Lecture 32: Space, Time, & Gravity: General Relativity
What is gravity? Newton left that question unanswered when he formulatedhis inverse square law of the gravitational force, framing no hypothesisfor what agency transmits gravity, only asserting it was an actionat a distance. Einstein brought gravity into relativity by answeringNewton's unanswered question with his General Relativity, our moderntheory of gravity. In Einstein's formulation, Matter tells spacetime howto curve, and curved spacetime tells matter how to move. This lecturepresents the basic picture of General Relativity, and introduces someof its observational consequences. The surprising conclusion is thatinstead of space and time being a backdrop for physics in Newton's view,united into spacetime by Relativity they are understood to be physicaland dynamic. This is important for understanding how the Universe asa whole works.Recorded 2006 February 21 in 1008 Evans Laboratory on the Columbus campusof The Ohio State University.
21 Feb 2006
Lecture 20: Black Holes
What happens if even Neutron Degeneracy pressure is insufficient tohalt the collapse of gravity? In that case, the object simply collapsesin upon itself, approaching a state of infinite density. Such an objecthas such strong gravity that nothing, not even light can escape from it.We call these Black Holes. This lecture describes the basic propertiesof 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 othersare 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 campusof The Ohio State University.
1 Feb 2006
Lecture 41: Dark Matter & Dark Energy
We are not made of the same matter as most of the Universe! Thissurprising conclusion, that the ordinary matter we are made of (protons,neutrons, and electrons) constitute only 13% or so of the total matterin the Universe, the rest being in the form of Dark Matter. Further,this dark matter is only about 30% of the combined matter and energydensity of the Universe, the remaining 70% of which appears to be a formof Dark Energy that fills the vacuum of space and acts in the presentday to accelerate the expansion of the Universe. This lecture willsummarize the state of our understanding of Dark Matter and Dark Energy,and look at the questions remaining to be answered in this active areaof current research. Recorded 2006 March 7 in 1008 Evans Laboratory onthe Columbus campus of The Ohio State University.
7 Mar 2006
Lecture 31: A Tale of Two World Views: Special Relativity
What are space and time? To begin our exploration of the evolvingUniverse, we must first understand what we mean by space and time.This lecture contrasts the Newtonian view of the World, with itsabsolute space and absolute time, with that of Einstein, who showedthat space and time were not absolute but relative constructs, andthat only spacetime, unified by light, was independent of the observer.This requires such non-intuitive notions as the speed of light beingthe same for all observers regardless of their motion, and thatobservers moving relative to each other will agree on the same physicallaws and speed of light, but disagree on lengths, times, masses, etc.measured by applying those laws. This sets the stage for Einstein'srevision of the Law of Gravity, General Relativity, which we willreview in the following lecture.Recorded 2006 February 20 in 1008 Evans Laboratory on the Columbus campusof The Ohio State University.
20 Feb 2006
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Lecture 18: Supernovae
Once a massive star builds a massive Iron/Nickel core at the end ofthe Silicon Burning day, it is doomed. A catastrophic core collapseis 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 heavymetals by the explosion debris. The fate of the collapsing core isthe subject of the next lecture in this series.Recorded 2006 January 30 in 1008 Evans Laboratory on the Columbus campusof The Ohio State University.
30 Jan 2006
Lecture 19: Extreme Stars: White Dwarfs & Neutron Stars
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 upby Neutron Degeneracy Pressure). We also the Chandrasekhar Mass forWhite Dwarfs, Type Ia Supernovae resulting from a white dwarf gettingtipped over the Chandrasekhar Mass, Pulsars (rapidly rotating magnetizedneutrons stars), and ask what happens when a neutron star gets tippedover its mass limit.Recorded 2006 January 31 in 1008 Evans Laboratory on the Columbus campusof The Ohio State University.
