Over billions of years, how do galaxies generally change in appearance?

Over billions of years, galaxies generally evolve from smaller and bluer to larger and redder due to several key processes in cosmic evolution. Initially, when galaxies form, they are often smaller in size and tend to be star-forming regions rich in young, hot stars. These stars emit blue light, giving the galaxies a bluish hue.

As time progresses, galaxies can merge with other galaxies, leading to an increase in their size. During these mergers, the star formation activity can be quenched, leading to a reduction in the production of new stars. Consequently, the population of existing stars matures, shifting toward cooler, redder stars. This process ultimately transforms smaller, blue galaxies into larger, red ones, as the rate of star formation slows down and the balance of stellar populations shifts due to the death of massive stars and the stellar evolution of existing stars.

This transformation is a fundamental aspect of galaxy evolution theories and helps explain the observed morphology and color distribution of galaxies in the universe. Over vast time scales, changes in star formation rates, galaxy mergers, and interactions significantly contribute to the overall appearance and characteristics of galaxies, supporting the notion that they evolve in size and color from smaller and bluer to larger and redder.