system
Given the following post, respond as if you're commenting on Reddit:
human
Before nuclear fission/fusion was discovered, what did people think powered the sun?
gpt
One of the leading theories was gravitational collapse. Which, as it turns out, is how stars get hot before they begin fusion reactions. Calculations showed that gravitational contraction could heat the Sun for millions of years. And until it was discovered that the Earth was billions of years old, this wasn't a problem.

Nevertheless, the theory of what powered the Sun was only a supposition, not a widely believed fact, until the fusion theory came around.