What is a Black Hole?
A black hole is a region of spacetime exhibiting such strong gravitational effects that nothing—not even particles and electromagnetic radiation such as light—can escape from inside it The theory of general relativity predicts that a sufficiently compact mass can deform spacetime to form a black hole. The boundary of the region from which no escape is possible is called the event horizon. Although crossing the event horizon has enormous effect on the fate of the object crossing it, it appears to have no locally detectable features. In many ways a black hole acts like an ideal black body, as it reflects no light. Moreover, quantum field theory in curved spacetime predicts that event horizons emit Hawking radiation, with the same spectrum as a black body of a temperature inversely proportional to its mass. This temperature is on the order of billionths of a kelvin for black holes of stellar mass, making it virtually impossible to observe.
Examples of Black Holes
1. Supermassive Black Holes: Supermassive black holes are the largest type of black hole, containing masses between millions and billions of times that of our Sun. These can be found in the center of most large galaxies, including our own Milky Way. For example, the supermassive black hole at the center of our galaxy, called Sagittarius A*, is estimated to be 4 million times the mass of our Sun.
2. Stellar-Mass Black Holes: Stellar-mass black holes form when a star at least 30 times the mass of our Sun collapses. These objects are typically found in binary systems where they interact with a companion star and often emit strong X-ray radiation. For example, A0620-00 is a binary located in the constellation Monoceros, and contains a black hole estimated to be 4 to 10 times the mass of our Sun.
3. Intermediate-Mass Black Holes: Intermediate-mass black holes contain masses between 100 and 100,000 times that of our Sun. These black holes are not as well studied as the two previous categories, since they are not as luminous and therefore harder to detect. An example of an intermediate-mass black hole is ESO 243-49 HLX-1, located around 300 million light-years away from Earth in the galaxy ESO 243-49.
4. Primordial Black Holes: Primordial black holes form from the collapse of extremely dense regions of the early universe and typically have masses much smaller than our Sun. For example, microlensing events detected by the Hubble Space Telescope may be due to primordial black holes with masses around 1/100th of our Sun.
5. Quasi-Stellar Objects (QSOs): Quasi-stellar objects are believed by some to be powered by supermassive black holes at their centers. These objects are very luminous and can emit strong X-ray radiation. An example of a QSO is 3C 273, located around 2.5 billion light-years from Earth in the constellation Virgo. This object is estimated to be over 1 billion times the mass of our Sun.