When astronomers refer to “compact objects,” they are generally referring to objects significantly more dense than a star or a planet. For example, white dwarfs or neutron stars are extremely dense stars that have collapsed, no longer able to produce a sufficient amount of pressure within to prevent their outer layers from falling into their centers. Under extreme conditions, these collapses can trigger the formation of a black hole – a region of space in which gravity is so strong that even light cannot escape.
Black Holes – Very Massive
Black holes with masses comparable to that of the Sun are scattered through the Milky Way and neighboring galaxies. Scientists also have found strong evidence of massive black holes – a million or more times more massive than the Sun – in the centers of many galaxies. In fact, one of these supermassive black holes sits at the heart of our very own Milky Way.
Detecting Black Holes
Compact objects are difficult to observe directly. Fortunately any ordinary matter falling toward them – or disappearing entirely into a black hole – tends to heat up and radiate in the process. Sometimes great streams, or “jets,” of matter and energy surge into space at velocities nearly equal to the speed of light. As matter falls onto a compact object, energy is often released in the form of X-rays and gamma-rays. Likewise, rotating neutron stars can produce copious amounts of high-energy radiation but because Earth's atmosphere absorbs most of this kind of radiation, observations must be made from space. Measurements have revealed that these phenomena are responsible for the highest energies yet detected in the universe.