A galaxy is a massive system comprising stars, stellar remnants, interstellar gas, dust, and dark matter held together by gravity. This distinguishes galaxies from smaller structures like star clusters.

The Milky Way is the galaxy that contains our Solar System and is well known as our home galaxy. Understanding this helps in grasping basic astronomical scales and the concept of galaxies.

Spiral galaxies are defined by their rotating disks with spiral arms and a central bulge. This classification helps distinguish them from elliptical or irregular galaxies.

Hydrogen is the most abundant element in the universe and fuels the nuclear fusion process in stars. Its prevalence is crucial in the formation and evolution of galaxies.

A light-year measures the distance that light travels in one year. This unit is commonly used in astronomy to describe vast interstellar and intergalactic spaces.

Galaxy clusters form as gravity pulls together galaxies embedded in dark matter halos. Dark matter's gravitational influence is a key driver in assembling these large-scale structures.

Dark matter does not emit light but has a strong gravitational effect, creating potential wells that attract ordinary matter. This gathering of matter facilitates the formation of stars and galaxies.

Dark Energy is the mysterious force driving the accelerated expansion of the universe. This accelerated expansion affects how galaxies form and evolve over cosmic time.

Mergers and collisions between galaxies can disrupt spiral structures, leading to the formation of elliptical galaxies. These violent interactions redistribute stars and gas, resulting in a more rounded galaxy shape.

Astronomers use the redshift of a galaxy's light along with Hubble's Law to estimate its distance. This method relies on the observation that more distant galaxies recede faster from us.

Cosmic reionization refers to the process where the first generations of stars and galaxies reionized neutral hydrogen. This change in the state of hydrogen enabled further cooling and star formation in the universe.

The Epoch of Reionization marks the time when the first galaxies and stars formed, illuminating the universe after its dark ages. This period was crucial for setting the stage for later cosmic evolution.

The Hubble Tuning Fork diagram is used to categorize galaxies by their visual morphology. It provides a framework that helps astronomers understand the evolutionary relationships between different galaxy types.

The discovery of Cosmic Microwave Background Radiation provided strong evidence for the Big Bang theory. This remnant radiation confirms that the universe began from a hot, dense state, setting the conditions for galaxy formation.

Computer simulations allow scientists to model complex physical processes such as gravity, gas dynamics, and dark matter interactions. These models are essential for understanding the dynamic and evolving nature of galaxy formation.

Hierarchical clustering is a bottom-up process where small galaxies and gas clouds merge to form larger galaxies over time. This concept is central to modern theories of cosmic structure formation.

Feedback from supernovae and active galactic nuclei inject energy into the surrounding medium, heating the gas and driving outflows. This process regulates star formation by preventing runaway cooling and collapse within galaxies.

Simulating galaxy formation requires accurately modeling both baryonic processes like star formation and the dynamics of dark matter. The interplay between these components presents significant challenges that require high computational capabilities.

Galaxies in dense clusters experience processes such as ram-pressure stripping and tidal interactions, which can remove gas and disturb their structure. These environmental effects significantly alter star formation rates and the evolutionary trajectory of galaxies.

Features such as tidal tails and stellar streams are strong indicators of past merger events. These structures result from gravitational disturbances during mergers and provide evidence of a galaxy's dynamic history.