
Concept explainers
The circumferences of the two orbits, the separation between the stars, the total mass of the system and the individual mass of each star.

Answer to Problem 18P
The circumferences of the two orbits are 2.84 AU and 1.42AU. The separation between the stars is 0.678 AU. The total mass of the system is 40.6 solar mass. The individual masses of each star are 13.5 solar mass and 27.1solar mass.
Explanation of Solution
Write the equation for the circumference of first orbit.
C1=v1T (I)
Here, C1 is the circumference of first star. v1 is the speed of first star and T is the orbital period.
Write the equation for the circumference of second orbit.
C2=v2T (II)
Here, C2 is the circumference of second star, v2 is the speed of second star and T is the orbital period.
Write the expression for the separation between two stars.
r=v1+v2ω (III)
Here, r is the separation between two stars and ω is the angular speed.
Write the expression for the angular speed.
ω=2πT (IV)
Rewrite the expression for the separation between the stars.
r=v1+v2(2π/T) (V)
Write the expression for the distance of first star by using equation (IV).
r1=v1ω=v1(2π/T) (VI)
Here, r1 is the distance of first star.
Write the expression for the distance of second star by using equation (IV).
r2=v2ω=v2(2π/T) (VII)
Here, r2 is the distance of second star.
Write the equation for the total mass of a visual binary system by using (V).
M=r3T2 (VIII)
Here, M is the total mass.
Write the expression for the total mass of the system.
M=MA+MB (IX)
Here, MA is the mass of first star and MB is the second star.
Write the expression for the center of mass by using equation (VI) and (VII).
xcm=MAr1−MBr2MA+MB=MA(v1(2π/T))−MB(v2(2π/T))MA+MB (X)
Here, xcm is the center of mass.
Conclusion:
Substitute, 154 km/s for v1, 32 days for T in equation (I) to find C1.
C1=[(154 km/s)(1×103m1 km)][(32 days)(24 hrs1 day)(3600 s1 hr)]=(4.26×1011 m)(6.68459×10−12 AU1 m)≈2.84 AU
Substitute, 77 km/s for v1, 32 days for T in equation (II) to find C2.
C2=[(77 km/s)(1×103m1 km)][(32 days)(24 hrs1 day)(3600 s1 hr)]=(2.13×1011 m)(6.68459×10−12 AU1 m)=1.42 AU
Substitute, 154 km/s for v1, 77 km/s for v2, 32 days for T in equation (V) to find r.
r=[(154 km/s)(1×103m1 km)]+[(77 km/s)(1×103m1 km)](2π/[(32 days)(24 hrs1 day)(3600 s1 hr)])=(1.01×1011m)(6.68459×10−12 AU1 m)=0.678 AU
Substitute, 0.678 AU for r, 32 days for T in equation (VIII) to find M.
M=(0.678 AU)3[(32 days)(0.00273973 year1 days)]2=40.6 solar mass
Substitute, 154 km/s for v1, 32 days for T in equation (IX) to find MA.
MA=[(154 km/s)(0.210805 AU/yr1 km/s)]3[(32 days)(0.00273973 year1 day)](2π)3=12.1
Substitute, 0 for xcm, 154 km/s for v1, 77 km/s for v2 in equation (X) to find relation between MA,MB.
0=MA(v1(2π/T))−MB(v2(2π/T))MA+MBMAv1−MBv2=0MA[(154 km/s)(0.210805 AU/yr1 km/s)]−MB[(77 km/s)(0.210805 AU/yr1 km/s)]=032.46MA−16.23MB=0 (XI)
Substitute, 40.6 solar mass for M in equation (IX) to find relation between MA,MB.
MA+MB=40.6 solar mass (XII)
Solving equation (XI) and (XII), value of the individual masses of star.
MA=13.5 solar mass , MB=27.1 solar mass
Thus, the circumferences of the two orbits are 2.84 AU and 1.42AU. The separation between the stars is 0.678 AU. The total mass of the system is 40.6 solar mass. The individual masses of each star are 13.5 solar mass and 27.1solar mass.
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