![]() In the past it was believed that supermassive black holes started as stellar mass black holes that accreted matter over time. ![]() Their enormous gravity causes nearby stars and gas to swirl around them, getting closer and closer… this ‘accretion’ of matter onto the black hole powers some of the most energetic objects in the Universe, including quasars and blazars. We know that supermassive black holes lurk at the centres of most galaxies, including the Milky Way. There are many remaining mysteries surrounding black holes, one being that they seem to be either ‘stellar mass’ (a few times the mass of the Sun) or ‘supermassive’ (millions, if not billions, of times the mass of the Sun). Future ESA missions such as Webb, LISA and Athena, could help prove or disprove these theories. Other black formation theories exist, for example that most black holes formed immediately after the Big Bang. Eventually a point is reached when even light cannot travel fast enough to escape – this is a black hole. If the compact core that remains is more than three times the mass of the Sun, it is crushed into a smaller and smaller volume, its gravitational attraction increases, and its escape velocity becomes larger. When the outer layers fall onto this compact core, they bounce back and are expelled into space as a supernova. But larger stars with iron cores begin to collapse, starting with the innermost layers. Stars up to about ten times the mass of the Sun expand to become red giants and then white dwarfs surrounded by planetary nebulas (which have nothing to do with planets). Eventually, the star has no material left that can be transformed into energy, but what happens next depends on the size of the star. During a star’s lifetime, it produces energy by fusing heavier and heavier elements in its core.
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