Astro 160: The Physics of Stars

where X is the mass fraction of the species, Z is the number of electrons, and A is the atomic numberof the species, thus we findµ= 1 Aµ e = A(3)X 1+Z XZa) What are the values of µ and µ e if the gas consists ofi) ionized H,Since we know thatthenX = 1 A = 1 Z = 1µ= 1 2 µ e = 1ii) 75 % (by mass) ionized H and 25 % (by mass) ionized He,We have to treat this case seperately since we have two species contributing to the mean molecularweightX = 0.75 Y = 0.25 A = 4 Z = 2thusand so we find1= X µ I A + Y A = 1 13(X +Y) =A 16µ= 1627 µ e = 8 71µ e= Z A (X +Y) = 7 8iii) ionized He,Since this is a pure fully ionized gas we can use Equation 1 withX = 1 A = 4 Z = 2we findiv) ionized O,Using Equation 1 withwe findv) ionized FeUsing Equation 1 withwe findµ= 4 3 µ e = 2X = 1 A = 16 Z = 8µ= 169µ e = 2X = 1 A = 56 Z = 26µ= 5627µ e = 281346