Astronomy 162 - Stars, Galaxies, & the Universe

Welcome to the Astronomy 162 Lecture Podcasts! This is a brief message from me explaining the podcasts, and welcoming new and old listeners. Recorded 2006 Mar 10 on the Columbus campus of The Ohio State University.

What are Spiral Galaxies? This lecture describes the basic properties of spiral galaxies, their patterns of rotation and how that lets us measure their masses, and the nature of the spiral arms as waves moving through the disk and triggering formation of new stars. Recorded 2006 February 10 in 1008 Evans Laboratory on the Columbus campus of The Ohio State University.

How did we come to understand that the Milky Way was just one of billions of other galaxies in a vast Universe? This lecture reviews the history of how we came to recognize that the spiral nebulae were, in fact, other milky ways like our own: vast systems of 100s of billions of stars located millions of parsecs away. The key to understanding their nature was finding the distances to the spiral nebulae compared to the size of our Galaxy. Recorded 2006 February 8 in 1008 Evans Laboratory on the Columbus campus of The Ohio State University.

How do we measure distances to astronomical objects that are too far away to use Trigonometric Parallaxes? This first lecture of Unit 4 reviews geometric methods like trigonometric parallaxes, and then introduces the idea of Standard Candles, and how they are used to develop methods for deriving Luminosity Distances based on the Inverse Square Law of Brightness. We will explore three luminosity-based distance methods useful for studying our Galaxy and nearby galaxies: Spectroscopic Parallaxes, Cepheid Variable Period-Luminosity Relation, and the RR Lyrae P-L Relation. Recorded 2006 February 6 in 1008 Evans Laboratory on the Columbus campus of The Ohio State University.

What happens if even Neutron Degeneracy pressure is insufficient to halt the collapse of gravity? In that case, the object simply collapses in upon itself, approaching a state of infinite density. Such an object has such strong gravity that nothing, not even light can escape from it. We call these Black Holes. This lecture describes the basic properties of black holes, takes an imaginary journey through the event horizon, and discusses observational evidence that stellar-mass black holes (the remnants of the evolution of very massive stars) actually exist, and ends with the suggestion that if Steven Hawking and others are right, black holes may not be so black after all. One Erratum: during the lecture while commenting on the fate of Karl Schwarzschild, for whom the Schwarzschild Radius is named, I incorrectly identify Henry Moseley (killed by a sniper during the Galipoli Campaign of WWI) as one of the discoverers of the neutron. Moseley was the person who discovered that "atomic number" corresponded to nuclear charge, and hence the number of protons in the nucleus. The discoverer of the neutron was James Chadwick, who died in 1974. Recorded 2006 February 1 in 1008 Evans Laboratory on the Columbus campus of The Ohio State University.

Once a massive star builds a massive Iron/Nickel core at the end of the Silicon Burning day, it is doomed. A catastrophic core collapse is followed by explosive ejection of the envelope in a Supernova. This lecture describes the stages of a core-bounce supernova explosion, and the subsequent seeding of the interstellar medium with heavy metals by the explosion debris. The fate of the collapsing core is the subject of the next lecture in this series. Recorded 2006 January 30 in 1008 Evans Laboratory on the Columbus campus of The Ohio State University.

What happens to a low-mass star (less than 4 solar masses) when it runs out of core Hydrogen and must leave the Main Sequence. This lecture describes the changes inside a low-mass star after Hydrogen exhaustion through the Red Giant, Horizontal Branch, Asymtotic Giant, and Planetary Nebula phases. In the end, we will see the star's envelope and core go their separate ways, the envelope gently puffed off into space, briefly flowering as a Planetary Nebula, and the Carbon-Oxygen core collapsing into a White Dwarf. Recorded 2006 January 26 in 1008 Evans Laboratory on the Columbus campus of The Ohio State University.

How do stars form? The Sun is old and in Hydrostatic and Thermal equilibrium. How did it get that way? This lecture presents the basic steps of star formation as a progress from cold interstellar Giant Molecular Clouds to Protostars in Hydrostatic Equilibrium, and then Pre-Main Sequence evolution which ends in ignition of core Hydrogen fusion and establishing Thermal Equilibrium on the Zero-Age Main Sequence. Recorded 2006 January 24 in 1008 Evans Laboratory on the Columbus campus of The Ohio State University.

How long can the Sun continue to shine, and what source of energy does it tap to keep shining? This lecture answers this question by introducing two important energy sources for stars: Gravitational Contraction otherwise known as the Kelvin-Helmholz Mechanism, and Nuclear Fusion. We will show that fusion of 4 Hydrogen nuclei into a Helium nucleus via the proton-proton chain liberates enough energy to provide for the Sun's Luminosity needs for about 10 Billion Years. Recorded 2006 January 19 in 1008 Evans Laboratory on the Columbus campus of The Ohio State University. Note: the recording mistakenly says January 18th. Oops.

How are all of the observed properties of stars (Luminosity, Mass, Radius, Temperature and Spectral Type) related to one another? This lecture introduces the Herzsprung-Russell Diagram, a plot of Luminosity versus Temperature for stars that is our most powerful tool for unlocking the secrets of the stars. Recorded 2006 January 17 in 1008 Evans Laboratory on the Columbus campus of The Ohio State University.

How do we measure the masses and radii of stars? This lecture describes the three basic types of binary stars, and how each are used to measure the masses of stars. Details of how to measure stellar radii are beyond the scope of this class, but we briefly describe the direct measurements of stellar radii. Recorded 2006 January 12 in 1008 Evans Laboratory on the Columbus campus of The Ohio State University.

The "fixed stars" are really in constant motion, but these motions are too small to see with the human eye in a human lifetime. This lecture introduces proper motions (apparent angular motion of the stars in the sky), radial velocities (motion towards or away from us measured using the Doppler Shift of the star's spectral lines), and true space velocities, measured by combining three key observables: the proper motion, radial velocity, and distance to the star. Recorded 2006 January 10 in 1008 Evans Laboratory on the Columbus campus of The Ohio State University.