This is the seventh 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.

We begin this incredibly important module by learning about stellar spectra. We’ll learn what the dark lines in the spectra of stars mean, and the history behind their understanding. Next, I discuss parallax and proper motion. The stars move in many ways; they are not fixed in the sky, but changes slowly over the centuries. Observationally, we see binary stars, which are not just pretty to see. This is the first of two videos about binary stars and their applications. We learn that there are many types of binary star, and some of the most important also very near to us. Next, we look at binary stars again; specifically, their eclipses, masses and planets. Binary stars are some of the most important stars to study, as they are the gateway to astrophysics. We can determine their masses and we can determine many other things about them. So, here we chat more about spectroscopic and eclipsing binaries, and how a planet about the size of Earth was found orbiting the star nearest to the Sun. On our next lecture, we focus on the mass of Proxima b and other stellar masses. We use the data from the Pale Red Dot's discovery of a planet around another star to show exactly how we link together the observational data of radial velocity observations to give us the mass and radius of a planet orbiting a distant star. We can't see the planet, but we see its effects. Therefore, we can deduce its properties. We use this to emphasize the importance of learning the masses of the stars. We then go on to the Hertzsprung-Russell Diagram. This is the most fundamental tool in all of astrophysics. It cannot be understated that this tool basically invented astrophysics, and can even be replicated with amateur-class telescopes. From here, we go on measure stellar radii. The sizes of stars are not just their masses, but their physical diameters, or more useful, their radii. With that done, we then measure the linked properties of stellar mass, luminosity and lifespan. Now we get to some real meat of astrophysics. We discuss the relationship between mass, luminosity and lifespan of main sequence stars. It is an amazing discovery that mass is known to be strictly correlated to luminosity for M-S star. And for M-S stars, the luminosity is tightly related to spectral type. So, just get the spectrum, and you know the mass! After that, I define the tool of spectroscopic parallax. All of astrophysics depends on knowing accurate distances. Here, we see how knowing the distance to a well-studied, nearby star cluster can help us begin to determine the size scale of the Universe. We see how parallaxes, and brightnesses in standard filters can get us all the way out to extremely far distances, and teach us about the nature of those distant places. Finally, I go through star clusters and stellar evolution, Star clusters are elegant and pretty things to see in a telescope, but that's not the whole story. We know that stars must change as they run out of hydrogen to burn in their cores, and those deep changes in the core of stars have observable consequences. So, we use HR Diagrams to map the changes and enormous computer simulations to test the physics.