“What’s the rarest thing you can see in the universe?” Astronomers now have a contender: a supermassive black hole tearing apart a colossal star and unleashing a flare so luminous it outshines 10 trillion Suns.
Long-term sky surveys began catching sight of an unremarkable point of light in a faraway galaxy back in 2018. That changed within months. The object, an active galactic nucleus, or AGN, given the designation J2245+3743, brightened by a factor of 40, peaking at 30 times the luminosity of any black hole flare ever observed. Lying about 10 billion light-years away, with an estimated central black hole mass of 500 million solar masses, light reaching Earth today left when the universe was only a third of its current age.
The team behind the discovery, led by Matthew Graham of Caltech, determined that the flare was the result of a tidal disruption event-a phenomenon in which a star ventures too close to a black hole and is destroyed by tidal forces. In this case, the doomed star was at least 30 times the mass of the Sun, making it one of the largest ever seen consumed in such an event. “Stars this massive are rare,” said K. E. Saavik Ford of the City University of New York. “But we think stars within the disk of an AGN can grow larger. The matter from the disk is dumped onto stars, causing them to grow in mass.”
The mechanics of a TDE are extreme: as the star approaches a black hole’s event horizon, the difference in gravitational pull between its near and far sides stretches it into a long filament a process astrophysicists call spaghettification. In some cases, compression perpendicular to the stretching can trigger “pancake detonation,” releasing bursts of thermonuclear energy. The stellar debris forms an accretion stream; some material is ejected, while the rest spirals inward, heating to millions of degrees and radiating across the electromagnetic spectrum.
The total energy output for J2245+3743 is estimated to be 10⁵⁴ ergs equivalent to converting the Sun’s entire mass into energy via Einstein’s E = mc². That’s millions of times more energy than a typical supernova and firmly in the realm of what researchers are now calling extreme nuclear transients (ENTs), a newly recognized class of ultra-bright, long-lived black hole flares.
It is rare to detect such an event inside an AGN. Already luminous because of the constant infall of gas and dust, the variability in AGNs often hides other transient phenomena. The brightness of this flare overwhelmed the AGN’s baseline glow. Early spectra, taken with the 200inch Hale Telescope, didn’t show anything particularly exciting-but follow-up observations with the W. M. Keck Observatory in 2023 revealed the truly exceptional nature of the flare. Data from NASA’s retired Wide-field Infrared Survey Explorer ruled out beaming effects, confirming that the light was emitted in all directions.
The apparent six-year duration of the flare is partly an illusion created by cosmological time dilation. As Graham explained, “Seven years here is two years there. We are watching the event play back at quarter speed.” This stretching of time occurs because the expansion of space elongates the wavelength of light and slows the perceived evolution of distant events.
This finding highlights the impact of long-term searches such as those currently being conducted by the Zwicky Transient Facility (ZTF) and Catalina Real-Time Transient Survey. Both projects survey the sky each night to establish archives that will enable astronomers to study the rise and fall of transient events over periods of years. “We never would have found this rare event in the first place if it weren’t for ZTF,” Graham said. The implications are profound.
Finding a giant star torn apart in an AGN’s disk suggests that such environments can harbor stellar giants, which in turn are capable of creating incredibly luminous TDEs. The Vera C. Rubin Observatory and other next-generation surveys likely will harvest many more of these rare cosmic feasts, providing new information about how black holes feed, the life cycles of stars in extreme conditions, and the growth of supermassive black holes earlier in the history of the universe.
