That's a star, baby! Or, well. almost.
The funny thing about stars is that they are remarkably simple. They're spherical plasma in a vacuum! Sure, they have all sorts of fun physics in the core, and stellar atmospheres are an incredibly complex environment, but as soon as you leave their immediate vicinity they become extremely easy to study.
You wanna know the best part? 90% of the time, knowing one specific thing about a star is enough to tell you everything you'd ever want to know.