They weren't different in the past by any significant amount, we know this from observational evidence.
In essence, the stars are a window into the past because of the speed of light. All stars, everywhere, no matter how different in time appear to operate according to the self same laws figured out right here on Earth.
I can almost hear you thinking "but what if the speed of light was different, that's circular reasoning". No, it isn't. For two reasons, firstly, we can check the speed of light itself by observations of movement in stellar objects and, secondly, the speed of light is not arbitrary. You've heard of E=mc squared, I assume? What this means is that a change in the speed of light will alter the very properties of matter, and that would show up in the behaviour of stars.