According to Levin, the number of black holes found in our galaxy is at least how many?

• A. About five
• B. At least twenty
• C. More than a hundred
• D. None

Answer: (B)

The main idea tested is how many black holes have been observationally identified in our galaxy and how we determine that number. We identify black holes mainly through indirect signs, because they don’t emit light on their own unless they’re in a system where they’re pulling in material from a companion star. In particular, we study X-ray binaries where the visible star’s orbit reveals the mass of the unseen partner. If that mass exceeds about three solar masses, it’s too heavy for a neutron star and is taken as strong evidence for a black hole. Using this dynamical method, astronomers have confirmed at least twenty black holes in the Milky Way. That’s why Levin quotes a number of at least twenty—this is a solid lower limit based on the systems we can dynamically weigh.

There are many more candidate systems and hidden populations, but without dynamical confirmation their nature remains uncertain, so the count stays at least twenty rather than a higher, more definite number. The other options don’t fit what the evidence shows: a very small count like five would ignore the numerous confirmed X-ray binaries with massive compact companions; none would contradict the reality that we have found black holes; and claiming more than a hundred would overstate the number that have been robustly confirmed given current detection methods and observational biases.