Apparent magnitude is a logarithmic scale to denote a star’s brightness. The idea is a couple of thousand years old, from back in the days when fancy telescopes were yet to be engineered and all a budding astronomer had to observe with was their own eyesight.
The brightest stars were called first magnitude stars, then the ones half as bright as first magnitude stars were second magnitude stars, and so on. So it’s actually a backward scale – the greater the number, the dimmer the star.
It’s ‘apparent’ magnitude because the stars are seen through the Earth’s atmosphere, which acts like a filter and removes some of the light, so it only describes how bright the star appears to an Earth-based observer.
More recently, a couple of hundred years ago, astronomers had new technology and therefore better ways of measuring the light from a star, and so the scale was fiddled with slightly. However, the main gist of it is the same.
The dimmest stars observed have an apparent magnitude of 30+ while the brightest (Sirius) is -1.4 - although really the brightest star observed is our own Sun, which comes in at a whopping -26.7 .
