1 Brightness of the stars
One of the first things that you may have noticed when looking at the stars is that some are brighter than others.
The brightness of a star is referred to as its magnitude and uses a scale based on a classification initially devised by the ancient Greek astronomer Hipparchus who produced one of the first star catalogues. Hipparchus grouped the stars into six categories, from the brightest visible to the naked eye to the faintest. The brightest stars he classified as first magnitude, the next group of slightly fainter stars as second magnitude, and so on down to sixth magnitude for the faintest stars visible to the naked eye. This system was later refined by Ptolemy and by Galileo and is still in use today, having been extended to include far fainter objects visible only in telescopes.