One of the more difficult tasks in astronomy is getting accurate physical measurements of stars. The most fundamental properties of a star are its mass, radius, and age — these tell you a huge amount about their behavior and how they work. We can determine some of these by using other properties; for example, the temperature of a star is generally directly measurable by taking a spectrum of it, and for stars like the Sun the temperature depends on the mass (more massive stars are hotter). So it's possible to figure some of these out.
But we rely on models a lot to do this. We understand the physics of how stars work, and can use that to figure out the age by looking at different properties. The problem is we can't always know we have the physics right! So the best way to get these three basic properties is to observe them directly.
But that's tough! Getting the mass of a single star sitting out in space by itself is a difficult thing to do, if not impossible. Same for its radius.
Still, sometimes nature is generous. If two stars are orbiting each other then their orbit depends on ...