Supermassive black hole

A supermassive black hole is the largest class of black hole in the universe. These astronomical objects form when matter undergoes gravitational collapse. This process creates a spheroidal region of space where gravity is so strong that nothing can escape, including light. Most large galaxies possess a supermassive black hole at their center. For instance, our Milky Way galaxy contains a supermassive black hole known as Sagittarius A*.

These giants are much larger than stellar-mass black holes. They are typically defined as having a mass exceeding 100,000 solar masses. Some reach masses of billions of times that of our Sun.

Supermassive black holes have unique physical properties that distinguish them from smaller black holes. One major difference is the strength of tidal forces near the event horizon. The event horizon is the boundary around the black hole. For these giants, the tidal forces are significantly weaker. A person at the event horizon would feel a pull between their head and feet similar to the pull felt on Earth's surface. This happens because tidal force is inversely proportional to the square of the black hole's mass. In smaller black holes, these forces become much more intense much sooner.

It is also surprising to consider the density of a supermassive black hole. Density is the mass divided by the volume within the Schwarzschild radius. The Schwarzschild radius is the distance from the center to the event horizon. Because this radius is directly proportional to mass, the volume increases very quickly. In fact, the volume is proportional to the cube of the radius. This means the average density of a supermassive black hole can actually be less than the density of water. As the mass increases, the average density decreases.

Scientists have different theories about how these giants begin. They might start as "seeds" from the first stars. These seeds would be the remains of massive stars that exploded. Another theory involves the direct collapse of large gas clouds. These clouds could form a "quasi-star" that eventually collapses into a black hole. Some models suggest that metal-free gas clouds can collapse directly into a black hole. These are called direct collapse black holes. They might reach masses of 10,000 to 100,000 solar masses without ever being a star.

Once formed, these black holes grow through accretion and merging. Accretion is the process of pulling in nearby interstellar gas. As gas falls toward the black hole, it can power active galactic nuclei (AGNs) or quasars. Quasars are extremely bright and energetic objects. Black holes also grow by merging with other black holes. Some researchers suggest that the most massive black holes might be "stupendously large black holes," or SLABs. These could have masses greater than 100 billion solar masses.

The history of studying these objects is quite long. In 1963, Maarten Schmidt studied the radio source 3C 273. He discovered it was a quasar moving away from Earth at high speeds. In 1964, Edwin Salpeter and Yakov Zeldovich proposed that matter falling onto compact objects explained quasars. Later, in 1974, Bruce Balick and Robert Brown discovered Sagittarius A*. This provided the first evidence of a supermassive black hole in our own galaxy. In 1994, the Hubble Space Telescope observed gas orbiting the center of Messier 87. The gas moved at 500 kilometers per second, proving a huge mass was present.

Modern technology has finally allowed us to see these objects more clearly. In 2019, the Event Horizon Telescope Collaboration released the first direct image of a black hole. This image showed the black hole at the center of the galaxy Messier 87.

This was a landmark moment in astronomy. In 2020, Andrea Ghez and Reinhard Genzel won the Nobel Prize in Physics. They were honored for discovering the supermassive compact object at the center of our galaxy. These discoveries connect our understanding of gravity, light, and the very structure of galaxies.