31 Jan 2006
Lecture 33: Einstein's Universe
What are the implications of Relativity for the Universe? This lectureintroduces the Cosmological Principle, which states that the Universe isHomogeneous and Isotropic on Large Scales. Applying this to histhen-new General Relativyt, Einstein got a surprise: the Universe musteither expand or contract in response to all the matter/energy thatfills it, something not observed in 1917. To attempt to stabilize theUniverse, he introduced a Cosmological Constant (Lambda), that was toprove his greatest blunder. Subsequent theoretical and observationalwork was to establish that the Universe is indeed expandingsystematically, if you look on scales large enough (the scale ofgalaxies). We will review observational evidence for the large-scaleHomogeneity and Isotropy of the Universe, Einstein's brilliant conjecture,and see how the Cosmological Constant maybe wasn't such a blunder afterall, as it has recently made a comeback of sorts. We'll explore thesethemes in greater detail in subsequent lectures.Recorded 2006 February 22 in 1008 Evans Laboratory on the Columbus campusof The Ohio State University.
22 Feb 2006
Lecture 42: Time Travel
Can we travel through time? This is not a frivilous, science-fictionkind of question. Certain restricted kinds of time travel are in factallowed by classical General Relativity. This lectures takes up thisquestion, and looks at some of the surprising answers that have beenfound. Recorded 2006 March 8 in 1008 Evans Laboratory on the Columbuscampus of The Ohio State University.
8 Mar 2006
Lecture 11: The Internal Structure of Stars
What are the physical laws that determine the internal structureof stars? We first introduce the Mass-Luminosity Relation forMain Sequence stars, as well as seeing how the mean density of starsdiffers for stars on different parts of the H-R diagram. We thenintroduce the Ideal Gas Law, which relates pressure, density, and temperature,and show how the internal structure of a star is determined bya continuous tug-of-war between internal pressure trying to blowthe star apart, and self-gravity trying to make it collapse. Thebalance between the two is the state of Hydrostatic Equilibrium. Howthe balance is maintained, and what happens when it is tipped infavor of either will determine the appearance and subsequent evolutionof the star. Recorded 2006 January 18 in 1008 Evans Laboratory on the Columbus campus of The Ohio State University.
18 Jan 2006
Lecture 36: The Big Bang
The Universe today is old, cold, low-density, and expanding. If we runthe expansion backwards, we will eventually find a Universe where allthe matter was in one place where the density and temperature are nearlyinfinite. We call this hot, dense initial state of the Universe the BigBang. This lecture introduces the Big Bang model of the expandinguniverse, and how the history of the Universe depends on two numbers:the curretn expansion rate (H0), and the relative density of matter andenergy (Omega0). Combined with observations, these give us an estimateof the age of the Universe of 14.0 +/- 1.4 Gyr. Recorded 2006 February 27 in1008 Evans Laboratory on the Columbus campus of The Ohio StateUniversity.
27 Feb 2006
Lecture 17: The Evolution of High-Mass Stars
What happens when a high-mass (more than 4 solar masses) Main Sequencestars runs out of Hydrogen in its core. At first the internal evolutionlooks like that of a low-mass star, but now we get first a Red Supergiantthen a sucession of blue and red supergiant phases as different nuclearfuels are tapped by the star for its energy. This lecture describesthe evolution of high-mass stars from the Main Sequence until theireventual ends.Recorded 2006 January 27 in 1008 Evans Laboratory on the Columbus campusof The Ohio State University.
27 Jan 2006
Lecture 30: Active Galaxies & Quasars
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 oftimes 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 GalacticNucleus (AGN) that can outshine an entire galaxy full of billions of stars.This lecture reviews the observed properties of Active Galaxies, theriddle of the Quasars, and the recognition that they are powered bythe accretion of matter onto supermassive black holes. The lecture endswith some open questions in this active area of current research.Recorded 2006 February 16 in 1008 Evans Laboratory on the Columbus campusof The Ohio State University.
16 Feb 2006
Lecture 23: The Milky Way
What is the Milky Way, and what is our place within it? This lectureintroduces the Milky Way, the bright band of light that crosses thesky, and describes how we came to our present understanding of the sizeand shape of the Milky Way Galaxy, and our location in it.Recorded 2006 February 7 in 1008 Evans Laboratory on the Columbus campusof The Ohio State University.
