 Stellar Apparent Magnitudes | Bright Stars | Basic Celestial Phenomena | Constellations | 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. Hint: Look at the star magnitude key on the back of your planisphere.

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.

 No. Description No. of stars Examples 6 6th magnitude stars are barely discernible to the naked eye under the best viewing conditions. Over 1000 NA 5 A star of the 5th magnitude is still dim, but 2.5 times brighter than a star of the 6th. Under 1000 Great Orion nebula 4 A star of the 4th magnitude is (2.5 x 2.5) = 6.3 times brighter than one of the 6th. Under 300 NA 3 3rd magnitude stars make a handy reference to evaluate observing conditions: if you can see them, conditions are good. Under 100 Three stars of Orion's head 2 2nd magnitude stars are relatively bright. You can easily become familiar with most of them. 30 Polaris (UMi) = 2 Regulus (Leo) = 1.5 1 1st magnitude stars are 100 times brighter than 6th magnitude stars. 20 Pollux (Gem) = 1.16 Aldebaran (Tau) = 1 Spica (Vir) = 1 Betelgeuse (Ori) = 0.8 Capella, Arcturus, Vega = 0.5 0 Stars brighter than first magnitude are given negative numbers. Sirius is the brightest star in the sky (other than the Sun!). All visible planets except Saturn can be brighter than Sirius. Among the planets, Venus is brightest. few Procyon = 0.5 Capella = 0 Arcturus = 0 Vega = 0 Sirius (CMa) = -1.42 Jupiter = -2.5 Venus = -2.5 to -4.4 (max.) Quarter Moon = -10 Full moon = -13 Sun = -26.7 