Great Courses, Great Episodes

The Great Courses offers a number of excellent courses on DVD (also streaming and audio only). Here are my favorite episodes. (Note: This is a work in progress and more entries will be added in the future.)

Course No. 153
Einstein’s Relativity and the Quantum Revolution: Modern Physics for Non-Scientists, 2nd Edition – Richard Wolfson
Lecture 8 – Uncommon Sense—Stretching Time
“Why does the simple statement of relativity—that the laws of physics are the same for all observers in uniform motion—lead directly to absurd-seeming situations that violate our commonsense notions of space and time?”
Lecture 9 – Muons and Time-Traveling Twins
“As a dramatic example of what relativity implies, you will consider a thought experiment involving a pair of twins, one of whom goes on a journey to the stars and returns to Earth younger than her sister!”
Lecture 12 – What about E=mc2 and is Everything Relative?
“Shortly after publishing his 1905 paper on special relativity, Einstein realized that his theory required a fundamental equivalence between mass and energy, which he expressed in the equation E=mc2. Among other things, this famous formula means that the energy contained in a single raisin could power a large city for an entire day.”
Lecture 16 – Into the Heart of Matter
“With this lecture, you turn from relativity to explore the universe at the smallest scales. By the early 1900s, Ernest Rutherford and colleagues showed that atoms consist of a positively charged nucleus surrounded by negatively charged electrons whirling around it. But Rutherford’s model could not explain all the observed phenomena.”
Lecture 19 – Quantum Uncertainty—Farewell to Determinism
“Quantization places severe limits on our ability to observe nature at the atomic scale because it implies that the act of observation disturbs that which is being observed. The result is Werner Heisenberg’s famous Uncertainty Principle. What exactly does this principle say, and what are the philosophical implications?”
Lecture 21 – Quantum Weirdness and Schrödinger’s Cat
“Wave-particle duality gives rise to strange phenomena, some of which are explored in Schrödinger’s famous ‘cat in the box’ example. Philosophical debate on Schrödinger’s cat still rages.”

Course No. 730
Symphonies of Beethoven – Robert Greenberg
Lecture 11 – Symphony No. 3—The “New Path”—Heroism and Self-Expression, III
“Lectures 9 through 12 focus on Symphony No. 3, the Eroica Symphony. This key work in Beethoven’s compositional revolution resulted from his crisis of going deaf. Beethoven’s struggle with his disability raised him to a new level of creativity. Symphony No. 3 parallels his heroic battle with and ultimate triumph over adversity. The symphony’s debt to Napoleon is discussed before an analysis.”
Lecture 13 – Symphony No. 4—Consolidation of the New Aesthetic, I
“Lectures 13 through 16 examine Symphony No. 4 in historical context and in its relationship to opera buffa. Symphony No. 4 is the most infrequently heard of his symphonies. We see how it represents a return to a Classical structure. Its framework is filled with iconoclastic rhythms, harmonies, and characteristic motivic developments that mark it as a product of Beethoven’s post-Eroica period.”
Lecture 23 – Symphony No. 7—The Symphony as Dance, I
Lecture 24 – Symphony No. 7—The Symphony as Dance, II
“Lectures 23 and 24 discuss Beethoven’s Symphony No. 7 with references to the historical and personal events surrounding its composition. The essence of the symphony is seen to be the power of rhythm, and originality is seen to be an important artistic goal for Beethoven.”
Lecture 31 – Symphony No. 9—The Symphony as the World, IV
“The last five lectures are devoted to Symphony No. 9, the most influential Western musical composition of the 19th century and the most influential symphony ever written. We see how this work obliterated distinctions between the instrumental symphony and dramatic vocal works such as opera. Also discussed are Beethoven’s fall from public favor in 1815, his disastrous relationship with his nephew Karl, his artistic rebirth around 1820, his late compositions, and his death in 1827.”

Course No. 759
Great Masters: Robert and Clara Schumann-Their Lives and Music – Robert Greenberg
“In Düsseldorf, Robert was inspired to write the Symphony No. 3 in E-flat Major, along with trios, sonatas, orchestral works, and pieces for chorus and voice and piano. Robert and Clara also met Johannes Brahms there; he became a lifelong friend and source of strength for Clara. In 1854, Robert attempted to drown himself in the Rhine and was taken to an asylum. He died there two years later. Clara managed to sustain the family through her concerts but was dealt even more pain by the early deaths of several of her children.”

