## F T

Thus a difference of one magnitude represents a ratio of 2.51 in flux. The logarithmic scale of astronomical magnitudes is actually a base 2.51 system.

This magnitude system is somewhat annoying. Fainter stars have larger numbers, and the logarithmic interval for unit magnitude (a factor of 2.51) is hardly conventional. A factor of 10, or 2, or e = 2.718, would be more natural. But one of the beauties of astronomy is its roots in human history; the oldest written records include astronomical references. The magnitude system is a daily tangible reminder of the observations of Hipparchus and astronomical history. But we could likewise perpetuate furlongs, rods, and pecks, and this definitely would not be convenient.

### Spectral color bands

The magnitude of a star can be measured in any of a number of wavelength bands. Thus when specifying a magnitude, the spectral band that was measured is usually specified. The "3-color" system mentioned above is the traditional system often used, namely the UBV system: ultraviolet, blue, visual (yellow). Each spectral

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