Astronomers believe they may have witnessed a massive black hole wake up and become active in a far-away galaxy.

The cosmic event was first spotted in 2019 when a US telescope detected an “unusual brightness” around 300 million light-years away.

Data showed the “calm” galaxy in the constellation Virgo had mysteriously started to glow.

An international team then tracked this “unprecedented behaviour” and found the galaxy was gradually growing brighter and radiating more light – unlike any other typical event seen before.

They believe the sudden brightness is being caused by a massive black hole waking up at the heart of the galaxy and “feasting” on the surrounding gas.

If the findings are validated in further studies, it would be the first time scientists have observed a black hole becoming active in real time.

Paula Sanchez Seez, an astronomer at the European Southern Observatory (ESO) in Germany, said: “This behaviour is unprecedented.

“Imagine you’ve been observing a distant galaxy for years, and it always seemed calm and inactive.

“Suddenly, its core starts showing dramatic changes in brightness, unlike any typical events we’ve seen before.”

Massive black holes exist at the centre of most galaxies and can have masses more than 100,000 times that of Sun.

The gravity is so strong in black holes that light cannot escape and the normal laws of physics break down.

For the study, the researchers compared the data from European Southern Observatory’s Very Large Telescope (VLT) to other Earth-based telescopes.

They found the galaxy, called SDSS1335+0728, to be emitting much more light at ultraviolet, optical, and infrared wavelengths than before.

Lorena Hernandez Garcia, of the Millennium Institute of Astrophysics and the University of Valparaiso in Chile, said: “The most tangible option to explain this phenomenon is that we are seeing how the core of the galaxy is beginning to show activity.

“If so, this would be the first time that we see the activation of a massive black hole in real time.”

The team said their findings, due to be published in the journal Astronomy & Astrophysics, will provide valuable insight into how black holes grow and evolve and help shed light on the fate of Sagittarius A – the massive black hole at the heart of the Milky Way.

But the researchers said follow-up observations are needed to rule out alternative explanations such tidal events, where a star gets too close to a black hole and is torn apart.

Claudio Ricci, an astronomer from the Diego Portales University in Chile, said: “These giant monsters usually are sleeping and not directly visible.

“In the case of SDSS1335+0728, we were able to observe the awakening of the massive black hole, which suddenly started to feast on gas available in its surroundings, becoming very bright.”