Concept explainers
(a)
The average separation between the two stars of the overcontact binary system W Ursae in kilometers. It is given that the two stars have mass 0.99 M⊙ and 0.62 M⊙, repectively.
(a)

Answer to Problem 42Q
Solution:
1.65×106 km
Explanation of Solution
Given data:
The mass of two stars are 0.99 M⊙ and 0.62 M⊙.
Formula used:
Write the expression for Newton’s form of Kepler’s third law:
(m1+m2)p2=a3
Here, m1 is the mass of the first star, m2 is the mass of the second star, p is the period of revolution of one star around the other in years, and a is the semi-major axis of the orbit of one star around the other. In the above formula, if the two masses are in terms of the solar mass (M⊙) and the period is in years, then the semi-major axis is in astronomical unit (au).
In case of binary stars, a becomes the distance between the two stars.
Explanation:
It is known that the period of the binary star system W Ursae is of 8 hours. Convert this time period in years.
8 hours=13 days=13×365 years=11095 years
Now, recall the expression for Newton’s form of Kepler’s third law.
(m1+m2)p2=a3
Substitute 0.99M⊙ for m1, 0.62M⊙ for m2, and 11095 years for p and calculate a.
a3=(0.99M⊙+0.62M⊙)(11095)2=1.34×10−6 (au)3a=1.10×10−2au
Convert a from au to km using the following conversion,
1 au=1.496×108 km
So, the distance between the two binary stars is,
a=(1.10×10−2)(1.496×108)=1.65×106 km
Conclusion:
Hence, the value of average separation between the stars is 1.65×106 km.
(b)
To check: That W Ursae is an overcontact binary star system, using the radii of its two stars given as 1.14 R⊙ and 0.83 R⊙ and the result obtained in part (a).
(b)

Answer to Problem 42Q
Solution:
W Ursae is an overcontact binary star system since the distance between the two binary stars in is almost comparable to the sum of their individual radii.
Explanation of Solution
Introduction:
An overcontact binary star system is one where the two stars are in contact with each other.
The radii of the two stars of W Ursae are 1.14 R⊙ and 0.83 R⊙, respectively. For this system to be an overcontact binary, the sum of these radii should be equal to the distance between the stars.
Explanation:
Caluclate the sum of the two given radii as ‘Rt’.
Rt=1.14R⊙+0.83R⊙=1.97R⊙
Convert unit of Rt to km.
Rt=(1.97)(6.96×105)=1.37×106km
Now, compare the value of Rt with the value calculated in part (a), and observe that they are close to each other. This proves that the system is an overcontact binary.
Conclusion:
Hence, it is found that the W Ursae is an overcontact binary star system as the sum of the radii of the two stars is almost the same as the distance between them.
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Chapter 19 Solutions
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