Course No. 1257
Mysteries of Modern Physics: Time – Sean Carroll
Lecture 10 – Playing with Entropy
“Sharpen your understanding of entropy by examining different macroscopic systems and asking, which has higher entropy and which has lower entropy? Also evaluate James Clerk Maxwell’s famous thought experiment about a demon who seemingly defies the principle that entropy always increases.”
Lecture 15 – The Perception of Time
“Turn to the way humans perceive time, which can vary greatly from clock time. In particular, focus on experiments that shed light on our time sense. For example, tests show that even though we think we perceive the present moment, we actually live 80 milliseconds in the past.”
Lecture 16 – Memory and Consciousness
“Remembering the past and projecting into the future are crucial for human consciousness, as shown by cases where these faculties are impaired. Investigate what happens in the brain when we remember, exploring different kinds of memory and the phenomena of false memories and false forgetting.”
Lecture 20 – Black Hole Entropy
“Stephen Hawking showed that black holes emit radiation and therefore have entropy. Since the entropy in the universe today is overwhelmingly in the form of black holes and there were no black holes in the early universe, entropy must have been much lower in the deep past.”
Lecture 21 – Evolution of the Universe
“Follow the history of the universe from just after the big bang to the far future, when the universe will consist of virtually empty space at maximum entropy. Learn what is well founded and what is less certain about this picture of a universe winding down.”

Course No. 1280
Physics and Our Universe: How It All Works – Richard Wolfson
Lecture 1 – The Fundamental Science

“Take a quick trip from the subatomic to the galactic realm as an introduction to physics, the science that explains physical reality at all scales. Professor Wolfson shows how physics is the fundamental science that underlies all the natural sciences. He also describes phenomena that are still beyond its explanatory power.”
Special Note: This entire series is outstanding! I will eventually be adding many of the episodes of this course as I rewatch them. (I watched this series before I began keeping track of “best” episodes.)

Course No. 1360
Introduction to Astrophysics – Joshua Winn
Lecture 5 – Newton’s Hardest Problem
“Continue your exploration of motion by discovering the law of gravity just as Newton might have—by analyzing Kepler’s laws with the aid of calculus (which Newton invented for the purpose). Look at a graphical method for understanding orbits, and consider the conservation laws of angular momentum and energy in light of Emmy Noether’s theory that links conservation laws and symmetry.”
Lecture 10 – Optical Telescopes
“Consider the problem of gleaning information from the severely limited number of optical photons originating from astronomical sources. Our eyes can only do it so well, and telescopes have several major advantages: increased light-gathering power, greater sensitivity of telescopic cameras and sensors such as charge-coupled devices (CCDs), and enhanced angular and spectral resolution.”
Lecture 11 – Radio and X-Ray Telescopes
“Non-visible wavelengths compose by far the largest part of the electromagnetic spectrum. Even so, many astronomers assumed there was nothing to see in these bands. The invention of radio and X-ray telescopes proved them spectacularly wrong. Examine the challenges of detecting and focusing radio and X-ray light, and the dazzling astronomical phenomena that radiate in these wavelengths.”
Lecture 12 – The Message in a Spectrum
“Starting with the spectrum of sunlight, notice that thin dark lines are present at certain wavelengths. These absorption lines reveal the composition and temperature of the Sun’s outer atmosphere, and similar lines characterize other stars. More diffuse phenomena such as nebulae produce bright emission lines against a dark spectrum. Probe the quantum and thermodynamic events implied by these clues.”
Lecture 13 – The Properties of Stars
“Take stock of the wide range of stellar luminosities, temperatures, masses, and radii using spectra and other data. In the process, construct the celebrated Hertzsprung–Russell diagram, with its main sequence of stars in the prime of life, including the Sun. Note that two out of three stars have companions. Investigate the orbital dynamics of these binary systems.”
Lecture 15 – Why Stars Shine
“Get a crash course in nuclear physics as you explore what makes stars shine. Zero in on the Sun, working out the mass it has consumed through nuclear fusion during its 4.5-billion-year history. While it’s natural to picture the Sun as a giant furnace of nuclear bombs going off non-stop, calculations show it’s more like a collection of toasters; the Sun is luminous simply because it’s so big.”
Lecture 16 – Simple Stellar Models
“Learn how stars work by delving into stellar structure, using the Sun as a model. Relying on several physical principles and sticking to order-of-magnitude calculations, determine the pressure and temperature at the center of the Sun, and the time it takes for energy generated in the interior to reach the surface, which amounts to thousands of years. Apply your conclusions to other stars.”
Lecture 17 – White Dwarfs
“Discover the fate of solar mass stars after they exhaust their nuclear fuel. The galaxies are teeming with these dim “white dwarfs” that pack the mass of the Sun into a sphere roughly the size of Earth. Venture into quantum theory to understand what keeps these exotic stars from collapsing into black holes, and learn about the Chandrasekhar limit, which determines a white dwarf’s maximum mass.”
Lecture 18 – When Stars Grow Old
“Trace stellar evolution from two points of view. First, dive into a protostar and witness events unfold as the star begins to contract and fuse hydrogen. Exhausting that, it fuses heavier elements and eventually collapses into a white dwarf—or something even denser. Next, view this story from the outside, seeing how stellar evolution looks to observers studying stars with telescopes.”
Lecture 19 – Supernovas and Neutron Stars
“Look inside a star that weighs several solar masses to chart its demise after fusing all possible nuclear fuel. Such stars end in a gigantic explosion called a supernova, blowing off outer material and producing a super-compact neutron star, a billion times denser than a white dwarf. Study the rapid spin of neutron stars and the energy they send beaming across the cosmos.”
Lecture 20 – Gravitational Waves
“Investigate the physics of gravitational waves, a phenomenon predicted by Einstein and long thought to be undetectable. It took one of the most violent events in the universe—colliding black holes—to generate gravitational waves that could be picked up by an experiment called LIGO on Earth, a billion light years away. This remarkable achievement won LIGO scientists the 2017 Nobel Prize in Physics.”

