Astronomy 162 - Stars, Galaxies, & the Universe

Where are Lectures 1-4? This is a good question, and one I've gotten from many listeners. Here's the answer. Recorded 2006 Nov 27 on the Columbus campus of The Ohio State University.

How do we measure the distances to the stars? This lecture introduces the method of trigonometric parallaxes and the units of the Parsec and Light Year. Recorded 2006 January 9 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.

How do we quantify stellar brightness? This lecture introduces the inverse square-law of apparent brightness, the relation between Luminosity and Apparent Brightness, introduces the stellar magnitude system, and discusses photometry and the how we measure apparent brightness in practice. Recorded 2006 January 11 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.

What do the spectra of stars look like, and what can they tell us about stellar properties? This lecture introduces the idea of stellar color, gives a brief overview of the history of stellar spectroscopy, and introduces spectral classification and the main stellar spectral types OBAFGKMLT. Recorded 2006 January 13 in 1008 Evans Laboratory on the Columbus campus of The Ohio State University.

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.

What are the physical laws that determine the internal structure of stars? We first introduce the Mass-Luminosity Relation for Main Sequence stars, as well as seeing how the mean density of stars differs for stars on different parts of the H-R diagram. We then introduce the Ideal Gas Law, which relates pressure, density, and temperature, and show how the internal structure of a star is determined by a continuous tug-of-war between internal pressure trying to blow the star apart, and self-gravity trying to make it collapse. The balance between the two is the state of Hydrostatic Equilibrium. How the balance is maintained, and what happens when it is tipped in favor of either will determine the appearance and subsequent evolution of the star. Recorded 2006 January 18 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 do stars generate energy in their cores, and once made, how is that energy transported to the surface where it can be radiated away as Luminosity? This lecture revisits nuclear fusion and the Kelvin-Helmholz Mechanism, and discusses the 3 ways energy can be transported in stars: Radiation, Convection, and Conduction. This will lead us to the concept of Thermal Equilibrium in stars, which is the last main piece of stellar physics we need before we can address the question of how stars are formed and evolve in the rest of this Unit. Recorded 2006 January 23 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.

What are the properties of stars on the Main Sequence? This lecture discusses what happens to a star after it alights onto the Main Sequence, burning H to He in its core, and maintaining a state of Hydrostatic and Thermal Equilibrium. We will see how the mass of a star determines its location along the Main Sequence and influences its energy generation and internal structure. We finally introduce the nuclear timescale and derive the Main-Sequence Lifetime for stars, and discuss its consequences. Recorded 2006 January 25 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.

What happens when a high-mass (more than 4 solar masses) Main Sequence stars runs out of Hydrogen in its core. At first the internal evolution looks like that of a low-mass star, but now we get first a Red Supergiant then a sucession of blue and red supergiant phases as different nuclear fuels are tapped by the star for its energy. This lecture describes the evolution of high-mass stars from the Main Sequence until their eventual ends. Recorded 2006 January 27 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 the cores left behind at the end of a star's life? This lecture introduces these stellar remnants: White Dwarfs (remnants of low-mass stars held up by Electron Degeneracy Pressure), and Neutron Stars (remnant cores of core-bounce supernovae held up by Neutron Degeneracy Pressure). We also the Chandrasekhar Mass for White Dwarfs, Type Ia Supernovae resulting from a white dwarf getting tipped over the Chandrasekhar Mass, Pulsars (rapidly rotating magnetized neutrons stars), and ask what happens when a neutron star gets tipped over its mass limit. Recorded 2006 January 31 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.

What are our observational tests of Stellar Evolution? This lecture discusses how we use Hertzsprung-Russell Diagrams of star clusters to test stellar evolution theory, and some of the conclusions we have drawn. In particular, we will see how the age of a star cluster can be estimated from the Main-Sequence Turn-Off for the cluster. We also introduce Open and Globular Clusters, and show how we apply stellar evolution theory to their H-R diagrams. Recorded 2006 February 2 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 is the Milky Way, and what is our place within it? This lecture introduces the Milky Way, the bright band of light that crosses the sky, and describes how we came to our present understanding of the size and shape of the Milky Way Galaxy, and our location in it. Recorded 2006 February 7 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.

