Active galaxy. Artwork of an active galaxy ejecting jets, seen from the surface of a nearby planet. Jets such as these have been seen in many galaxies. They are thought to form due to a super- massive black hole at the galaxy's centre. Most galaxies are thought to harbour such objects. The huge black hole accretes matter from surrounding space, which forms an accretion disc around it. Charged particle radiation is prevented from leaving the hole by this disc, so it is blasted out from the hole's poles, at nearly the speed of light.