Math Problem Statement

The spectral lines of two stars in a particular eclipsing binary system shift back and forth with a period of 6 months. The lines of both stars shift by equal amounts, and the amount of the Doppler shift indicates that each star has an orbital speed of 5.0×10^5 m/s relative to the other. What are the masses of the two stars? Assume that each of the two stars traces a circular orbit around their center of mass.

Solution

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Problem:

The spectral lines of two stars in an eclipsing binary system shift periodically due to their motion. Each star orbits their common center of mass with a period of 6 months. The orbital speed of each star is given as 5.0×105m/s5.0 \times 10^5 \, \text{m/s}. The task is to calculate the masses of the two stars, assuming they are of equal mass (M1=M2M_1 = M_2) and express the answer using two significant figures.


Step-by-Step Solution:

1. Binary System Dynamics:

The stars orbit their common center of mass. According to Kepler's Third Law and Newton's Law of Gravitation: M1+M2=4π2r3GT2M_1 + M_2 = \frac{4 \pi^2 r^3}{G T^2} where:

  • rr = radius of the circular orbit,
  • G=6.674×1011m3kg1s2G = 6.674 \times 10^{-11} \, \text{m}^3 \, \text{kg}^{-1} \, \text{s}^{-2} (gravitational constant),
  • T=6months=6×30×24×3600sT = 6 \, \text{months} = 6 \times 30 \times 24 \times 3600 \, \text{s},
  • M1=M2=MM_1 = M_2 = M (equal masses).

2. Orbital Velocity Relationship:

For a circular orbit: v=2πrTv = \frac{2 \pi r}{T} Rearranging for rr: r=vT2πr = \frac{v T}{2 \pi} Substitute v=5.0×105m/sv = 5.0 \times 10^5 \, \text{m/s} and T=6monthsT = 6 \, \text{months}.


3. Substitute Values:

Using rr in the mass equation: M1+M2=4π2r3GT2M_1 + M_2 = \frac{4 \pi^2 r^3}{G T^2}

Since M1=M2=MM_1 = M_2 = M: 2M=4π2r3GT22M = \frac{4 \pi^2 r^3}{G T^2}

Solve this equation step by step for MM.


Let me compute the numerical values.import math

Given values

v = 5.0e5 # Orbital speed (m/s) T = 6 * 30 * 24 * 3600 # Orbital period in seconds (6 months) G = 6.674e-11 # Gravitational constant (m^3 kg^-1 s^-2)

Calculate orbital radius (r)

r = (v * T) / (2 * math.pi)

Apply mass equation: 2M = (4 * pi^2 * r^3) / (G * T^2)

total_mass = (4 * math.pi2 * r3) / (G * T**2) individual_mass = total_mass / 2 # Since M1 = M2

Format to 2 significant figures

individual_mass_rounded = round(individual_mass, 2)

individual_mass_rounded2.3179293479970372e+332.3179293479970372e+33successThe mass of each star in the binary system is approximately 2.32×1033kg2.32 \times 10^{33} \, \text{kg}, expressed to two significant figures.


Do you have any further questions or want a detailed explanation of any part of this solution?

Here are 5 related questions to consider:

  1. How does the Doppler effect help determine orbital velocities in binary systems?
  2. What changes if the masses of the stars are not equal?
  3. How does the orbital period affect the masses in a binary system?
  4. What other methods exist to determine the masses of stars?
  5. How do eccentric (non-circular) orbits affect this calculation?

Tip: Always double-check the period's unit conversions (e.g., months to seconds) to avoid errors in astronomical calculations.

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Math Problem Analysis

Mathematical Concepts

Kepler's Third Law
Circular Motion Dynamics
Gravitational Force
Unit Conversion

Formulas

M1 + M2 = (4π^2r^3) / (GT^2)
v = 2πr / T
r = (vT) / (2π)

Theorems

Kepler's Third Law
Newton's Law of Gravitation

Suitable Grade Level

College-level (Physics/Astronomy)