By engaging with all the videos within this series, you will effectively complete a full undergraduate course in astronomy, equipping yourself with the knowledge and skills necessary to navigate the night sky with confidence, learning all the basics and many advanced topics! Let’s explore colliding and interacting galaxies. In our previous session, we discussed the Hubble classification of galaxies and noticed some peculiar ones, which appeared distorted. This time, we’ll uncover the secrets behind these appearances, emphasizing the significance of galaxy collisions and mergers. To understand galaxy interactions, we need to understand their spatial distribution. Unlike individual stars, galaxies are relatively close to each other. While stars are separated by vast distances, galaxies are often just 20 times their diameter apart. This proximity makes encounters and collisions frequent. Galaxies interact through gravitational forces, not electromagnetic or nuclear forces. As two galaxies approach, they exert differential gravitational pulls, causing distortions called tidal interactions. These interactions often lead to the formation of peculiar galaxies with distorted shapes and massive star formations. Throughout this lecture, we’ll see various stunning examples of interacting galaxies, illustrating how these gravitational dances unfold over billions of years. Despite their proximity, stars rarely collide due to their immense separation on a stellar scale. However, the gravitational pull profoundly alters the galaxies’ structures, leading to spectacular star formations and tidal tails. We’ll explore iconic interactions like the Whirlpool Galaxy and its elliptical counterpart, revealing how gravitational forces enact stellar births and distort galactic shapes. Iconic astronomer Halton Arp cataloged many such peculiar galaxies, now known as the Arp Catalog. Further examples include the Tadpole Galaxy and the Mice Galaxies, each displaying dramatic tidal tails and rapid star formation. Advanced computer simulations, employing Newton’s laws of motion and considering dark matter, accurately predict and understand these phenomena. These simulations track countless particles representing star clusters and gas clouds, comparing scientific predictions with observable data. We’ll discuss real-time cosmic events like the anticipated collision between our Milky Way and the Andromeda Galaxy, which spans over billions of years. These interactions, starting as beautiful cosmic dances, merge galaxies into elliptical giants. Computer simulations like those by John Dubinski highlight the intricate details of these interactions, from direct collisions triggering starbursts to the stripping away of stars into tidal streams. Galactic cannibalism, where a larger galaxy absorbs a smaller one, plays a crucial role in the universe’s evolution. We witness how starbursts triggered by collisions cause brief periods of intense luminosity before depleting the galaxies’ star-forming gas. We also explore spectacular phenomena like ring galaxies formed by smaller galaxies impacting larger ones. This lecture underscores the immense interplay of forces shaping the cosmos, providing a glimpse into the galaxies’ dramatic interactions through stunning astrophotography and meticulous scientific simulations. Overall, the segment emphasizes clear definitions, underlying geometry, and practical observing guidance so viewers can connect the concept to the real sky.