Masses of Binary Stars: Calculate from Motion

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The discussion centers on calculating the masses of two stars in a binary system based on their orbital characteristics. Observations indicate that one star has a circular orbit with a radius of 8 x 10^11 m and the other with a radius of 11 x 10^11 m, completing a revolution every 48 years. Participants express confusion about how to start the calculations, with one attempting to derive velocities from the circumferences of the orbits. Clarification is provided that the initial assumptions about the question may be incorrect, emphasizing the need for accurate interpretation of the given data. Ultimately, the focus remains on determining the masses of the stars using the provided orbital information.
fball558
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mass of stars?

Homework Statement



About half of the visible "stars" are actually binary star systems, two stars that orbit each other with no other objects nearby. Consider the motion of the center of mass of a binary star system. For a particular binary star system, telescopic observations repeated over many years show that one of the stars (whose unknown mass we'll call M1) has a circular orbit with radius R1 = 8 E 11 m, while the other star (whose unknown mass we'll call M2) has a circular orbit of radius R2 = 11 E 11 m about the same point. Make a sketch of the orbits, and show the positions of the two stars on these orbits at some instant. Label the two stars as to which is which, and label their orbital radii. Indicate on your sketch the location of the center of mass of the system. (Do this on paper; you will not be asked to turn it in. )(c) This double star system is observed to complete one revolution in 48 years. What are the masses of the two stars?

The Attempt at a Solution



i have no idea where to start. i asked 2 of my under grad TA's and they did not know how to do it so though i would come here.
i assumed that the orbits were perfect circles and them found circumference and divided by 48 years. i converted this to m/s and that should be the velocity.
V1 = 3318.44
V2 = 4562.85
 

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Hi fball558! :smile:

erm … neither you nor the TAs are reading the question :redface:

it clearly states:
fball558 said:
… one of the stars (whose unknown mass we'll call M1) has a circular orbit with radius R1 = 8 E 11 m, while the other star (whose unknown mass we'll call M2) has a circular orbit of radius R2 = 11 E 11 m about the same point

sooo … another diagram? :wink:
 
The book claims the answer is that all the magnitudes are the same because "the gravitational force on the penguin is the same". I'm having trouble understanding this. I thought the buoyant force was equal to the weight of the fluid displaced. Weight depends on mass which depends on density. Therefore, due to the differing densities the buoyant force will be different in each case? Is this incorrect?

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