Type I vs Type II Stars

Chemical composition has virtually no effect on the overall evolution of a star.  Where chemical composition comes into play is how the star looks, or to put it another way, chemical composition primarily affects the atmosphere of a star, not its interior.  Astronomers traditionally divide stars into two rough classes, called Type I and Type II.  Type I stars are richer in "dirt" (that is, elements heavier than helium) than Type II stars are, primarily because the Type I stars are younger.  As time goes by, the interstellar medium is becoming more "polluted" with heavier elements as a result of supernova explosions, so the older Type II stars are "cleaner" than the Type I stars because they were formed from older, cleaner gas.  Our Sun is a relatively dirty, Type I star – which is probably a very, very good thing, because otherwise there might not have been enough "dirt" floating around the early solar system to form the Earth!  The Sun is composed of about 1.5% "dirt", whereas Type II stars can have as little as one quarter-millionth as much as this.

By the way – Astronomers normally refer to the "dirt" in stars as "metals", but everyone knows that is inaccurate, so in this essay I will just use the more prosaic (and more accurate) term "dirt".

The cleaner atmospheres of Type II stars means that they are also clearer.  The radiation produced by a star has more of a tendency to simply shine throught the outer layers of a Type II than it does in a Type I.  As a result, the atmospheres of Type II stars absorb less heat and thus expand less than the atmospheres of Type I stars.  A Type II star, for any given mass, is therefore smaller in radius and hotter than its Type I analog.

In short, if the Sun were a Type II star, it would put out the same energy, but it would be more compact and have a higher surface temperature.  Other than their ages, this is the only essential difference between Type I and Type II stars.