Black holes are objects so dense, and with so much mass, that even light cannot escape their gravity.The existence of black holes has been theorised for more than 200 years. It is impossible to observe them directly, and astronomers had no way to test their theories until Hubble arrived.The high resolution of Hubble made it possible to see the effects of the gravitational attraction of some of these objects on their surroundings. Hubble has also proved that super massive black holes are most likely present at the centres of most, if not all, large galaxies. This has important implications for the theories of galaxy formation and evolution.Black holes exist in different sizes. Stellar black holes, which are around the mass of our Sun, form when very large stars explode as supernovae at the end of their lives. The star's core collapses as the outer layers are blown away, leaving a small but extremely dense ball.Supermassive black holes, many millions of times the mass of our Sun, are of more mysterious origin, and are found at the centre of galaxies. It is in the study of super massive black holes that Hubble has made its biggest contribution.
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