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## Main Question or Discussion Point

Say I have an X-ray binary system of a B2 main-sequence star with an unseen companion (i.e., black hole). They have a separation of 20 million km and an orbital period of 4 days.

How do I figure out what the semi-major axis is? I need it for the formula for Kepler's Version of Newton's Third Law:

[tex]M_{1} + M_{2}[/tex] = (4π²)÷G × (a³)÷(p²)

where G is the gravitational constant, p is the period (which is given to me) and a is the semi-major axis. My question asks to find the sum of the masses, so I need to plug something in for a in the equation to get that answer. Can I just use the 20 million km separation?

Thanks! :)

How do I figure out what the semi-major axis is? I need it for the formula for Kepler's Version of Newton's Third Law:

[tex]M_{1} + M_{2}[/tex] = (4π²)÷G × (a³)÷(p²)

where G is the gravitational constant, p is the period (which is given to me) and a is the semi-major axis. My question asks to find the sum of the masses, so I need to plug something in for a in the equation to get that answer. Can I just use the 20 million km separation?

Thanks! :)

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