Apparent Magnitude

The number of [stars] seen by the naked eye at once is seldom much above a thousand; though from their scintillation, and the indistinct manner in which they are viewed, they appear to be almost infinite.
Cycle of Celestial Objects, William Henry Smyth

The apparent brightness of a visible star, called apparent magnitude, is designated by a number usually falling between 0 and 6. A star with an apparent magnitude between 3.5 and 4.5 is called a 4th magnitude star. If you have a planisphere, look at the star magnitude key on the back.

Remember these key points about magnitude numbers:

1. The brighter the star, the lower its magnitude number: a first magnitude star is brighter than a second or third magnitude star, etc. Occasionally a magnitude may even be expressed as a negative value, and these are the brightest magnitudes of all.
2. Each integer difference of magnitude represents a change in apparent brightness of 2.5 times.
   • For example, a star of the 5th magnitude is 2.5 times brighter than one of the 6th magnitude.
   • A difference of two magnitudes, similarly, means that one star is 2.5 * 2.5 = 6.25 times brighter than the other. Therefore a 1st magnitude star is 2.5 times brighter than a 2nd magnitude star, and 6.25 times brighter than a 3rd magnitude star.
   • In general, one star is 2.5 raised to the n power brighter than another, if n = the difference between their magnitudes (remember, the brighter star has the lower magnitude number). That is, a 1st magnitude star is 2.5⁵= 100 times brighter than a 6th magnitude star.
3. There are a total of about 3000 stars potentially visible to the unaided eye, from magnitude 6 (very dim) to magnitude 1 or 0 (very bright). Only about one third of these are visible at a given time of year from a given hemisphere in the best of viewing conditions.