This is the thirteenth lecture series of my complete online introductory undergraduate college course. This video series was used at William Paterson University and CUNY Hunter in online classes as well as to supplement in-person course material. Notes and links are present in the videos at the start of each lecture.
Here, we learn about galaxies in the universe. The Hubble Sequence of Galaxies helps us organize them by appearance. When you don't know anything, first put them in buckets of "this one is like that one", and hope correlations appear. Here, they do! Edwin Hubble took many pictures, and found that they grouped into two rough groups with what he thought were transitions between them. Some of his intuitions held up over time, others did not. Next, galaxies are not small compared to the distances between them. Therefore, they crash together over hundreds of millions of years, creating new stars and disrupting their shapes. These dances of destruction take billions of years and cause catastrophic star formation. They fling stars out into the cosmos, and disperse the gas between the stars. Interactions prove that galaxies change with time, not only their appearances, but what they are made of. We then use many tools to get us out to the farthest reaches of the Cosmos. Herein, we summarize the various ways we learn to measure these gargantuan lengths. There are many steps that we use to scale the heights, from parallax, to star clusters, to variable stars and supernovae, to spinning galaxies, to redshift. Each measurement builds on the previous. It is truly astounding that we can learn the distances of the remotest points of creation. Edwin Hubble discovered the expansion of the Cosmos by seeking the distance to the Andromeda Galaxy. What is the cosmic redshift and how was it found? How we actually know what redshift is, and what its implications are, is one of the most important results of all of science: that the universe is expanding. And we measure it by seeing how fast our distant neighbors are rushing away. We see that gGalaxies appear in groups and clusters. Their mutual gravity reaches out across the unimaginably huge distances to pull them together over cosmic time. As a result, these groups are rich with interactions and varied appearances. We look again at our Local Group, and we find tiny analogs to it. We also find the beauty of the Virgo and Coma Clusters, vast arrays of galaxies all moving at extreme speeds. We find monster galaxies that cannibalize others and we find the largest gravitationally bound structures of the cosmos. These clusters are held together by dark matter, and stir up the intracluster gas to emit x-rays. Clusters are part of, and compose, superclusters, with ours being the gigantic Laniakea Supercluster. In between the clusters and superclusters are great voids, empty space with nothing it. We see on larger scales that superclusters form great filaments and walls, with the Sloan Great Wall being an excellent example. Where the island universes come from and how they grow over time has been an important recent study. Galaxies start as small clouds and undergo mergers and collisions over their billions of years of “life.” We see evidence for the changes in galaxies as we do what are called “pencil-beam" surveys, like the Hubble Deep Field and its antecedents. We see the effects on galaxies as they collide in clusters of galaxies. By taking huge surveys, we can map the changes through time that all galaxies undergo. Some galaxies have cores that outshine the entire rest of the galaxy. What are these amazing phenomena, and how do they manifest? We find the hyper-bright quasars, the cores of galaxies from an era long gone ago, which were powered by a supermassive black hole eating a star a day.