Course No. 1456
Discrete Mathematics – Arthur T. Benjamin
Lecture 8 – Linear Recurrences and Fibonacci Numbers
“Investigate some interesting properties of Fibonacci numbers, which are defined using the concept of linear recurrence. In the 13th century, the Italian mathematician Leonardo of Pisa, called Fibonacci, used this sequence to solve a problem of idealized reproduction in rabbits.”
Lecture 15 – Open Secrets—Public Key Cryptography
“The idea behind public key cryptography sounds impossible: The key for encoding a secret message is publicized for all to know, yet only the recipient can reverse the procedure. Learn how this approach, widely used over the Internet, relies on Euler’s theorem in number theory.”
Lecture 16 – The Birth of Graph Theory
“This lecture introduces the last major section of the course, graph theory, covering the basic definitions, notations, and theorems. The first theorem of graph theory is yet another contribution by Euler, and you see how it applies to the popular puzzle of drawing a given shape without lifting the pencil or retracing any edge.”
Lecture 18 – Social Networks and Stable Marriages
“Apply graph theory to social networks, investigating such issues as the handshake theorem, Ramsey’s theorem, and the stable marriage theorem, which proves that in any equal collection of eligible men and women, at least one pairing exists for each person so that no extramarital affairs will take place.”
Lecture 20 – Weighted Graphs and Minimum Spanning Trees
“When you call someone on a cell phone, you can think of yourself as a leaf on a giant ‘tree’—a connected graph with no cycles. Trees have a very simple yet powerful structure that make them useful for organizing all sorts of information.”
Lecture 22 – Coloring Graphs and Maps
“According to the four-color theorem, any map can be colored in such a way that no adjacent regions are assigned the same color and, at most, four colors suffice. Learn how this problem went unsolved for centuries and has only been proved recently with computer assistance.”

Course No. 1495
Introduction to Number Theory – Edward B. Burger
Lecture 12 – The RSA Encryption Scheme
“We continue our consideration of cryptography and examine how Fermat’s 350-year-old theorem about primes applies to the modern technological world, as seen in modern banking and credit card encryption.”
Lecture 22 – Writing Real Numbers as Continued Fractions
“Real numbers are often expressed as endless decimals. Here we study an algorithm for writing real numbers as an intriguing repeated fraction-within-a-fraction expansion. Along the way, we encounter new insights about the hidden structure within the real numbers.”
Lecture 24 – A Journey’s End and the Journey Ahead
“In this final lecture, we take a step back to view the entire panorama of number theory and celebrate some of the synergistic moments when seemingly unrelated ideas came together to tell a unified story of number.”

Course No. 1802
The Search for Exoplanets: What Astronomers Know – Joshua Winn
Lecture 4 – Pioneers of Planet Searching

“Chart the history of exoplanet hunting – from a famous false signal in the 1960s, through ambiguous discoveries in the 1980s, to the big breakthrough in the 1990s, when dozens of exoplanets turned up. Astronomers were stunned to find planets unlike anything in the solar system.”
Special Note: This entire series is outstanding! I will eventually be adding most of the episodes of this course as I rewatch them. (I watched this series before I began keeping track of “best” episodes.)

