The mysterious brightening of a galaxy far, far away has been traced to the heart of the star system and the sudden awakening of a giant black hole 1m times more massive than the sun. Decades of observations found nothing remarkable about the distant galaxy in the constellation of Virgo, but that changed at the end of 2019 when astronomers noticed a dramatic surge in its luminosity that persists to this day. Researchers now believe they are witnessing changes that have never been seen before, with the black hole at the galaxy’s core putting on an extreme cosmic light show as vast amounts of material fall into it. “We discovered this source at the moment it started to show these variations in luminosity,” said Dr Paula Sánchez-Sáez, a staff astronomer at the European Southern Observatory headquarters in Garching, Germany. “It’s the first time we’ve see this in real time.” The galaxy, which goes by the snappy codename SDSS1335+0728 and lies 300m light years away, was flagged to astronomers in December 2019 when an observatory in California called the Zwicky Transient Facility recorded a sudden rise in its brightness. The alert prompted a flurry of new observations and checks of archived measurements from ground- and space-based telescopes to understand more about the galaxy and its past behaviour. The scientists discovered the galaxy had recently doubled in brightness in mid-infrared wavelengths, become four times brighter in the ultraviolet, and at least 10 times brighter in the X-ray range. What triggered the sudden brightening is unclear, but writing in Astronomy and Astrophysics, the researchers say the most likely explanation is the creation of an “active galactic nucleus” where a vast black hole at the centre of a galaxy starts actively consuming the material around it. Active galactic nuclei emit a broad spectrum of light as gas around the black hole heats up and glows, and surrounding dust particles absorb some wavelengths and re-radiate others. But it is not the only possibility. The team has not ruled out an exotic form of “tidal disruption event”, a highly restrained phrase to describe a star that is ripped apart after straying too close to a black hole. Tidal disruption events tend to be brief affairs, brightening a galaxy for no more than a few hundred days, but more measurements are needed to rule out the process. “With the data we have at the moment, it’s impossible to disentangle which of these scenarios is real,” said Sánchez-Sáez. “We need to keep monitoring the source.”