Example 13.1 The observed period of a cepheid is 20 days and its mean apparent magnitude m = 20. From Fig. 13.4, its absolute magnitude is M 5. According to (4.11), the distance of the cepheid is r = 10 x 10(m - M)/5 = 10 x 10(20 + 5)/5

Example 13.2 The brightness of a cepheid varies 2 mag. If the effective temperature is 6000 K at the maximum and 5000 K at the minimum, how much does the radius change?

The luminosity varies between

In magnitudes the difference is

4n RmingTmm

4nR2 aT4

max"-1 max

This gives lg

whence Rm

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