The stability of galaxies and the reason why they do not collapse into their central black holes is due to a combination of factors:
Angular momentum: Galaxies, including their central black holes, possess a significant amount of angular momentum. Angular momentum is a property associated with rotational motion and is conserved in isolated systems. The angular momentum of the material in a galaxy keeps it in orbit around the central black hole rather than falling directly into it. This angular momentum prevents the material from collapsing all the way to the center.
Central mass concentration: While black holes have an extremely strong gravitational pull, they are not the only component in a galaxy. Galaxies consist of vast amounts of stars, gas, dust, and dark matter, all distributed throughout their structure. The mass distribution is not solely concentrated in the central black hole. The gravitational forces from all the mass in the galaxy counterbalance the gravitational pull of the central black hole, allowing the galaxy to maintain its overall structure.
Orbital stability: The motion of stars and other material within a galaxy is governed by the laws of orbital dynamics. These objects move in elliptical or nearly circular orbits around the central black hole. As long as the energy of the objects in the galaxy is conserved and their orbits remain stable, they will continue to orbit the black hole without falling into it. Disruptions to the orbits can occur due to interactions with other galaxies, but on average, galaxies remain stable over long periods of time.
It is worth noting that while galaxies can maintain stability over billions of years, there are instances where galactic interactions or mergers can cause material to fall into the central black hole, leading to the growth and activity of the black hole. This can result in phenomena such as active galactic nuclei or quasars, where material accretes onto the black hole, releasing vast amounts of energy.
Overall, the stability of galaxies over long periods of time is a result of the delicate balance between the gravitational forces of the central black hole and the overall mass distribution of the galaxy.