视星等（英语：apparent magnitude，符号：m）最早是由古希腊天文学家喜帕恰斯制定的，他把自己编制的星表中的1022颗恒星按照亮度划分为6个等级，即1等星到6等星。1850年英国天文学家普森发现1等星要比6等星亮100倍。根据这个关系，星等被量化。重新定义后的星等，每级之间亮度则相差2.512倍，1勒克司（亮度单位）的视星等为-13.98。

但1到6的星等并不能描述当时发现的所有天体的亮度，天文学家延展本来的等级──引入「负星等」概念。这样整个视星等体系一直沿用至今。如牛郎星为0.77，织女星为0.03，除了太阳之外最亮的恒星天狼星为−1.45，太阳为−26.7，满月为−12.8，金星最亮时为−4.89。现在地面上最大的望远镜可看到24等星，而哈勃望远镜则可以看到30等星。

The apparent magnitude (m) of a celestial object is a number that is a measure of its brightness as seen by an observer on Earth. The smaller the number, the brighter a star appears. The Sun, at apparent magnitude of −27, is the brightest object in the sky. It is adjusted to the value it would have in the absence of the atmosphere. The brighter an object appears, the lower its magnitude value (i.e. inverse relation). In addition, the magnitude scale is logarithmic: a difference of one in magnitude corresponds to a change in brightness by a factor of or about 2.512.