How do black holes grow their mass in galaxies?

• A. By accreting gas only.
• B. By mergers with other black holes only.
• C. By accretion, mergers, and capture of stars and compact objects.
• D. By absorbing dark energy.

Answer: (C)

Black holes in galaxies grow by multiple channels that work together over cosmic time. Gas that and flows into the central region can be pulled across the event horizon, increasing the black hole’s mass as it accretes material (this is what powers active galactic nuclei and quasars when fueling is strong). When galaxies merge, their central black holes can form a bound pair and eventually coalesce, adding the mass of the second hole to the growing black hole. In dense galactic centers, stars and compact objects that pass close enough can be swallowed or tidally disrupted, letting the black hole gain mass from these encounters as well. The idea that dark energy feeds black holes isn’t part of the standard growth channels, so absorbing dark energy isn’t considered a meaningful mechanism for increasing black hole mass. Because growth occurs through all three processes—accretion, mergers, and capture of stars and compact objects—the best answer reflects this combined picture.