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About Philip A. Stahl
Expertise
I specialize in stellar and solar astrophysics. Can answer any questions pertaining to these areas, the spectroscopic analysis of stars – as well as the magneto-hydrodynamics of sunspots and solar flares. Sorry – No homework problems done or research projects! I will provide hints on solutions.

Experience
Have published papers on the relationship between sunspot morphology and solar flares; discovery of SID flares related to this, constructed computerized stellar models; MHD research.

Organizations
American Astronomical Society (Solar physics and Dynamical astronomy divisions), American Geophysical Union, American Mathematical Society, Intertel.

Publications
Solar Physics, Journal of the Royal Astronomical Society of Canada, Journal of the Barbados Astronomical Society, Meudon Solar Flare Proceedings (Meudon, France)

Education/Credentials
B.A. degree in Astronomy; M.Phil. degree in Physics - specializing in solar physics.

Awards and Honors
Postgraduate research award- Barbados government; Studentship Award in Solar Physics - American Astronomical Society

You are here: Experts > Science > Physics > Astrophysics > Binary star problem

Astrophysics - Binary star problem

Expert: Philip A. Stahl - 11/3/2009

Question
Can you help with this problem? A visual binary has a period of 100 years, and the semi-major axis of the relative orbit is 1."00.

If one of the stars is identical to the Sun and has an apparent visual magnitude of +8.0, what is the mass of the other star?

Thanks!

Answer
Hello,

This looks like a fairly basic HW problem, so I will just get you oriented to solving it yourself. If you need more help after my hints, you can get back to me.

First, if one of the stars is *identical* to the Sun, it means it must have the same absolute magnitude, M = +4.8.

You will also need the distance modulus in this problem, and you will find an illustrated example of it in the latter part of a previously addressed problem here:

http://en.allexperts.com/q/Astrophysics-3368/Gray-atmosphere-distance-modulus.ht...

Now, for a semi-major axis in astronomical units A, and for which a" denotes the angular separation, one has:

A = a"/p

The only thing remaining is to relate A to the period P, and the sum of masses of the system (M1 + M2) using Kepler's law relating the period squared to the semi-major axis cubed.

I believe you ought to be able to get it from here. If you want you may send the solution and I will tell you if you are on the right trail, have it correct...or not.

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