The first thing you notice on looking sky on a clear night is a hodgepodge of stars of varying brightness. How do we describe the brightness of stars and planets.
APPARENT MAGNITUDE Our system of magnitudes are divided into six brightness groups, the brightest stars being 1 st magnitude and the faintest 6th magnitude. A 1st magnitude star is 100 times brighter than the faintest star visible without a telescope. A 2nd magnitude star is 2.5 times fainter than a 1 st magnitude star and a 3rd magnitude star, is 2.5 times fainter. Polaris, the North Star, is a 2nd magnitude. Most of the stars in the Big Dipper are also about 2nd magnitude. Vega, the brightest star in the Summer Triangle, is 0 magnitude. The brightest star in the sky, Sirius, has a minus magnitude of -1.4. The Moon and Sun are brighter, at - 12.6 (full Moon) and the sun -26.8.
The stars differ in magnitude for two reasons, some are closer to us than others, and some really are brighter than others. In order to describe a star's brightness, astronomers have defined absolute magnitude as the apparent magnitude a star would have if it were 10 parsecs or 33 light years from us.
STAR COLORS
A look skyward will show, the stars differ in brightness, and color. If you look at Antares in Scorpius, Aldebaran in Taurus, or Betelgeuse in Orion, you will see that all these stars are reddish, a subtle red rather than the red of traffic lights. Vega in Lyra and Rigel in Orion are a delicate blue. A star's color gives us a clue to its nature. Blue signifies that the star is hotter than the Sun, red means it is cooler.
