You can do spectroscopy on the light emitted from stars and see what elements it's made of.
This, combined with our understanding of physics, both offer overwhelming support for the notion that stars are powered by nuclear fusion.
This, combined with our understanding of physics, both offer overwhelming support for the notion that stars are powered by nuclear fusion.
So it's a theory, not a proven "fact." Can we know for 100% sure before we replicate a sun and see the expected behavior and output?
I'm just taking issue with the liberal use of the word "fact" in my comment's parent. It seems to have become as diluted as "literally."
https://en.wikipedia.org/wiki/Theory "In modern science, the term "theory" refers to scientific theories, a well-confirmed type of explanation of nature, made in a way consistent with scientific method, and fulfilling the criteria required by modern science. Such theories are described in such a way that any scientist in the field is in a position to understand and either provide empirical support ("verify") or empirically contradict ("falsify") it. Scientific theories are the most reliable, rigorous, and comprehensive form of scientific knowledge, in contrast to more common uses of the word "theory" that imply that something is unproven or speculative (which is better characterized by the word 'hypothesis')."
Note that I'm not disputing our understanding of how stars work. The point is whether our understanding is based on reproducibility.