Galaxies: Birth, Evolution, and Destiny

Galaxies begin as quantum fluctuations post-Big Bang. Gravity amplifies these irregularities, allowing gas to collapse into halos of dark matter, giving rise to the first stars and protogalaxies in a process called hierarchical merging.

Early galaxies are chaotic. Intense star formation leads to rapid stellar evolution. Two types of stellar populations form: Population III, metal-free stars, and Population II, slightly metal-rich, influencing subsequent galaxy characteristics through supernovae.

Galaxies evolve through internal and external processes. Major mergers catalyze starbursts and active galactic nuclei formation. Meanwhile, secular evolution, like bar formation in spirals, reshapes galaxies without dramatic external influence.

Star formation in galaxies eventually ceases. Mechanisms like virial shock heating of infalling gas, active galactic nuclei feedback, or the depletion of cold gas reservoirs lead to a galaxy's transition from 'blue and active' to 'red and dead'.

Galaxy interactions are common. Larger galaxies often cannibalize smaller companions, a process observed in the extended stellar halos of many galaxies, including our own Milky Way, which contains remnants of devoured dwarf galaxies.

Galaxies are recycling powerhouses. Supernovae and stellar winds expel gas into the intergalactic medium, which can be re-accreted. This cosmic recycling can spark new star formation in neighboring galaxies, perpetuating the cycle of galactic life.

Galaxy growth exhibits 'downsizing'. Surprisingly, massive galaxies formed their stars early and quickly, while smaller galaxies continue to form stars. This inverse relationship challenges early understanding of galaxy formation and remains an active research area.

Some galaxies, known as ultra-diffuse galaxies, are nearly invisible due to their low star density, yet they contain massive amounts of dark matter.