Andromeda is the nearest bright spiral galaxy to the Milky Way, and a near twin in terms of stellar and gas content. This lecture discusses the idea of stellar populations and chemical evolution in galaxies as determined by combining observations from within (the Milky Way) and without (Andromeda). At the end, two other features of these galaxies, their supermassive central black holes, is introduced, setting up a question to be addressed in later lectures. Recorded 2006 February 9 in 1008 Evans Laboratory 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.

What are the different types of galaxies? What hints can they give us as to the structure and evolution of galaxies? This lecture introduces the Hubble Classification System for galaxies, and describes the properties of each major class. This detailed overview gives us some tantalizing clues as to the formation and evolution of galaxies that will be picked up in subsequent lectures. Recorded 2006 February 13 in 1008 Evans Laboratory on the Columbus campus of The Ohio State University.

Galaxies are found in groups and clusters, and these are only the start of a hierarchy of cosmic structures up to the largest scales observed. This lecture introduces the properties of groups and clusters of galaxies, superclusters (clusters of clusters), and large scale structure with filaments of superclusters surrouning vast voids. We start with our Local Group, and then expand our view to encompass the depths of intergalactic space. Recorded 2006 February 14 in 1008 Evans Laboratory on the Columbus campus of The Ohio State University.

What happens if two galaxies collide? The average distance between bright galaxies is only about 20 times their size, so over the history of the Universe (14 Billion years), we expect that most bright galaxies will have had at least one close gravitational encounter with a neighboring galaxy. This lecture explores what happens when two galaxies undergo interactions ranging from passing tidal interactions to head-on collisions, all the way to multiple collisions and galaxy "cannibalism" in the centers of large clusters. While at first glance galaxy interactions explain rare "peculiar" galaxies, on closer examination we find that galaxy interactions and mergers are central to understanding the assembly and evolution of galaxies. At the end, we take a speculative look at the distant future 3-4 Billions years hence if in fact Andromeda and the Milky Way are on a collision course. Recorded 2006 February 15 in 1008 Evans Laboratory on the Columbus campus of The Ohio State University.

What are Active Galaxies and Quasars? We have good reason to think that buried deep in the hearts of nearly every (?) bright galaxy is a supermassive black hole with masses of millions or even billions of times the mass of the Sun. Most, like the one in our Milky Way, are quiescent, but in about 1% of galaxies, they are fed enough matter (up to about a sun's worth per year), and light up as an Active Galactic Nucleus (AGN) that can outshine an entire galaxy full of billions of stars. This lecture reviews the observed properties of Active Galaxies, the riddle of the Quasars, and the recognition that they are powered by the accretion of matter onto supermassive black holes. The lecture ends with some open questions in this active area of current research. Recorded 2006 February 16 in 1008 Evans Laboratory on the Columbus campus of The Ohio State University.

What are space and time? To begin our exploration of the evolving Universe, we must first understand what we mean by space and time. This lecture contrasts the Newtonian view of the World, with its absolute space and absolute time, with that of Einstein, who showed that space and time were not absolute but relative constructs, and that only spacetime, unified by light, was independent of the observer. This requires such non-intuitive notions as the speed of light being the same for all observers regardless of their motion, and that observers moving relative to each other will agree on the same physical laws and speed of light, but disagree on lengths, times, masses, etc. measured by applying those laws. This sets the stage for Einstein's revision of the Law of Gravity, General Relativity, which we will review in the following lecture. Recorded 2006 February 20 in 1008 Evans Laboratory on the Columbus campus of The Ohio State University.

