A given star, at a given time, has a certain temperature and a certain luminosity. It turns out if you plot temperature vs. luminosity of a whole bunch of stars, they aren’t distributed randomly — there is a certain pattern that is apparent. The pattern is dominated by a swath that goes through the center of the plot. This swath is called the Main Sequence.
Stars spend most of their life on the main sequence.
The problem is, there are two things that can make a star appear bright or dim — the intrinsic brightness of the star and how far away the star is. If you know a bunch of star are all roughly the same distance away, like in clusters of stars, you can assume all the differences in brightness you observe are due to intrinsic brightness differences in the star. Then rather than plotting luminosity and temperature, you get the same result plotting the color of the star vs. the brightness of the star.
I tried this little experiment with my telescope on a cluster known as M38. The resulting plot showing the main sequence looks like this:
So in a very minor sense, using my own equipment, I have proven a basic observational fact in stellar evolution: the existence of the main sequence.