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Until now we have discussed dim stars and bright stars without being very exact about what we mean by "dim" and "bright". The Greek skygazer Hipparchos made a list of a thousand stars and their brightness two thousand years ago. He gave the brightest stars a value of one and the very dimmest stars a value of six. These values are called the apparent magnitude of the stars.
The word "apparent" is used because the brightness of the stars that Hipparchos saw and that we see is dependent on their distance from the Earth and not only their inherent brightness. Remember that the higher the magnitude is, the dimmer the object is and the more difficult it is to see it. Although Hipparchos worked without any modern instruments, his calculations were fairly accurate and needed to be only slightly modified. A star with magnitude one is 2.512 times as bright as a star with magnitude two; a star with magnitude two is 2.512 times as bright as a star with magnitude three, and so on. A magnitude one star is exactly 100 times as bright as a magnitude six star. Theoretically, on a clear night a person can see a star or any other object with a magnitude of six. Do not expect to see magnitude six stars unless there is no interfering moonlight and all viewing conditions are excellent.
There are exceptions. Ray E. Fabre was able to see magnitude seven stars and probably more one night. He happened to be standing near the peak of an extinct Hawaiian volcano Mauna Kea. No situation is perfect, and after a few minutes the sky turned “strangely gray” as the lack of oxygen at the high altitude began to affect his vision.
Although in most cases we can see only objects of magnitudes down to six, instruments are able to measure the brightness of much dimmer and brighter objects than the stars visible to our naked eyes. For example, Pluto, the most distant planet from the Sun, can only be seen through a large telescope, and has a magnitude of 15. To measure very bright objects, negative magnitudes are used. The more negative a magnitude, the brighter the object. The brightest star, Sirius, has a magnitude of -1.4. The magnitude of the full Moon –12.7, while that of the Sun by far the brightest object in the sky, is –26.7. Remember that all these figure are of apparent magnitude, which measures the brightness of an object as seen from the Earth. If our Sun and Sirius were the same distance from us, Sirius would appear much brighter than the Sun.
How bright an object will appear to you depends on how good the viewing conditions are. There are nights when one can see only the few very bright stars with negative magnitudes. What remains constant are the differences in brightness between the stars. A magnitude three star will always appear almost half as bright as a magnitude two star. The stars with magnitude two and less are the signposts of the sky. The patterns of the sky created by the stars not only have abstract shapes, but a sort of “texture” provided by stars of different magnitudes and even varying colors. From night to night, a single star will have a different degree of brightness, but the relative brightness of the stars in the sky to each other remains fixed and this is what we are able to identify.