Course No. 1830
Cosmology: The History and Nature of Our Universe – Mark Whittle
Lecture 3 – Overall Cosmic Properties

“The universe is lumpy at the scale of galaxies and galaxy clusters. But at larger scales it seems to be smooth and similar in all directions. This property of homogeneity and isotropy is called the cosmological principle.”
Lecture 4 – The Stuff of the Universe
“The most familiar constituents of the universe are atomic matter and light. Neutrinos make up another component. But by far the bulk of the universe—96%—is dark energy and dark matter. The relative amounts of these constituents have changed as the universe has expanded.”
Lecture 6 – Measuring Distances
“Astronomers use a ‘distance ladder’ of overlapping techniques to determine distances in the universe. Triangulation works for nearby stars. For progressively farther objects, observers use pulsating stars, the rotation of galaxies, and a special class of supernova explosions.”
Lecture 8 – Distances, Appearances, and Horizons
“Defining distances in cosmology is tricky, since an object’s distance continually increases with cosmic expansion. There are three important distances to consider: the emission distance, when the light set out; the current distance, when the light arrives; and the distance the light has traveled.”
Lecture 10 – Cosmic Geometry – Triangles in the Sky
“Einstein’s theory of gravity suggests that space could be positively or negatively curved, so that giant billion-light-year triangles might have angles that don’t add up to 180°. This lecture discusses the success at measuring the curvature of the universe in 1998.”
Lecture 11 – Cosmic Expansion – Keeping Track of Energy
“Has the universe’s rate of expansion always been the same? You answer this question by applying Newton’s law of gravity to an expanding sphere of matter, finding that the expansion was faster in the past and slows down over time.”
Lecture 12 – Cosmic Acceleration – Falling Outward
“You investigate why the three great eras of cosmic history—radiation, matter, and dark energy—have three characteristic kinds of expansion. These are rapid deceleration, modest deceleration, and exponential acceleration. The last is propelled by dark energy, which makes the universe fall outward.”
Lecture 13 – The Cosmic Microwave Background
“By looking sufficiently far away, and hence back in time, we can witness the ‘flash’ from the big bang itself. This arrives from all directions as a feeble glow of microwave radiation called the cosmic microwave background (CMB), discovered by chance in 1964.”
Lecture 22 – The Galaxy Web – A Relic of Primordial Sound
“A simulated intergalactic trip shows you the three-dimensional distribution of galaxies in our region of the universe. On the largest scale, galaxies form a weblike pattern that matches the peaks and troughs of the primordial sound in the early universe.”
Lecture 24 – Understanding Element Abundances
“The theory of atom genesis in the interiors of stars is confirmed by the proportions of each element throughout the cosmos. The relative abundances hardly vary from place to place, so that gold isn’t rare just on earth, it’s rare everywhere.”
Lecture 27 – Physics at Ultrahigh Temperatures
“This lecture begins your investigation of the universe during its first second, which is an immense tract of time in nature. To understand what happened, you need to know how nature behaves at ultrahigh energy and density. Fortunately, the physics is much simpler than you might think.”
Lecture 29 – Back to the GUT – Matter and Forces Emerge
“You venture into the bizarre world of the opening nanosecond. There are two primary themes: the birth of matter and the birth of forces. Near one nanosecond, the universe was filled with a dense broth of the most elementary particles. As temperatures dropped, particles began to form.”
Lecture 30 – Puzzling Problems Remain
“Although the standard big bang theory was amazingly successful, it couldn’t explain several fundamental properties of the universe: Its geometry is Euclidean, it’s smooth on the largest scales, and it was born slightly lumpy on smaller scales. The theory of cosmic inflation offers a comprehensive solution.”
Lecture 31 – Inflation Provides the Solution
“This lecture shows how the early universe might enter a brief phase of exponentially accelerating expansion, or inflation, providing a mechanism to launch the standard hot big bang universe. This picture also solves the flatness, horizon, and monopole problems that plagued the standard big-bang theory.”
Lecture 33 – Inflation’s Stunning Creativity
“All the matter and energy in stars and galaxies is exactly balanced by all the negative energy stored in the gravitational fields between the galaxies. Inflation is the mechanism that takes nothing and makes a universe—not just our universe, but potentially many.”
Lecture 34 – Fine Tuning and Anthropic Arguments
“Why does the universe have the properties it does and not some different set of laws? One approach is to see the laws as inevitable if life ever evolves to ask such questions. This position is called the anthropic argument, and its validity is hotly debated.”

