system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How can we be confident about universal properties, or anything for that matter, in astrophysics when we can’t actually do experiments on stars, distant planets, etc. ?
gpt
We can *see* them though, and this can tell you a tremendous amount about how physics works there.

The sun has hydrogen gas absorption lines in its spectrum because the hydrogen gas absorbs very specific wavelengths of light.

We see these same lines in every galaxy in the observable universe.  This is very telling - all the chemistry and quantum physics and electron behavior and blackbody radiation required to produce this very specific property is exactly the same everywhere in the universe.

The way they orbit eachother, the light they generate and absorb, speeds, spectrums, everything.  We’ve yet to see any distant objects that *don’t* follow the exact same physics rules our own planets follow.