe.
While novae is Latin for "new star", these explosions actually happen on different types of old stars at the end of their life.
White dwarfs, the dead core of Sun-like stars that have expended all their hydrogen, are one of the main types of stars involved in stellar explosions.
When white dwarf stars, which are about the size of Earth, are close to another star, they suck hydrogen gas off their companion like vampires.
The gas builds up on the surface of the white dwarf, causing the star to heat up, triggering a runaway thermonuclear reaction.
The hotter it gets, the more hydrogen fusion occurs.
If enough gas builds up on the star it reaches a threshold called the Chandrasekhar limit, and ignites and blows up in a cataclysmic process known as a type 1a supernova.
If the white dwarf is further away from its companion or it sucks up less gas, the entire shell around the star ignites, but the star itself does not blow up. This event is known as a classic nova.
Mysterious spikes detected by TESS
While trawling through data from NASA's Transiting Exoplanet Survey Satellite (TESS), Dr Scaringi and his colleagues detected a series of very fast, rapid bursts of energy coming from a white dwarf star.
While these bursts were a million times less powerful than novae explosions, they still packed a punch, burning through 20,000 trillion tonnes of material – roughly the equivalent of a hefty asteroid – in just hours.
Soon after, the team found another white dwarf in the TESS data doing the same thing, and a third white dwarf was confirmed with observations using the European Southern Observatory's Very Large Telescope in Chile.
"We didn't really expect these thermonuclear explosions to happen locally on the poles.
"We thought before that material would spread around the whole surface, rather than remain confined."
"It just goes to show … the universe is very dynamic."
So what's going on?
In a separate paper accepted for publishing in the Monthly Notices of the Royal Astronomical Society, Dr Scaringi and his team suggest micronovae can happen on white dwarfs that have strong magnetic fields at their poles.
The magnetic fields funnel material to the poles in columns, stopping the gas from spreading sideways when it hits the surface of the star.
"As material piles up into this column, it gets hotter and hotter and hotter until the column ignites," he said.
"That is very new, we never thought that that would actually be a possibility."
It's also possible this phenomenon is very common.
"The conclusion is that maybe these things are quite regular and quite normal if you look at the right systems," Dr Scaringi said.
"But they are just so hard to find because you have to be looking at the right time, otherwise you'll just miss it.
"We will be excited to find more of these, we think there should be plenty more."
Putting together a picture
The new discovery was exciting, commented Brad Tucker, an astronomer at the Australian National University.
"It's cool because, hey, who doesn't like a good explosion?" said Dr Tucker, who studies supernovae lurking in TESS data.
"If we really want to understand the end stages of stars, we really need to understand the nitty-gritty detail.
"And this [discovery] is very relevant for what I do in the work on a supernova because the mechanism is essentially the same.
"This sort of puts this whole picture together of what happens."
While Dr Tucker said it made sense that there would be smaller explosions like this on white dwarfs, the discovery was only possible because of TESS.
NASA's planet-hunting telescope looks at large slabs of sky every 30 minutes and can detect when stars change in brightness.
"The power of TESS is you see these really short time-scale events that are just physically impossible to see from the ground," Dr Tucker said.
Whether white dwarfs that have micronovae can build up enough mass to become novae, or eventually blow up as supernovas, is still an open question, Dr Scaringi said.
"Who knows what the implications for this [are], but for now, we just wonder about the universe, and appreciate that."