Course No. 1866
The Remarkable Science of Ancient Astronomy – Bradley E. Schaefer
Lecture 10 – Origins of Western Constellations
“The human propensity for pattern recognition and storytelling has led every culture to invent constellations. Trace the birth of the star groups known in the West, many of which originated in ancient Mesopotamia. At least one constellation is almost certainly more than 14,000 years old and may be humanity’s oldest surviving creative work.”

Course No. 1878
Radio Astronomy: Observing the Invisible Universe – Felix J. Lockman
Lecture 5 – Radio Telescopes and How They Work
“Radio telescopes are so large because radio waves contain such a small amount of energy. For example, the signal from a standard cell phone measured one kilometer away is five million billion times stronger than the radio signals received from a bright quasar. Learn how each of these fascinating instruments is designed to meet a specific scientific goal—accounting for their wide variation in form and size.”
Lecture 7 – Tour of the Green Bank Observatory
“The Green Bank Observatory is located within the 13,000-acre National Radio Quiet Zone straddling the border of Virginia and West Virginia. Come tour this fascinating facility where astronomers discovered radiation belts around Jupiter, the black hole at the center of our galaxy, and the first known interstellar organic molecule, and began the search for extra-terrestrial life.”
Lecture 8 – Tour of the Green Bank Telescope
“At 17 million pounds, and with more than 2,000 surface panels that can be repositioned in real time, this telescope is one of the largest moveable, land-based objects ever built. The dish could contain two side-by-side football fields, but when its panels are brought into focus, the surface has errors no larger than the thickness of a business card. Welcome to this rare insider’s view.”
Lecture 9 – Hydrogen and the Structure of Galaxies
“Using the laws of physics and electromagnetic radiation, astronomers can ‘weigh’ a galaxy by studying the distribution of its rotating hydrogen. But when they do this, it soon becomes clear something is very wrong: A huge proportion of the galaxy’s mass has simply gone missing. Welcome to the topsy-turvy world of dark matter, which we now believe accounts for a whopping 90 percent of our own Milky Way.”
Lecture 10 – Pulsars: Clocks in Space
“In the mid-1960s, astronomers discovered signals with predictable periodicity but no known source. In case these signals indicated extraterrestrial life, they were initially labeled LGM, Little Green Men. But research revealed the source of the pulsing radiation to be neutron stars. Learn how a star with a diameter of only a few kilometers and a mass similar to that of our Sun can spin around hundreds of times per second.”
Lecture 11 – Pulsars and Gravity
“A pulsar’s spin begins with its birth in a supernova and can be altered by transfer of mass from a companion star. Learn how pulsars, these precise interstellar clocks, are used to confirm Einstein’s prediction of gravitational waves by observations of a double-neutron-star system, and how we pull the pulsar signal out of the noise.”
Lecture 12 – Pulsars and the 300-Foot Telescope
“Humans constantly use radio transmission these days, for everything from military communications to garage-door openers. How can scientists determine which signals come from Earth and which come from space? Learn how the 300-foot telescope, located in two radio quiet zones, was built quickly and cheaply. It ended up studying pulsars and hydrogen in distant galaxies, and made the case for dark matter.”
Lecture 16 – Radio Stars and Early Interferometers
“When radio astronomers discovered a sky full of small radio sources of unknown origin, they built telescopes using multiple antennas to try to understand them. Learn how and why interferometers were developed and how they have helped astronomers study quasars—those massively bright, star-like objects that scientists now know only occur in galaxies whose gas is falling into a supermassive black hole.”
Lecture 18 – Active Galactic Nuclei and the VLA
“The need for a new generation of radio interferometers to untangle extragalactic radio sources led to the development of the Very Large Array (VLA) in New Mexico. With its twenty-seven radio antennas in a Y-shaped configuration, it gives both high sensitivity and high angular resolution. The VLA provided a deeper and clearer look at galaxies than ever before, and the results were astonishing.”
Lecture 19 – A Telescope as Big as the Earth
“Learn how astronomers use very-long-baseline interferometry (VLBI) with telescopes thousands of miles apart to essentially create a radio telescope as big as the Earth. With VLBI, scientists not only look deep into galactic centers, study cosmic radio sources, and weigh black holes, but also more accurately tell time, study plate tectonics, and more—right here on planet Earth.”
Lecture 20 – Galaxies and Their Gas
“In visible light, scientists had described galaxies as ‘island universes’. But since the advent of radio astronomy, we’ve seen galaxies connected by streams of neutral hydrogen, interacting with and ripping the gases from each other. Now astronomers have come to understand that these strong environmental interactions are not a secondary feature—they are key to a galaxy’s basic structure and appearance.”
Lecture 21 – Interstellar Molecular Clouds
“In the late 1960s, interstellar ammonia and water vapor were detected. Soon came formaldehyde, carbon monoxide, and the discovery of giant molecular clouds where we now know stars and planets are formed. With improvements in radio astronomy technology, today’s scientists can watch the process of star formation in other systems. The initial results are stunning.”
Lecture 22 – Star Formation and ALMA
“With an array of 66 radio antennas located in the high Chilean desert above much of the earth’s atmosphere, the Atacama Large Millimeter/submillimeter Array (ALMA) is a radio telescope tuned to the higher frequencies of radio waves. Designed to examine some of the most distant and ancient galaxies ever seen, ALMA has not only revealed new stars in the making, but planetary systems as well.”
Lecture 23 – Interstellar Chemistry and Life
“Interstellar clouds favor formation of carbon-based molecules over any other kind—not at all what statistical models predicted. In fact, interstellar clouds contain a profusion of chemicals similar to those that occur naturally on Earth. If planets are formed in this rich soup of organic molecules, is it possible life does not have to start from scratch on each planet?”
Lecture 24 – The Future of Radio Astronomy
“Learn about the newest radio telescopes and the exhilarating questions they plan to address: Did life begin in space? What is dark matter? And a new question that has just arisen in the past few years: What are fast radio bursts? No matter how powerful these new telescopes are, radio astronomers will continue pushing the limits to tell us more and more about the universe that is our home.”

