system
Given the following post, respond as if you're commenting on Reddit:
human
How did scientists think the sun worked before the discovery of nuclear fusion?
gpt
In the 19th century, Lord Kelvin (the one the temperature's named after), thought the sun emitted light and heat through gravitational contraction. The sun's gravity would move all matter closer to the core, thereby decreasing their potential energy and releasing energy in some form.

Calculations based on this 'determined' that the sun was about a hundred million years old, and definitely no older than half a billion years, which was already disputable in his own time (geologists were much closer to the actual value in their conclusions).