Perhaps one of the most intriguing concepts of astrophysics is that involving black holes, regions of space so dense and with such an overwhelming force of gravity that nothing can escape them once something enters their neighbourhood (or event horizon) and this includes light, thus for the term "black hole". In other words, the escape velocity of a black hole is not only great but exceeds the speed of light. The event horizon is often described by the Schwarzschild radius and is dependent on the mass of the black hole with more massive black holes having a greater event horizon or Schwarzschild radius. Matter and/or radiation which infringes the Schwarzschild radius is consumed by the black hole without any visible event being possible to an observer outside the Schwarzschild radius.

Since nothing can escape a black hole once it enters its event horizon, an observer outside the event horizon cannot observe directly the black hole. However, the indirect consequences of a black hole on its immediate region provide the necessary evidence for its existence as well as its characteristics (ex. mass). For example, black holes are quite often associated with binary stars and where one of the member stars collapses, thus forming the black hole and then starting to accredit material from the other member star with X-rays being emitted when the matter is about to cross the event horizon.

Black holes are characterized into one of four types based on their mass and which in turn defines their Schwarzschild radius since these two parameters are interrelated. Supermassive black holes are characterized with a mass ranging from 10⁵ to 10⁹ solar masses and have a Schwarzschild radius of 0.001 to 10 astronomical units; they are believed to exist at the center of large galaxies and to be created by the cannibalization of many smaller black holes and other stars and matter. The next category is that of intermediate mass black holes whose mass is equivalent to approximately 1000 solar masses with a Schwarzschild radius of about 1000 meters (similar to the radius of Earth) and which are also believed to be the net aggregate of smaller black holes. Stellar mass black holes are lighter with a mass of approximately 10 solar masses and a Schwarzschild radius of only 30 km and can be formed by the collapse of individual stars or a member from a binary neutron star. The final category is that of primordial black holes with a mass equivalent to one lunar mass and a Schwarzschild radius of 0.1 mm and which were created during the early stages of the universe and/or are produced by cosmic rays and other high-energy events.

Note: Cygnus X-1 has been regarded for a long time as the best and most probable candidate for a black hole. It lies at a distance of approximately 6,000 light-years away in the constellation of Cygnus. HDE 226868 is a blue supergiant with a mass of about 30 solar masses and which is locked by another "star" with a mass of between 5 and 10 solar masses in a binary formation and with a period of 5.6 days. Since such a mass precludes any type of white dwarf (or neutron star), the only remaining alternative is a black hole and which at one time was a member of this binary system and prior to its collapse. Cygnus X-1 was discovered in the early 1970's owing to its x-ray emissions. The bright central star in the image below is HDE 226868 and the companion to the black hole whereas the emission nebulosity to the upper left is Sh2-101.