Course No. 1884
Experiencing Hubble: Understanding the Greatest Images of the Universe – David M. Meyer
Lecture 5 – The Cat’s Eye Nebula – A Stellar Demise
“Turning from star birth to star death, get a preview of the sun’s distant future by examining the Cat’s Eye Nebula. Such planetary nebulae (which have nothing to do with planets) are the exposed debris of dying stars and are among the most beautiful objects in the Hubble gallery.”
Lecture 7 – The Sombrero Galaxy – An Island Universe
“In the 1920s, astronomer Edwin Hubble discovered the true nature of galaxies as ‘island universes’. Some 80 years later, the telescope named in his honor has made thousands of breathtaking pictures of galaxies. Focus on one in particular—an edge-on view of the striking Sombrero galaxy.”
Lecture 8 – Hubble’s View of Galaxies Near and Far
“Hubble’s image of the nearby galaxy NGC 3370 includes many faint galaxies in the background, exemplifying the telescope’s mission to establish an accurate distance scale to galaxies near and far—along with the related expansion rate of the universe. Discover how Hubble’s success has led to the concept of dark energy.”
Lecture 10 – Abell 2218 – A Massive Gravitational Lens
“One of the consequences of Einstein’s general theory of relativity is evident in Hubble’s picture of the galaxy cluster Abell 2218. Investigate the physics of this phenomenon, called gravitational lensing, and discover how Hubble has used it to study extremely distant galaxies as well as dark matter.”

Course No. 3130
Origin of Civilization – Scott MacEachern
Lecture 36 – Great Zimbabwe and Its Successors
“Few archaeological sites have been subjected to the degree of abuse and misrepresentation sustained by Great Zimbabwe in southeastern Africa. Nevertheless, this lecture unpacks the intriguing history of this urban center and the insights it can provide into the development of the elite.”

Course No. 3900
Ancient Civilizations of North America – Edwin Barnhart
Lecture 12 – The Wider Mississippian World
“After the fall of Cahokia, witness how Mississippian civilization flourished across eastern North America with tens of thousands of pyramid-building communities and a population in the millions. Look at the ways they were connected through their commonly held belief in a three-tiered world, as reflected in their artwork. Major sites like Spiro, Moundville, and Etowah all faded out just around 100 years before European contact, obscuring our understanding.”
Lecture 13 – De Soto Versus the Mississippians
“In 1539, Hernando de Soto of Spain landed seven ships with 600 men and hundreds of animals in present-day Florida. Follow his fruitless search for another Inca or Aztec Empire, as he instead encounters hundreds of Mississippian cities through which he led a three-year reign of terror across the land-looting, raping, disfiguring, murdering, and enslaving native peoples by the thousands.”
Lecture 19 – The Chaco Phenomenon
“Chaco Canyon contains the most sophisticated architecture ever built in ancient North America—14 Great Houses, four Great Kivas, hundreds of smaller settlements, an extensive road system, and a massive trade network. But who led these great building projects? And why do we find so little evidence of human habitation in what seems to be a major center of culture? Answer these questions and more.”
Lecture 24 – The Iroquois and Algonquians before Contact
“At the time of European contact, two main groups existed in the northeast—the hunter-gatherer Algonquian and the agrarian Iroquois. Delve into how the Iroquois created the first North American democracy as a solution to their increasing internal conflicts. Today, we know much of the U.S. Constitution is modeled on the Iroquois’ “Great League of Peace” and its 117 articles of confederation, as formally acknowledged by the U.S. in 1988.”

