What was Murdin's estimate of the mass of the invisible partner (in solar masses)?

• A. 3
• B. 6
• C. 9
• D. 12

Answer: (B)

In this kind of binary system, you can figure out the mass of an invisible partner from how the visible star moves. Measure the orbital period and how fast the visible star shifts in velocity toward and away from us (its radial velocity). Those observations, together with Kepler’s laws, let you compute the mass function, which gives a minimum possible mass for the unseen companion (the true mass can be higher if the orbit isn’t viewed edge-on). Murdin’s analysis using that motion data pointed to about six solar masses for the invisible partner. That scale is well above the maximum mass for a neutron star, so it supports the interpretation of a black hole. If the data forced a much smaller mass like three solar masses, it would be near the neutron-star limit and harder to reconcile with the observed motion; much larger values would require even stronger gravitational effects not seen in the measurements.