FIGURE 3.10 The characteristic light curve of a Cepheid variable. This star has a period of approximately 5.5 days (log 0.74), which implies that it has an absolute magnitude of approximately −1.5. Recalling from Figure 3.9 that the magnitude scale is a logarithmic scale of luminosity and that stars whose absolute magnitude differs by 5 differ in intrinsic luminosity by a factor of 100, we can calculate the distance to the star. The average apparent magnitude (visual magnitude in the figure) is 3.75, making the difference between absolute and apparent magnitude slightly more than 5. Therefore, the star appears approximately 100 times less luminous than it would if it were at the standard distance of 10 parsecs. Applying the inverse square law (luminosity decreases in proportion to 1/r2), one can infer that this star would be slightly more than 100 parsecs away. SOURCE: Abell, 1969, p. 480.