The reason the magnitudes go into the minus's, is because hundreds of years ago, the astronomers made a system for the brightness of every star in the sky, with mag. +5 being the faintest then could see with the naked eye and mag. 0 being the brightest. Now a days, some of the stars in our sky are brighter than the ones back then. So the astronomers had to "update" the system with the magnitudes of the brighter stars. So instead of changing the whole system around, they just went into the minus's.