The difference between the apparent magnitude (m) and absolute magnitude (M) is called the distance modulus (m - M). In formula form:
A star's apparent magnitude can be measured directly. For a distant star whose parallax cannot be measured but whose absolute magnitude is known, as from consideration of its spectrum, the distance modulus can be used to calculate distance.
Refer to Figure 3.15. Give the distance modulus of the stars that are (a) closer _; (b) farther away_
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