1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Two body orbital system question

  1. Jan 25, 2007 #1
    1. The problem statement, all variables and given/known data
    The observed period of the binary system Cygnus X-1, presumed to be a bright star and a black hole, is equal to 5.6 days. If the mass of the visable (the star) is 20*Mass of the Sun, and the black hole has a mass 16*Mass of the Sun, show taht the semimajor axis of the orbit of the black hole relative to the visable star is roughly 1/5 the distance from the Earth to the sun.

    2. Relevant equations
    Period (using AU) relationship to semimajor axis of a system

    T = [m,sub 1 + m, sub 2]^(-1/2)*a^3/2, where 'a' is the semi-major axis and both values of 'm' are given in units of mass of the sun.

    Reduced mass: (m,sub 1)*(m,sub 2)/((m, sub 1)+(m, sub 2))

    Postion vector of particle 1 relative to particle 2

    R= (r(from center of mass), sub 1)*(1+ ratio of masses)

    3. The attempt at a solution

    I am not entirly sure where to go with this, however, I did calculate some values so far:

    u= 8.888*Mass of Sun
    a= .020384 AU, note that the value of this semi-major axis is for the binary system as a whole, not relative to either one of the points.

    Any ideas where to go with all of this?

    (I am still working at it, but a hint would be nice)
  2. jcsd
  3. Jan 26, 2007 #2


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Binary stars obey Kepler's laws.
  4. Jan 27, 2007 #3
    I will be retracting my question....

    After stareing at the question for a good 3 hours, my brain kicked in and noticed that I had done a simple math mistake (seriously, I moved the decimal place over 1 too many). I guess thats what I get when I do homework late at night.

    sorry of the stupid question.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?

Similar Discussions: Two body orbital system question