How Long Could Humans Survive if There Was No Sun?

Sometimes it is fun to contemplate the impossible, like what would happen to Earth if the sun just disappeared. Questions like these lead people to examine the influence of things that they take for granted, like the warmth and energy of the sun. If the sun were suddenly absent after some cosmic magic trick, humans would know about it 8 1/2 minutes later, when the last of the light from the gone-missing sun hit the Earth. After that, life on the planet would become increasingly perilous, but the line where human life becomes impossible is a little blurry.

Once the Earth went dark, all photosynthesis would stop. The energy from photosynthesis is the basis for almost all of the food chains on Earth, and without it, everything begins to collapse. Global food stores are currently at a record low, so the population could not survive very long on current reserves, likely a few months to a year. The food shortage would be secondary to other changes making life inhospitable around the globe.

Without the light from the sun, Earth would lose its heat source. After a week, the warmest places on the planet would be around negative 18 degrees Celsius (zero degrees Fahrenheit). After a few months to a year, the average temperature would be around negative 73 degrees Celsius (negative 100 degrees Fahrenheit). This is still relatively warm compared to space because of the residual heat in Earth’s core, but it's certainly too cold for humans to survive for long. Living on the surface of the Earth would be nearly impossible unless you lived directly over a thermal feature. Life at the bottom of the sea next to deep sea vents would also be a little warmer.

As soon as the Earth goes dark, it will also come unmoored from the gravitational pull that the sun recently exerted and will start moving off into space in a straight line from the point in its orbit that it happened to be at. The Earth would continue to rotate because its rotational inertia is not tied to the gravitational effects of the sun. The new path would not immediately imperil human life, but it would make it much more possible for us to run into another cosmic body -- say, Jupiter. A collision like that would kill any remaining humans.

Most likely all humans would die from cold and hunger in the weeks and months after the sun’s disappearance. Because this is a hypothetical, though, there is a theoretical possibility that humans could harness geothermal or nuclear energy to stay warm and create light to grow food. Given that possibility, a small contingent of humans could survive indefinitely in an anthropogenic bubble. After several million years, the planet would cool further to a base level of negative 240 degrees Celsius (negative 400 degrees Fahrenheit), and that would likely preclude any type of life on Earth.