At 6,000 light-years away, Cygnus X-1 corresponds to approximately how many solar masses?

• A. 8
• B. 15
• C. 30
• D. 50

Answer: (B)

Binary orbital dynamics let us infer the mass of an unseen companion by watching how the visible star moves in its orbit. By measuring the companion star’s radial velocity over the orbital period, we get the mass function, which provides a minimum mass for the unseen object. For Cygnus X-1, these measurements indicate the companion is too massive to be a neutron star. When you also account for how tilted the orbit is relative to us and other observational constraints, the mass converges to about fifteen solar masses. That’s why the value near 15 solar masses is the best match. The distance of roughly 6,000 light-years doesn’t set the mass itself; the dynamical measurements do, and they point to a stellar-mass black hole of roughly fifteen solar masses.