When you look at the sky at night, three things become obvious. The first is that the stars twinkle, the second is that it is easy for us to arrange some into recognizable patterns, and the third is that not all the stars look the same. Some are bright; some are less bright, and some almost too dim to see with the naked eye.
Due to the different brightness, astronomers have had to devise some means of expression the variation precisely. The attempt to classify such differences began some two thousand years ago when the ancient Greek astronomer Hipparchos produced a star catalogue in which he introduced the idea of importance, or magnitude, for the stars he listed.
Hipparchos thought the brightest stars to be of first importance and termed them magnitude 1. Those somewhat dimmer he considered of second importance, or magnitude 2, and so on. In the event he divided the stars visible to eye into six classes of magnitude. Although he did not know it, his six divisions were based on the way the human eye recognizes a brightness difference where one object seems half as bright as another does. It has subsequently been found also that Hipparchos' six magnitudes give a difference between magnitude 1 and 6 of 100 times.
In the intervening two thousand years the subject of magnitudes has been much refined. Yet astronomers still use the same basic system, which is really a measure of dimness - the higher the magnitude number, the dimmer the star. To be more precise, a star of magnitude 2 is 2.51 times dimmer than one of magnitude 1, and a star of magnitude 3 is 2.51 times dimmer than magnitude 2. For extremely bright stars, the planets and the Sun and Moon, it is necessary to introduce magnitude zero and even negative magnitudes: Sirius is described as having a magnitude of -1.46, while the Sun is -26.85.
Most stars do not have names, but even if they do, you will find that the brightest are also designated by a Greek letter, starting with a (alpha) for the brightest, b (beta) for the next brightest, and so on down the alphabet. Since there are more stars to be designated than there are Greek letters, each of the remaining stars is assigned a number.
Stars and other celestial objects often have catalogue numbers as well. Thus Sirius is also a (alpha) Canis Majoris; the second magnitude star in Orion's left foot is k (kappa) Orionis, while the fifth magnitude star just above it to the left is 55 Orionis.
(left) A star map of the constellation Gemini (the Twins) is shown, above. The two bright stars Castor and Pollux are clearly visible. Castor has an apparent magnitude of +1.58 and is blue-white in colour. Pollux is slightly brighter at +1.14 but appears redder. Ancient records indicate that Castor may once have brighter than Pollux. (right) Gemini, shown in the photograph, contains the two-first magnitude stars Castor and Pollux. This fine zodiacal constellation is one of the easiest to recognize.
