When a star wanders too close to a massive black hole, the star can be ripped apart and a part of it (half) can be swallowed (accreted) by the black hole. This means that a black hole that was previously faint (or even undetected) can suddenly become very bright in X-rays, before fading over about one year. Even though this only happens once every 6000 years or so in any one galaxy, the number of galaxies means that a tidal disruption event is always happening somewhere in the observable Universe. Identifying tidal disruption events is one way to find black holes that were previously unknown.