Transcript: The magnitude scale is defined in such a way as a magnitude difference of five magnitudes corresponds to a factor of a hundred in apparent brightness. Two and a half magnitude difference corresponds to a factor of 10 in apparent brightness. Lower numbers in the magnitude scale are brighter, which is of course the opposite of a scale set by the number of photons per second. Zero on the magnitude scale is defined by the bright star Vega. The magnitude scale can be illustrated by some magnitude differences and corresponding brightness ratios of typical situations. Two bright stars that are identical, seen at the same distance, have a magnitude difference of zero; their brightnesses are equal. Magnitude difference of one, or a factor of 2.5 in apparent brightness, is the minimum difference visible by eye between stars in the night sky. Magnitude difference of 4, or a brightness ratio of 40, corresponds to the limit of the naked eye relative to binoculars. Magnitude difference of 5, or a factor of 100, is a range between the brightest and the faintest stars in the sky. A factor of 104, or 10 magnitudes, is the ratio between the full moon and Mars. Fifteen magnitudes, or a factor of 106 in apparent brightness, is the ratio between the brightest star and Pluto. Twenty magnitudes, or 108 in apparent brightness, is the limit between binocular vision and the Hubble Space Telescope, and twenty-five magnitudes, 1010 in apparent brightness ratio, is the ratio between the Sun and the brightest star in the night sky.