# Astrophysics: Finding the mass of a hidden star in a binary system

1. Dec 1, 2009

### knowlewj01

1. The problem statement, all variables and given/known data

The spectral lines in a low mass main sequence star show sinusodal velocity variations with an amplitude of 500 km/s and a time period of 10 hours

calculate a lower limit to the mass of the unseen binary companion

2. Relevant equations

M1 + M2 = $$\frac{4\pi^2a^3}{GP^2}$$

M1r1 = M2r2

a = r1 + r2

3. The attempt at a solution

The redshifted and blueshifted spectral lines show that the star is travelling at 500km/s

in a time period of 10 hoiurs (= 36 000 seconds)

Distance travelled in 1 orbit = 18 000 000 km

radius of orbit = $$\frac{18 000 000}{2\pi}$$ = 28274334 km

assume that the hidden object is much more massive.

so a = radius of this orbit = 28274334000 m

M1 >> M2 therefore r1 << r2

M1 = $$\frac{4\pi^2r^3}{GP^2}$$

M1 = 1.30x10^13 kg

this is wrong,

think i may have made a mistake when i said that the hidden object is much more massive, can this be solved without making an assumption?

2. Dec 1, 2009

### Staff: Mentor

I think you can calculate ratio of both masses, then use information about mass of the observed star ("low mass main sequence star") to estimate mass of the other one. But that's just intuition, I can be easily wrong.