Black holes are one of the most fascinating and mysterious astronomical objects. They were hypothesised for the first time in 1916, a year after Albert Einstein’s publication of his general theory of relativity, when the German astronomer Karl Schwarzschild presented the first exact solution of the theory’s equations, known as “Einstein equations”. His solution also predicted the existence of objects whose gravitational field is so intense that it prevents matter and light from escaping once a border called the “event horizon” (the term “black hole” would be coined several decades later) has been passed. For many years, the majority of scientists considered black holes to be a simple mathematical curiosity, without astrophysical reality. However, beginning from the second half of last century, theoretical and observational progress in astrophysics and cosmology gave more and more credit to the hypothesis that black holes may be real objects, both as final stages of the evolution of massive stars and in the guise of enormous “attractors” at the centre of many galaxies (in the latter case, you talk about “supermassive” black holes).
