Why hasn’t the sun burned out yet?

This was the most frequently asked question in the 19th century.                                          At the time, Humans understood  only two ways the sun could be generating energy: either it was creating heat and light through gravitational contractions————pulling itself in at the center and emitting energy (in the form of heat that we feel on Earth), therefore getting smaller over time—————or it was literally on fire, like the chemical reaction we see on Earth when we light a match or start a campfire.