Since many stars are hundreds of light years away from the earth and therefore, what we observe of them today is really their distant past, how can we say anything with certainty about their composition, size and nature? Betelguese, for example, is said to be in the last stage of stellar evolution, but taking into consideration its approximately 640 light years of distance from the earth, aren't we actually making assertions about the star that are 640 years old? If that is the case, how can we claim to know the actual present status of the star?
Well, we don't claim to know the status of the star at the present time. If the star is close enough (within our galaxy, or local neighbourhood), we measure the distance in light years, or parsecs. If they're further away than that, it's easier to quantify their distance in terms of a redshift.
When we say that a star has some properties (size, temperature etc.) in that statement is the knowledge that the light received is delayed, so it won't reflect the state of the star right now.
Even though knowing the state of a star as it was several thousand years ago may not seem like it's a useful thing, it's the only tool we have to probe the evolution of stars, and the evolution of the universe. Since the further away something is, it will reflect conditions of the universe at earlier and earlier times. Which is where we can test cosmological theories of the evolution of the universe, or our theories of the evolution of galactic or stellar systems.