It is useful to describe the brightness of stars in terms of their flux, the measure of how much light arrives here on Earth from that star, or by their luminosity, the measure of how much energy they radiate. Astronomers, however, also use a historical description of a star’s brightness known as magnitude.

Ancient Greek astronomers established the original magnitude system, whereby the brightest stars visible to the naked eye were categorized as “1st magnitude,” the next brightest stars “2nd magnitude,” and so forth. The faintest, barely visible stars were labeled “6th magnitude.” After telescopes were invented, many more stars fainter than 6th magnitude were discovered. Astronomers thus extended the magnitude beyond first and sixth magnitudes, following a mathematical formula on a logarithmic scale.

Due to this historical origin of the magnitude system, brighter objects have a lower magnitude number, while fainter objects have a higher magnitude number. This means that negative magnitudes are brighter than positive magnitudes. Like a lot of things with long histories, the astronomical magnitude system is backward and counterintuitive, but it works and persists to this day.