7 Feb 2006
Lecture 14: Star Formation
How do stars form? The Sun is old and in Hydrostatic andThermal equilibrium. How did it get that way? This lecturepresents the basic steps of star formation as a progress fromcold interstellar Giant Molecular Clouds to Protostars inHydrostatic Equilibrium, and then Pre-Main Sequence evolution which ends in ignition of core Hydrogen fusion and establishingThermal Equilibrium on the Zero-Age Main Sequence.Recorded 2006 January 24 in 1008 Evans Laboratory on the Columbus campusof The Ohio State University.
24 Jan 2006
Lecture 07: Stellar Brightness
How do we quantify stellar brightness? This lecture introducesthe inverse square-law of apparent brightness, the relation betweenLuminosity and Apparent Brightness, introduces the stellar magnitude system, and discusses photometry and the how we measure apparent brightness in practice. Recorded2006 January 11 in 1008 Evans Laboratory on the Columbus campus ofThe Ohio State University.
11 Jan 2006
Lecture 38: The First Three Minutes
What was the Universe like from the earliest phases immediately afterthe Big Bang to the present day? This lecture reviews the physics ofmatter, and follows the evolution of the expanding Universe from thefirst instants after the Big Bang, when all 4 forces of nature wereunified in a single grand-unified superforce until the emergence of thevisible Universe we see around us today. Recorded 2006 March 1 in 1008Evans Laboratory on the Columbus campus of The Ohio State University.
1 Mar 2006
Lecture 39: The Fate of the Universe
What is the ultimate fate of the Universe? The ultimate fate ofthe Big Bang is either expansion to a maximum size followedby re-collapse (the Big Crunch) or eternal expansion into a cold,dark, disordered state (the Big Chill). Which of these is ourfuture depends on the current density of matter and energy in theUniverse, Omega0. This lecture examines our current knowledge ofthe matter and energy content of the Universe, which leads to thesurprising discovery that we live in a Universe thatis Flat (Omega0=1), Infinite, and Accelerating! We will end the lectureby exploring the possible fate of an infinite accelerating Universe.Recorded 2006 March 2 in 1008 Evans Laboratory on the Columbus campusof The Ohio State University.
2 Mar 2006
Lecture 22: The Cosmic Distance Problem
How do we measure distances to astronomical objects that are too faraway to use Trigonometric Parallaxes? This first lecture of Unit 4reviews geometric methods like trigonometric parallaxes, and thenintroduces the idea of Standard Candles, and how they are used todevelop methods for deriving Luminosity Distances based on the InverseSquare Law of Brightness. We will explore three luminosity-baseddistance 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 EvansLaboratory on the Columbus campus of The Ohio State University.
6 Feb 2006
Lecture 16: The Evolution of Low-Mass Stars
What happens to a low-mass star (less than 4 solar masses) whenit runs out of core Hydrogen and must leave the Main Sequence.This lecture describes the changes inside a low-mass star afterHydrogen exhaustion through the Red Giant, Horizontal Branch,Asymtotic Giant, and Planetary Nebula phases. In the end, we willsee the star's envelope and core go their separate ways, theenvelope gently puffed off into space, briefly flowering as aPlanetary Nebula, and the Carbon-Oxygen core collapsing intoa White Dwarf.Recorded 2006 January 26 in 1008 Evans Laboratory on the Columbus campusof The Ohio State University.
26 Jan 2006
Lecture 08: Stellar Masses & Radii
How do we measure the masses and radii of stars? This lecturedescribes the three basic types of binary stars, and how each are used to measure the masses of stars. Details of how to measurestellar radii are beyond the scope of this class, but we brieflydescribe the direct measurements of stellar radii.Recorded 2006 January 12 in 1008 Evans Laboratory on the Columbus campusof The Ohio State University.
12 Jan 2006