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Ratio of Total Angular Momentum to Star Angular Momentum in Binary Star System

A binary star consists of two stars A (mass 2.2Ms2.2\text{M}_\text{s}) and B (mass 11Ms11\text{M}_\text{s}), where Ms\text{M}_\text{s} is the mass of the sun. They are separated by distance dd and are rotating about their centre of mass, which is stationary. The ratio of the total angular momentum of the binary star to the angular momentum of star B about the centre of mass is

Official Numerical Answer6

Step-by-Step Solution

Let the masses of star A and star B be denoted by mAm_A and mBm_B, respectively:
mA=2.2 Msm_A = 2.2\,\text{M}_\text{s}
mB=11 Msm_B = 11\,\text{M}_\text{s}

The two stars orbit their stationary centre of mass (CM) with a common angular velocity ω\omega.

Let the distance of star A and star B from the centre of mass be rAr_A and rBr_B, respectively. By the definition of the centre of mass:
mArA=mBrB  ⟹  rArB=mBmAm_A r_A = m_B r_B \implies \frac{r_A}{r_B} = \frac{m_B}{m_A}

The angular momentum of star A about the centre of mass is:
LA=mArA2ωL_A = m_A r_A^2 \omega

The angular momentum of star B about the centre of mass is:
LB=mBrB2ωL_B = m_B r_B^2 \omega

The total angular momentum of the binary star system is:
Ltotal=LA+LBL_{\text{total}} = L_A + L_B

The ratio of the total angular momentum of the system to the angular momentum of star B is:
LtotalLB=LA+LBLB=LALB+1\frac{L_{\text{total}}}{L_B} = \frac{L_A + L_B}{L_B} = \frac{L_A}{L_B} + 1

Substituting the expressions for LAL_A and LBL_B:
LALB=mArA2ωmBrB2ω=mAmB(rArB)2=mAmB(mBmA)2=mBmA\frac{L_A}{L_B} = \frac{m_A r_A^2 \omega}{m_B r_B^2 \omega} = \frac{m_A}{m_B} \left(\frac{r_A}{r_B}\right)^2 = \frac{m_A}{m_B} \left(\frac{m_B}{m_A}\right)^2 = \frac{m_B}{m_A}

Substituting the given values of the masses:
LALB=11 Ms2.2 Ms=5\frac{L_A}{L_B} = \frac{11\,\text{M}_\text{s}}{2.2\,\text{M}_\text{s}} = 5

Therefore, the required ratio is:
LtotalLB=5+1=6\frac{L_{\text{total}}}{L_B} = 5 + 1 = 6