astronomers have identified a number of black hole candidates. One might wonder how they can detect such coal black objects against the backdrop of darkest space. Like a ghost sitting on a seesaw and lifting a startled child resting on the other end, astronomers have sensed these unseen bodies through the reactions of those around them. Many black hole candidates have been found in binary star systems by noting their actions on visible stars. Black holes are thought to victimize their companions by absorbing their material in a process called “accretion.” As such captured matter plunges into the black hole’s bottomless gravitational well, it reaches ultrahigh temperatures, causing it to emit highly energetic radiation, mainly in the form of X-rays. Astronomers have recorded such characteristic signals, leading them to conclude that black holes likely exist.

Black holes, according to current thinking, comprise one of three possible end points for stellar evolution. When a star’s primary source of energy—its nuclear fuel—becomes exhausted, its central core collapses and its outer envelope expands. The peripheral material exudes into space—either in a gradual dissipation (for lighter stars) or in a catastrophic supernova explosion (for heavier stars). In the former case, the remaining core settles down into a hot, tiny beacon, called a white dwarf. Such will be the fate of the Sun.

A star between 1.4 and 3 times the mass of the Sun, however, suffers a far more turbulent fate. Its core implodes so suddenly and energetically that the very atoms inside it are completely pulverized. Throughout the collapsing body, positive protons and negative electrons fuse into neutrons. This happens simultaneously with the supernova explosion of the outer shell—similar to the pulling back of the undertow when ocean waves are building up. The core—an ultradense amalgamation of neutrons known as a neutron star— remains as a relic of the catastrophe.

Finally, if a star is more than three times heavier than the Sun, its violent transformation is even more powerful. Not just the core’s atoms but also its elementary particle constituents are utterly



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