A supermassive black hole (SMBH) is an extremely massive type of black hole, often found at the centers of galaxies, including our own Milky Way. These black holes are millions to billions of times the mass of the Sun, vastly outweighing the smaller stellar black holes, which form from collapsing massive stars.

Key Characteristics of Supermassive Black Holes:

1. Size and Mass: They can range in mass from about 100,000 to 10 billion solar masses, though their physical size is not directly proportional to their mass because of the nature of black holes.
2. Event Horizon: The event horizon (the boundary beyond which nothing can escape) of a supermassive black hole can be extremely large, spanning several times the distance between the Earth and the Sun.
3. Formation: The origin of supermassive black holes is still an area of research. They may form from the collapse of massive gas clouds in the early universe or grow from smaller black holes by accreting material over time.
4. Location: Almost all galaxies, including our Milky Way, have a supermassive black hole at their center. The SMBH at the center of the Milky Way is called Sagittarius A*, and it weighs around 4 million times the mass of the Sun.
5. Accretion Disks and Quasars: Many supermassive black holes have surrounding accretion disks, which are disks of gas, dust, and other material that spiral into the black hole. As this material gets close to the event horizon, it heats up and emits intense radiation, sometimes forming quasars, which are some of the brightest objects in the universe.
6. Gravitational Influence: SMBHs have an enormous gravitational influence, affecting the orbits of stars, gas, and dust in the central regions of galaxies. They play a key role in the evolution of galaxies and may regulate star formation through energetic outflows of gas.

Supermassive black holes are critical to our understanding of galaxy formation and the large-scale structure of the universe.