Course No. 4215
An Introduction to Formal Logic – Steven Gimbel
Lecture 8 – Induction in Polls and Science
“Probe two activities that could not exist without induction: polling and scientific reasoning. Neither provides absolute proof in its field of analysis, but if faults such as those in Lecture 7 are avoided, the conclusions can be impressively reliable.”

Course No. 7210
The Symphony – Robert Greenberg
Lecture 24 – Dmitri Shostakovich and His Tenth Symphony

“Dmitri Shostakovich was used and abused by the Soviet powers during much of his life. Somehow, he survived—even as his Tenth Symphony made dangerously implicit criticisms of the Soviet government.”

Course No. 7261
Understanding the Fundamentals of Music – Robert Greenberg
Lecture 9 – Intervals and Tunings

“Resuming our focus on pitch, we will turn once more to Pythagoras, and his investigation into what is now known as the overtone series. This paves the way for an examination of intervals, the evolution of tuning systems, and an introduction to the major pitch collections.”

Course No. 7270
The Concerto – Robert Greenberg
Lecture 13 – Tchaikovsky
“Excoriated by colleagues and critics alike, Tchaikovsky’s concerti ultimately triumphed to become cornerstones of the repertoire. This lecture explores his Piano Concerto no. 1 in B flat Minor, op. 23; Piano Concerto no. 2 in G Major, op. 44; and Violin Concerto in D Major, op. 35, arguably his single greatest work and one of the greatest concerti of the 19th century.”
Lecture 14 – Brahms and the Symphonic Concerto
“Johannes Brahms’s compositional style is a synthesis of the clear and concise musical forms and genres of the Classical and Baroque eras, and the melodic, harmonic, and expressive palette of the Romantic era in which he lived. This lecture examines in depth his monumental Piano Concerto no. 2 in B flat Major, op. 83.”

Course No. 30110
England, the 1960s, and the Triumph of the Beatles – Michael Shelden
Lecture 8 – The Englishness of A Hard Day’s Night
“In summer 1964, the cinematic Beatles vehicle A Hard Day’s Night broke almost every rule in Hollywood at the time. Professor Shelden reveals what lies underneath the film’s surface charm and musical numbers: an overall attitude of irreverence and defiance in the face of authority, and a challenge for audiences to think for themselves.”
Lecture 12 – Hello, Goodbye: The End of the 1960s
“In their last years together, all four of the Beatles seemed headed in new directions as they grew up—and apart. Nevertheless, witness how these final years brought a range of sounds, including protest songs, mystic melodies, anthems of friendship, and an iconic double album called simply, The Beatles, but better known as the ‘White Album.'”

Notes from AAS 234

I attended the 234th meeting of the American Astronomical Society (AAS), held in St. Louis, Missouri, June 9-13, 2019. Here are some highlights from that meeting.

Day 1 – Monday, June 10, 2019

Research Notes of the AAS is a non-peer-reviewed, indexed and secure record of works in progress, comments and clarifications, null results, or timely reports of observations in astronomy and astrophysics. RNAAS.

The Bulletin of the American Astronomical Society is the publication for science meeting abstracts, obituaries, commentary articles about the discipline, and white papers of broad interest to our community. BAAS.

We still have many unanswered questions about galaxy formation. The rate of star formation in galaxies and central black hole accretion activity was highest between 10 and 11 billion years ago. This corresponds to redshift z around 2 to 3, referred to as “cosmic high noon”. This is the ideal epoch for us to answer our questions about galaxy formation. Near-infrared spectroscopy is important to the study of galaxies during this epoch, and we are quite limited in what we can do from terrestrial observatories. Space based telescopes are needed, and the James Webb Space Telescope (JWST) will be key.

Galaxies are not closed boxes. We need to understand how inflows and outflows affect their evolution (“galactic metabolism”).

There are five international space treaties, with the Outer Space Treaty of 1967 being the first and most important. The United States has signed four of the five treaties. The Moon Agreement of 1979 which states that no entity can own any part of the Moon does not include the United States as one of the signatories.

