Hubble astronomers have watched a knot in the jet of gas and radiation from M87's black hole that appears to brighten steadily for several years, then fade, only to brighten again. While researchers can't be sure what's causing this phenomenon, some speculate the jet is hitting interstellar material. Another hypothesis is that the jet's magnetic field lines are being squeezed together and unleashing a large amount of energy.
Video Credit: NASA, ESA, Z. Levay, G. Bacon, and M. Estacion (STScI)