![]() This one can be correctly described by means of numerical simulations, taking into account the complex distortions that the strong gravitational field prints in spacetime and light rays trajectories. So striking they are, these images fail to report the astrophysical reality. Many of you certainly saw didactic or artistic representations of a black hole in popular science magazines, in the form of a black sphere floating in the middle of a circular whirlwind of brilliant gas. Thus, as soon as the basics of black holes astrophysics developed in the 1970’s, the scientists logically wondered what could look like a black hole. ![]() As a matter of fact,, if a black hole remains by itself invisible, it “switches on” in its characteristic way the materials it attracts, and distorts the background starry field by gravitational lensing. Giant black holes, such as those currently lurking at the centers of galaxies, can be also surrounded by a stellar cluster, whose orbital dynamics is strongly influenced. Swirling in a spiral motion, the gas forms a hot accretion disk within which it emits a characteristic spectrum of electromagnetic radiation. However, in typical astrophysical conditions, whatever its size and mass (ranging from stellar to galactic scales) a black hole is rarely bare but is dressed in gaseous material. Due to strong gravitational lensing, such a « bare » black hole could leave an observable imprint on a starry background. Seen in projection on a sky background, the event horizon would have the aspect of a perfectly circular black disk if the black hole is static (the so-called Schwarzschild solution) or of a slightly distorted one if it is in rotation (the Kerr solution). Contrarily to non-collapsed celestial bodies, their surface is neither solid nor gaseous it is an immaterial border called the event horizon, beyond which gravity is so strong that nothing can escape, not even light. ![]() According to the laws of General Relativity, they are by themselves invisible. Black holes are to many the most mysterious objects in space. ![]()
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