What would happen if the Sun became a black hole?

A black hole's gravity at a given distance depends only on its mass, and replacing the Sun with a black hole of one solar mass leaves the gravitational field beyond the current solar radius completely unchanged. Earth would continue in exactly the same orbit, at exactly the same distance, with the same year. What would change is everything to do with light and heat: no sunlight, so temperatures falling as described for the Sun simply switching off, and no solar wind. The Schwarzschild radius for one solar mass is about 2.95 kilometres, so this object would be under six kilometres across and effectively invisible from Earth except by its lensing of background stars. There is no danger of being pulled in — the frequently imagined vacuum-cleaner behaviour of black holes is a misconception, since they attract exactly as much as any object of the same mass. As it happens the Sun is far too small ever to become one.

How confident this is

This is a reasoned extrapolation rather than an observation. The mechanism is understood, but the scale of the consequences depends on assumptions that could reasonably be argued differently.

Physics

• What would happen if an asteroid hit the Earth? It depends enormously on size — most burn up, a 20-metre one broke windows across a city, and a 10-kilometre one ended the dinosaurs.
• What would happen if gravity disappeared? The Earth itself would come apart — gravity is the only thing holding a planet together.
• What would happen if you fell into a black hole? Into a small one, tidal forces would stretch you apart before you arrived; into a supermassive one, you would cross the horizon without noticing.
• What would happen if you fell into a hole through the Earth? You would oscillate from one side to the other and back, with a one-way trip of about 38 minutes.
• What would happen if you travelled at the speed of light? You cannot — accelerating any mass to light speed requires infinite energy. Approaching it, time dilates dramatically.
• What would happen if gravity doubled? You would weigh twice as much, most tall structures would need rebuilding, and no bird larger than a sparrow could fly.
• What would happen if you were shrunk to the size of an ant? The square-cube law would break you: you would freeze, suffocate, and be unable to drink from a surface of water.
• What would happen if the speed of light were slower? Chemistry, nuclear fusion and the mass-energy relationship would all change — stars would not work as they do.
• What would happen if a gamma-ray burst hit the Earth? A nearby one aimed at us would strip a large fraction of the ozone layer within seconds, with the damage arriving as ultraviolet over the following years.