What is gravity? Newton left that question unanswered when he formulated his inverse square law of the gravitational force, framing no hypothesis for what agency transmits gravity, only asserting it was an action at a distance. Einstein brought gravity into relativity by answering Newton's unanswered question with his General Relativity, our modern theory of gravity. In Einstein's formulation, Matter tells spacetime how to curve, and curved spacetime tells matter how to move. This lecture presents the basic picture of General Relativity, and introduces some of its observational consequences. The surprising conclusion is that instead of space and time being a backdrop for physics in Newton's view, united into spacetime by Relativity they are understood to be physical and dynamic. This is important for understanding how the Universe as a whole works. Recorded 2006 February 21 in 1008 Evans Laboratory on the Columbus campus of The Ohio State University.

What are the implications of Relativity for the Universe? This lecture introduces the Cosmological Principle, which states that the Universe is Homogeneous and Isotropic on Large Scales. Applying this to his then-new General Relativyt, Einstein got a surprise: the Universe must either expand or contract in response to all the matter/energy that fills it, something not observed in 1917. To attempt to stabilize the Universe, he introduced a Cosmological Constant (Lambda), that was to prove his greatest blunder. Subsequent theoretical and observational work was to establish that the Universe is indeed expanding systematically, if you look on scales large enough (the scale of galaxies). We will review observational evidence for the large-scale Homogeneity and Isotropy of the Universe, Einstein's brilliant conjecture, and see how the Cosmological Constant maybe wasn't such a blunder after all, as it has recently made a comeback of sorts. We'll explore these themes in greater detail in subsequent lectures. Recorded 2006 February 22 in 1008 Evans Laboratory on the Columbus campus of The Ohio State University.

How did we discover that the Universe is Expanding? What does it mean that it is expanding? This lecture introduces Hubble's Law, the observational evidence that the Universe is systematically expanding. As galaxies get more distant from us, the apparent speed of recession gets larger in proportion. The proportionality is the rate of expansion, called the Hubble Parameter (H0). This leads us to the idea of expanding space, and the Cosmological Redshift, which combined with the Hubble Law allows us a way to estimate very large cosmic distances. We will take up the more thorny issue of the Cosmic Distance Scale in tomorrow's lecture. Recorded 2006 February 23 in 1008 Evans Laboratory on the Columbus campus of The Ohio State University.

How do we measure distances on cosmic scales? This lecture describes the rungs in the cosmic distance ladder from measuring the AU in our own Solar System out into the Hubble expansion of the universe. These distances form the basis of the measurements that let us piece together the present, past, and future history of the expanding Universe, setting the stage for next week's lectures. Recorded 2006 February 24 in 1008 Evans Laboratory on the Columbus campus of The Ohio State University.

The Universe today is old, cold, low-density, and expanding. If we run the expansion backwards, we will eventually find a Universe where all the matter was in one place where the density and temperature are nearly infinite. We call this hot, dense initial state of the Universe the Big Bang. This lecture introduces the Big Bang model of the expanding universe, and how the history of the Universe depends on two numbers: the curretn expansion rate (H0), and the relative density of matter and energy (Omega0). Combined with observations, these give us an estimate of the age of the Universe of 14.0 +/- 1.4 Gyr. Recorded 2006 February 27 in 1008 Evans Laboratory on the Columbus campus of The Ohio State University.

Is there any evidence that the Universe was very hot and dense in the distant past as predicted by the Big Bang model of the expanding Universe? This lecture examines observational tests of the Big Bang Model. We have already covered expansion in the previous lecture. Today we look at Primordial Nucleosynthesis, the creation of light elements from fusion during the first 3-4 minutes of the hot phases of the Big Bang, and the Cosmic Background Radiation, the relic blackbody radiation remaining from when the Universe became transparent to light 300,000 years after the Big Bang. Both predictions of the Big Bang Model have been spectacularly confirmed by observations of the present-day Universe. These give us confidence that the Big Bang, in broad outline, is the correct physical model of our expanding Universe. Recorded 2006 February 28 in 1008 Evans Laboratory on the Columbus campus of The Ohio State University.

