Scientists Accidentally Discover 'Millinovas,' a New Class of Cosmic Explosion Hotter and Brighter Than the Sun

Scientists have accidentally discovered a new class of cosmic explosions called "millinovas" that burn hotter than the hottest known star and shine 100 times brighter than the Sun, according to Space.com. The accidental discovery of this new class of cosmic explosion came during a research project that was supposed to look for primordial black holes in the dark matter halo of the Milky Way.
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— Nereide (@Nereide) January 6, 2025
An international research team discovered 29 objects in two satellite galaxies of the Milky Way, the Magellanic Clouds, that display unique characteristics in X-ray emission.
The team named them *millinovae, as their peak brightness is roughly a thousand times lower than… pic.twitter.com/A1jCu2P6kk
A research team led by Przemek Mróz of the University of Warsaw stumbled upon these unusual stellar activities while analyzing 20 years of data from the Optical Gravitational Lensing Experiment (OGLE). "We came across a group of outbursting variable stars exhibiting very characteristic triangle-shaped symmetrical outbursts that did not resemble any previously known variable stars," Mróz said, putting into light how strange this was.
Mira variable stars regularly pulsate due to the entire star expanding and contracting over a period of months. pic.twitter.com/9Py2OCjZqw
— Andrew Rader (@marsrader) May 18, 2016
The discovery team found 28 millinovae in our galactic neighbors, the Large and Small Magellanic Clouds. These cosmic blasts forge temperatures over 600,000 degrees Celsius—about three times hotter than WR 102, which was thought to be the hottest star in the universe. One such event, labeled OGLE-mNOVA-11, caught the interest of the researchers when it exploded late in 2023, enabling detailed observation with the Southern African Large Telescope and NASA's Neil Gehrels Swift Observatory.
Indeed, it is at just that point—the origin story—that millinovae become interesting. These explosions are part of binary star systems: a white dwarf orbits close with a subgiant companion star. The proximity of these stellar partners—orbiting at a distance of only a few days—means that the white dwarf is perfectly positioned to steal material from its swollen companion and create these brilliant explosions. The discovery has opened new avenues in understanding the evolution of stars. Some millinovas blow up again every few years, while others have been seen to erupt just once during the period of study. These may be precursors of Type Ia supernovae—violent stellar explosions that astronomers use as "standard candles" to measure cosmic distances, as per the Daily Galaxy.
The research team proposed two theories for the strong X-ray emission of millinova events, which is a consequence of the incoming stellar material hitting the surface of the white dwarf at an equatorial belt. They could also be a consequence of minor surface thermonuclear reactions of the white dwarf as it accumulates material from its companion. Unlike classical novas or Type Ia supernovas, these explosions are rather weak, with little to no matter being ejected from the white dwarf's surface. The spectroscopic analysis of OGLE-mNOVA-11 revealed distinct emission lines from helium, carbon, and nitrogen ionized atoms, providing crucial insights into the extreme conditions present during these cosmic events.

These observations helped distinguish millinovas from other known stellar phenomena and confirmed their unique place in the hierarchy of cosmic explosions. Analysis allowed the real detection of pronounced emission lines during their most active moments stemming from helium- and carbon-nitrogen-ionized atoms throughout the spectrum associated with OGLE-mNOVAR-11.