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An accretion disk is a disk-shaped flow of gas, plasma, dust, or other matter that spirals into a black hole. As matter in the accretion disk spirals inward, it heats up and can emit intense radiation, often in the X-ray spectrum.

Gravitational waves are ripples in spacetime caused by some of the most violent and energetic processes in the universe, such as the merging of black holes. The detection of these waves opens a new window in astronomy.

The black hole information paradox arises from the conundrum that physical information could permanently disappear in a black hole, violating the quantum mechanical principle that information cannot be lost. Hawking radiation exacerbates this paradox.

At the center of a black hole lies the singularity, a point where gravitational forces compress matter to infinite density and zero volume, and where the curvature of spacetime becomes infinite.

A black hole is a region of spacetime where gravity is so strong that nothing, not even light, can escape from it. The boundary surrounding a black hole is called the event horizon.

Stellar black holes form when massive stars (>5-10 times the mass of the Sun) collapse under their own gravity at the end of their life cycles. They typically have up to 20 times the mass of the Sun.

The Schwarzschild radius is the radius of the event horizon of a non-rotating black hole. It's directly proportional to the mass of the black hole and can be calculated using the formula

The photonsphere is a spherical region around a black hole where gravity is just right for photons (light particles) to orbit the black hole at a constant distance. This is theoretically possible but highly unstable.

The Event Horizon Telescope is a global network of radio telescopes that combines data to create a virtual Earth-sized telescope, capable of imaging the immediate environment around a black hole to observe phenomena like the event horizon.

Hawking radiation is theoretical radiation predicted by Stephen Hawking, which is emitted by black holes due to quantum effects near the event horizon. It suggests that black holes are not completely black and can emit small amounts of thermal radiation.

Supermassive black holes, found at the centers of most galaxies, including the Milky Way, can have millions to billions of times the mass of the Sun. Their formation is still a matter of active research.