Black Hole

Black Holes  are regions of space-time that are exhibit such strong gravitational forces that nothing (except perhaps Hawking radiation ) can escape them. They are formed from the collapse of stars with masses greater than 20 M☉, and can weigh upwards of 5 M☉.

Although they cannot be directly observed (as light itself is captured by their gravity), they can be detected through gravitational lensing: where their gravity bends light around them, allowing an observer to see objects such as stars and galaxies (and duplicate images of these objects) behind the black hole.

Physical Properties
The event horizon, or Schwarzschild radius  (the radius of a sphere such that the escape velocity from that sphere is equal to the speed of light), of a black hole is given by the following equation:



The radius of the photon sphere (the point at which light is captured in an orbit around the black hole) is given by:



More interesting and complex black hole properties, such as Hawking Radiation and entropy, can be calculated here.

Supermassive Black Hole
Most, if not all galaxies, have a supermassive black hole (SMBH) at their centre. The supermassive black holes at the centres of galaxies can have masses upwards of 100,000 M☉. The SMBH at the centre of the Milky Way, Sagittarius A*, has a mass of about 4.3 million M☉. The interaction of a galaxy's SMBH with an accretion disk is the driving mechanism behind active galactic nuclei.

Worldbuilding in Practice
"The Palag Uu are a Bandu militant group who control a sizeable region of space on the fringes of the Florian Systems Alliance. Reportedly, they have dropped asteroids on colonies, tortured multitudes of civilians and are a major player in the galactic slave trade. However, no tale of their cruelty is complete without the mention of Torfa's Pit." "Torfa's Pit is a black hole of about 14 M☉, with a radius of about 41 km, and is located right in the middle of Palag Uu space. The galaxy is full of rumours that, roughly 80 years ago, the Palag Uu general Torfa executed close to 1 thousand prisoners by loading them on a near-derelict ship and crashing it into the black hole. Since then, the Palag Uu have occasionally released videos of such executions, usually of aid workers and journalists who get captured in Palag Uu territory."