Armor with stars in collection?

So what do the stars mean, I assume its not just that theyre super rare or legendary because the nightfall armor and helmet have stars on them and theyre not rare or legendary, does it mean I have to do something special to unlock them?

Those are Mythic items, which are super-mega-ultra-rare and hard to get. They apply to super-hard-to-get RNG drops and legacy unlocks (Mark VI, Helioskrill, Nightfall).