Also, don't you mean stellar spectroscopy? Mass spectrometry is when you fire ions through an electric field to determine their mass-to-charge ratio. Doesn't seem even a little bit applicable to far-away objects.
Think about our knowledge of the Earth rotating around the sun vs. the sun rotating around the Earth, or flat Earth vs a spheroid Earth, our models increase in accuracy as their testability increases in tandem.
Our current working hypothesis(es) and theories have not been invalidated so far and thus, they depict the most accurate model we have.
But it’s just a model: data that we get artists to turn into pretty “space” pictures.
If the public had a real understanding of astronomy and space exploration it would neither be seen as interesting or worth funding.
Sure. This is actually pretty simple.
Stars visible with naked eye are relatively close to us. We can measure parallax (https://en.wikipedia.org/wiki/Stellar_parallax) and this gives you the distance to the star directly. Once you know the apparent brightness and the distance you can calculate the actual brightness.
"The Hubble telescope WFC3 now has a precision of 20 to 40 microarcseconds, enabling reliable distance measurements up to 3,066 parsecs (10,000 ly) for a small number of stars."
Vast majority of visible stars fall within couple thousand light years, the only ones that are further are extremely bright, extremely rare and extremely short lived.
I can imagine that scientists have figured out accurate ways to measure the "actual" brightness of stars and including our sun, and maybe in retrospect using some simple insights and tricks, but it's not obviously straightforward at all.
Let's hypothesize that our sun is the most massive star in the galaxy, and what we look at in the sky are the rest of the stars in decreasing order of size, with the largest the most visible. How is it straightforward to conclude that that's not true? And are we declaring Newton's accomplishments straightforward?