"Imagine you have been observing a distant galaxy for years, and the galaxy always appears calm and inactive," he said as quoted from Live Science. "Suddenly, its core starts showing a dramatic change in brightness, unlike any ordinary event we have seen before," he added.

The cause of this sudden increase in brightness was initially unclear, but in their article in Astronomy and Astrophysics, the researchers suggested that the most likely possibility is the formation of an active galactic nucleus where a large black hole at the center of the galaxy begins actively consuming surrounding matter.

Meanwhile, the core of an active galaxy emits a wide spectrum of light when the gas around the black hole is heated and shines, although that is not the only possibility. The research team also considers the possibility that stars can be torn apart when approaching a very large black hole.