What was the Universe like from the earliest phases immediately after the Big Bang to the present day? This lecture reviews the physics of matter, and follows the evolution of the expanding Universe from the first instants after the Big Bang, when all 4 forces of nature were unified in a single grand-unified superforce until the emergence of the visible Universe we see around us today. Recorded 2006 March 1 in 1008 Evans Laboratory on the Columbus campus of The Ohio State University.

What is the ultimate fate of the Universe? The ultimate fate of the Big Bang is either expansion to a maximum size followed by re-collapse (the Big Crunch) or eternal expansion into a cold, dark, disordered state (the Big Chill). Which of these is our future depends on the current density of matter and energy in the Universe, Omega0. This lecture examines our current knowledge of the matter and energy content of the Universe, which leads to the surprising discovery that we live in a Universe that is Flat (Omega0=1), Infinite, and Accelerating! We will end the lecture by exploring the possible fate of an infinite accelerating Universe. Recorded 2006 March 2 in 1008 Evans Laboratory on the Columbus campus of The Ohio State University.

How will the Sun evolve? The Sun is now a middle-aged, low-mass, Main Sequence star in a state of hydrostatic and thermal equilibrium that has consumed about half of the Hydrogen available for fusion in its core. What will its subsequent evolution be as its core runs out of Hydrogen? This lecture describes our current state of understanding of the expected evolution of the Sun, informed by a combination of state-of-the-art solar models and stellar evolution codes, and data gathered from observations of nearby stars in our Galaxy. We will trace the future history of the Sun from the present until it begins its final phase as a fading White Dwarf some 8 Billion years in the future. Along the way, we'll also ask what will become of Earth. Recorded 2006 March 6 in 1008 Evans Laboratory on the Columbus campus of The Ohio State University.

We are not made of the same matter as most of the Universe! This surprising conclusion, that the ordinary matter we are made of (protons, neutrons, and electrons) constitute only 13% or so of the total matter in the Universe, the rest being in the form of Dark Matter. Further, this dark matter is only about 30% of the combined matter and energy density of the Universe, the remaining 70% of which appears to be a form of Dark Energy that fills the vacuum of space and acts in the present day to accelerate the expansion of the Universe. This lecture will summarize the state of our understanding of Dark Matter and Dark Energy, and look at the questions remaining to be answered in this active area of current research. Recorded 2006 March 7 in 1008 Evans Laboratory on the Columbus campus of The Ohio State University.

Can we travel through time? This is not a frivilous, science-fiction kind of question. Certain restricted kinds of time travel are in fact allowed by classical General Relativity. This lectures takes up this question, and looks at some of the surprising answers that have been found. Recorded 2006 March 8 in 1008 Evans Laboratory on the Columbus campus of The Ohio State University.

Are we alone in the Universe? This is the first part of a 2-part lecture that will explore the question of life and the Universe. We will look at the conditions needed for life, and address the question of how often we expect those conditions to be satisfied in our own Galaxy. In this part, we introduce the Drake Equation and make some basic estimates. To be honest, it was supposed to be one lecture, but I ran over time and ran into the bell. Oops! Very embarrasing. Tomorrow's lecture will finish up our discussion of life in the Universe, and then wrap up Astronomy 162 for the quarter. Recorded 2006 March 9 in 1008 Evans Laboratory on the Columbus campus of The Ohio State University.

How can we search for extraterrestrial intelligence, and what are we looking for? This second part of a 2-part lecture picks up where we left off yesterday by examining SETI, the Search for ExtraTerrestrial Intelligence, and reviews what we might look for and how. We will use this as a point of departure to then briefly review where we have come and what we have learned in Astronomy 162, bringing this course to a close for Winter Quarter 2006. Recorded 2006 Mar 10 in 1008 Evans Laboratory on the Columbus campus of The Ohio State University.

A new podcast, Astronomy 141, Life in the Universe, is available for those interested in continuing an exploration of topics in modern astronomy.