Galaxies and black holes are incredibly massive objects in the universe, and their evolution and stability are determined by the interplay of various factors. While they may appear to remain as they are for long periods of time, they do undergo changes and interactions over extremely long timescales.
Galaxies are vast systems of stars, gas, dust, and dark matter, bound together by gravity. Their overall structure and stability are maintained by the balance between gravity and other forces. Gravity pulls matter together, causing the formation of stars and the aggregation of material into galactic structures. On the other hand, other forces, such as pressure from stellar radiation, supernova explosions, and the rotation of the galaxy, counteract the force of gravity, preventing the galaxy from collapsing or dispersing.
Black holes, on the other hand, are regions of extremely dense matter where gravity is so intense that nothing, not even light, can escape their gravitational pull. They form when massive stars collapse under their own gravity during supernova explosions or through the merging of smaller black holes. Once a black hole forms, it continues to exist and exert its gravitational influence on its surroundings.
While galaxies and black holes are relatively stable over long timescales, they can still undergo changes and interactions. Galaxies can collide and merge with one another, causing the formation of larger galaxies. Such interactions can disrupt the structure of individual galaxies, triggering star formation, and influencing the distribution of matter within them.
Black holes can also grow in size by accreting matter from their surroundings. When matter falls into a black hole's gravitational field, it forms an accretion disk around the black hole, releasing immense amounts of energy as it spirals inward. This process, known as accretion, can fuel the growth of black holes and lead to the formation of active galactic nuclei and quasars, which are among the brightest objects in the universe.
However, it is important to note that the timescales over which these changes occur are extremely long compared to human lifetimes or even the age of the universe. In the grand scheme of things, galaxies and black holes do evolve and change, but their fundamental nature and presence can persist for billions or even trillions of years.