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What you will learn in Black Holes and Extreme Objects

1. Escape velocity and light
A black hole is not an object with unusual gravity. It is an ordinary amount of gravity in an unusually small space, and the difference matters.
2. The Schwarzschild radius
Black holes are supposed to be the densest things there are. The largest one we have photographed is less dense than air, and the arithmetic that says so is three lines long.
3. Falling in
Cross the horizon of a supermassive black hole and nothing happens. No wall, no surface, no local marker of any kind. What happens afterwards is not survivable; the crossing itself is unremarkable.
4. Why quasars are bright
The Sun converts seven parts in a thousand of the mass it burns. Matter falling onto a spinning black hole gives up a third of it, and that ratio is why a region the size of the solar system can outshine a galaxy.
5. Neutron stars and pulsars
Between a white dwarf and a black hole there is one more stop, and it is the densest thing that is still made of matter: a solar mass and a half inside a city.
6. Hearing a collision
Three solar masses of rest energy left the system in a fifth of a second. What reached Earth moved a four-kilometre ruler by a thousandth of the width of a proton.
7. The picture that looks the same at every scale
What the Event Horizon Telescope photographed is not the event horizon. It is a shadow two and a half times wider, and the gap between the two is the whole reason the picture is bent into a ring at all.

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1. Escape velocity and light
A black hole is not an object with unusual gravity. It is an ordinary amount of gravity in an unusually small space, and the difference matters.
2. The Schwarzschild radius
Black holes are supposed to be the densest things there are. The largest one we have photographed is less dense than air, and the arithmetic that says so is three lines long.
3. Falling in
Cross the horizon of a supermassive black hole and nothing happens. No wall, no surface, no local marker of any kind. What happens afterwards is not survivable; the crossing itself is unremarkable.
4. Why quasars are bright
The Sun converts seven parts in a thousand of the mass it burns. Matter falling onto a spinning black hole gives up a third of it, and that ratio is why a region the size of the solar system can outshine a galaxy.
5. Neutron stars and pulsars
Between a white dwarf and a black hole there is one more stop, and it is the densest thing that is still made of matter: a solar mass and a half inside a city.
6. Hearing a collision
Three solar masses of rest energy left the system in a fifth of a second. What reached Earth moved a four-kilometre ruler by a thousandth of the width of a proton.
7. The picture that looks the same at every scale
What the Event Horizon Telescope photographed is not the event horizon. It is a shadow two and a half times wider, and the gap between the two is the whole reason the picture is bent into a ring at all.