Why does the brightest star get the lowest number in astronomy?

Because the scale began as a ranking, like first and sixth place: the brightest stars were "first magnitude" and the faintest visible ones "sixth". A star's apparent magnitude is the number for how bright it looks from Earth, and it still runs backwards: the brighter the object, the lower, and sometimes negative, its number. Venus can reach magnitude −4.2 and Sirius, the brightest star in the night sky, sits at −1.46, while the faintest stars a sharp eye can pick out on a dark night hover around magnitude 6.5.

The backwards numbering is a fossil from antiquity. Ancient skywatchers sorted the stars they could see into six rough grades, from the showiest "first magnitude" stars down to the barely visible "sixth magnitude" ones at the edge of human sight. Ptolemy's Almagest spread this system, and it is usually credited to Hipparchus, though his original catalogue is lost; the one text of his that survives shows he did not even use numbers for brightness, just words like "big" or "faint."

That loose, subjective grading stayed in use until 1856, when the astronomer Norman Pogson turned it into actual mathematics. He defined a first-magnitude star as exactly 100 times brighter than a sixth-magnitude one, which meant each single step of magnitude had to represent a fixed brightness ratio: the fifth root of 100, about 2.512, now called Pogson's Ratio. A magnitude 2 star is 2.512 times brighter than a magnitude 3 star, 6.31 times brighter than a magnitude 4 star, and precisely 100 times brighter than a magnitude 7 star.

Because brighter keeps meaning smaller, the scale had to be allowed to run below zero for the most dazzling objects, and the Sun, by far the brightest thing in the sky, comes out at a striking −26.832. At the other extreme, the Hubble Space Telescope has photographed objects as faint as magnitude +31.5. Amateur astronomers still use this same ancient-turned-precise scale today, tracking the faintest magnitude they can see by eye as a simple gauge of how much light pollution is washing out their sky.