Here’s What Happens When A Dormant Black Hole Becomes Active Again

The whole process of a black hole devouring a star can last for a surprisingly large amount of time. In a Nature paper published in 2017, scientists detailed the findings of a tidal disruption event that likely lasted a whole decade. As mentioned above, the radiations emitted (especially in the X-ray region) serve as definitive proof of a black hole’s existence.

Finding a dormant black hole is tough because of the lack of any such bright radiation. But once it wakes up, it does so in a rather violent fashion. In fact, ASASSN-15lh — once known as the brightest ever supernova observed to date — later turned out to be a tidal disruption event. Then there’s one of the most interesting black holes of all, right in Earth’s own galactic backyard.

The supermassive black hole at the center of the Milky Way galaxy is also dormant. This black hole goes by the name Sagittarius A* (also known as A-star) and was seen for the first time in 2022. 

But it was not dormant some three centuries ago when it let out a massive burst of X-rays. And some 3.5 million years ago, it emitted a flare of radiation so bright that it was clearly visible from the Earth, according to research published in The Astrophysical Journal. At the moment it is flickering, but there is no conclusive evidence of it becoming destructively active for some time to come.