A unit of length often used by astronomers to measure distances in “nearby” space is the parsec (pc), defined as the distance at which a star subtends a parallax angle of one arc second due to Earth’s orbit around the Sun (see Equation 13-11 and Example 13-4). The practical limit of such measurements is 0.01 arc second. 

(a) How many light-years is 1 pc? 

(b) If the density of stars in the Sun’s region of the Milky Way is 0.08 star/pc³, how many stars could, in principle, have their distances from us measured by the trigonometric parallax method?

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1 AU r 13-11