Hipparchus was one of the greatest of the Greek astronomers. He cataloged over 1,000 stars and developed the mathematical science of trigonometry. The amazing thing is that he studied the sky by looking through a tube in order to focus his attention in only one area. He was forced to do this because telescopes were not invented until over a millennium and a half later.
He went on to divide the visible stars into six different categories according to their brightness. He called the brightest stars "first magnitude" and the faintest "sixth magnitude". Since hardly any astronomy equipment was invented during his prior or during his lifetime this system required a lot of guesswork.
During the 1850's this magnitude system was made more scientific. It turned out that some of the brightest objects in the sky exceed the first magnitude. So these stars are given a rating of zero and recently negative magnitudes have been added. Sirius, the brightest of all stars, as seen from Earth, has a magnitude of -1.46. A magnitude number greater than 6 means that an object is visible only through a telescope.