As theorists first presented it, a black hole represents the death throes of a large star weighing perhaps three times as much as our sun. Stars usually obtain their energy from nuclear fusion, the conversion of atomic nuclei of hydrogen into nuclei of helium (fusion powers the awesome hydrogen bomb). The hydrogen lasts for billions of years in most stars, but eventually it runs out. At that point, gravity takes over. The monstrously large star starts to compress under the pull of its own gravity, becoming denser and denser and smaller and smaller. Eventually, the star might end up no larger than Manhattan Island, but with a density billions of times that of lead. It has become, in effect, a ball of immense gravitational force. Its gravitational pull is so powerful that no dust, gas, or even radiation nearby can avoid being pulled into the black hole. The force is powerful enough to keep anything from leaving the black hole—including light. In other words, it is theoretically, as well as physically, impossible to see a black hole. The only way to do it would be to approach so closely that one is swallowed up by the hole, disappearing into its core before one even has the chance to write down what it looked like.