How, not that long ago, the center of the Milky Way exploded

Right now, despite its 4.2 million solar masses, Sagittarius A*, the gigantic black hole at the center of the Milky Way, appears to be a harmless, sleeping giant. But that wasn’t always the case. If one of our ancestors, Australopithecus, had been able to observe the skies over Africa 3.5 million years ago (thus, long after the extinction of the dinosaurs) just as intensively as we can, he might have been able to witness a gigantic, approximately 300,000-year-long explosion in the center of t...