Stars are categorized into different "magnitudes" according to their brightness; magnitude zero stars are the brightest, magnitude one the next brightest, and so on. Individuals with 20 / 20 vision can see magnitude six stars with the naked eye.
星等則依亮度區分，若以一等星為準，更亮的為零等星、進而負一等星，由正而負，亮度遞增，視力 1.0 的人以肉眼可以看到晦暗的六等星。