World Science Scholars
2.2 The Science of Black Holes

The Schwarzchild radius, also known as the event horizon, is the point of no return for a black hole.drop-down
  • The Schwarzchild radius is proportional to the mass of the black hole, and is given by r=2Gm/c^2 for a non-rotating black hole.
  • There is an outer radius, called the photosphere, where light can be trapped. Somebody standing there, facing 90 degrees away from the black hole, would be looking at the back of their own head.

Supermassive black holes are black holes with masses in excess of 1,000,000 times that of the sun.drop-down
  • The region of influence of a supermassive black hole is the area surrounding it where its gravity is the dominant force. Beyond this region, the gravity of stars is the dominant force.
  • The region of influence is given by Gm/v^2, where v is the average speed of stars in that region.

Black holes grow through the accretion of gas in their vicinity.drop-down
  • As the gas approaches the black hole, it accelerates to very high velocities and emits x-rays, UV radiation, and visible light. This is known as a quasar.
  • The average time it takes for a black to double in size is known as the Saltpeter time, and is equal to roughly 40,000,000 years.
  • The Eddington luminosity is the maximum luminosity that a quasar can reach before the radiation pressure balances out the gravity of the black hole and is proportional to the mass of the black hole. 
  • Quasars in the early universe emit radiation equal to the Eddington luminosity, which allows us to infer their mass. Some have been observed with masses 1,000,000,000 that of the Sun. 

There are three primary challenges in understanding the lifecycle of supermassive black holes that Professor Natarajan would like to understand.drop-down
  • Formation: Where do the seeds of supermassive black holes come from?
  • Fueling: How does gas make it to the black hole?
  • Feedback: What is the impact of the supermassive black hole on the surrounding galaxy?

In the nearby universe, almost all supermassive black holes are in a fasting state.drop-down
  • This is because they have exhausted their supply of gas.
  • In the early universe, most of the black holes that we observe are in a feeding state, emitting radiation at or near their Eddington luminosities.

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