Here's why:
* The vertical axis on an H-R diagram represents luminosity. Luminosity is the total amount of energy a star emits per second.
* Absolute magnitude is a measure of a star's intrinsic brightness. A lower absolute magnitude means a brighter star.
* Stars with higher luminosity are intrinsically brighter. Therefore, as you move up the vertical axis (higher luminosity), the absolute magnitude decreases (brighter).
Think of it this way: The top of the H-R diagram represents the most luminous stars, which also have the lowest absolute magnitudes. The bottom of the H-R diagram represents the least luminous stars, which have the highest absolute magnitudes.