U.S. Code 51303, adopted in 2015, identifies asteroid resource and space resource rights, and states that “A United States citizen engaged in commercial recovery of an asteroid resource or a space resource under this chapter shall be entitled to any asteroid resource or space resource obtained, including to possess, own, transport, use, and sell the asteroid resource or space resource obtained in accordance with applicable law, including the international obligations of the United States.”

So, unfortunately, U.S. law does allow a commercial entity to own an asteroid, but you have to get there first before you can claim it. The large metallic asteroid 16 Psyche is highly valuable and will probably be owned by some corporation in the not-too-distant future.

Space law often relies upon maritime law as a model.

Astronomer Vayu Gokhale from Truman State University gave an interesting iPoster Plus presentation on how he and his students are operating three automated and continuous zenithal sky brightness measurement stations using narrow-field Sky Quality Meters (SQMs) from Unihedron. Even measurements when it is cloudy are of value, as clouds reflect light pollution back towards the ground. Adding cloud type and height would allow us to make better use of cloudy-night sky brightness measurements. In a light-polluted area, the darkest place is the zenith, and clouds make the sky brighter. In an un-light-polluted area, the darkest place is the horizon, and clouds make the sky darker.

A number of precision radial velocity instruments for exoplanet discovery and characterization will begin operations soon or are already in operation: NEID, HARPS, ESPRESSO, EXPRES, and iLocater, to name a few.

Dark matter: clumps together under gravity, does not emit, reflect, or absorb electromagnetic radiation, and does not interact with normal matter in any way that causes the normal matter to emit, reflect, or absorb electromagnetic radiation. The ratio between dark matter and normal (baryonic matter) in our universe is 5.36 ± 0.05 (Planck 2018).

What is dark matter? It could be a new particle. If so, can we detect its non-gravitational interactions? It could be macroscopic objects, perhaps primordial black holes. Or, it could be a mixture of both. Another possibility is that a modification to the laws of gravitation will be needed to mimic the effects of dark matter.

How “dark” is dark matter? Does it interact at all (besides gravitationally)? Can dark matter annihilate or decay? Even if dark matter started hot, it cools down rapidly as the universe expands.

Primordial black holes could have masses ranging anywhere between 10-16 and 1010 solar masses. LIGO is possibility sensitive to colliding primordial black holes with masses in the range of a few to a few hundred solar masses. Primordial black holes are a fascinating dark matter candidate, with broad phenomenology.

The Cosmic Microwave Background (CMB) is a nearly perfect blackbody with distortions < 1 part in 10,000. What this tells us is that nothing dramatically heated or cooled photons after 2 months after the Big Bang. Anisotropies are variances in the CMB temperature, and the angular power spectrum is variance of CMB temperature as a function of angular scale. CMB anisotropies are very sensitive to the ionization history of the universe. How the universe recombined plays a key role in CMB anisotropies.

Hydrogen: not such a simple atom.

The CMB is polarized. The polarization is caused by Mie scattering of photons.

At the NASA Town Hall, we learned about current and future missions: TESS, SPHEREx, HabEx, LUVOIR, Lynx, Origins Space Telescope (OST).

The highest image rate of standard CCD and CMOS video cameras for asteroid occultation work is 30 frames (60 fields) per second, providing time resolution of 0.017 seconds per field. Adaptive optics and autoguider imaging devices often have a higher sampling rate, and such a camera could perhaps be easily modified to be used for occultation work. A time-inserter would need to be added to the camera (either on-board or GPS-based), and improvements in quantum efficiency (because of the shorter exposures) would benefit from newer imaging technologies such as a Geiger-mode avalanche photodiode (APD); or the Single-photon avalanche detector (SPAD), which are frequently used in chemistry.

Gregory Simonian, graduate student at Ohio State, presented “Double Trouble: Biases Caused by Binaries in Large Stellar Rotation datasets”. The Kepler data yielded 34,030 rotation periods through starspot variability. However, the rapid rotators are mostly binaries. In the Kepler dataset, many rapid rotators have a spin period of the stars equal to the orbital period of the binary. These eclipsing binaries, also known as photometric binaries because they are detected through changes in brightness during eclipses and transits, need to be treated separately in stellar rotation datasets.

Granulation was discovered by William Herschel in 1801 and are vertical flows in the solar photosphere on the order of 1000 m/s, and 1000 km horizontal scale. Supergranulation (Hart 1954, Leighton et al. 1962) are horizontal motions in the photosphere of 300 to 500 m/s with a horizontal scale on the order of 30,000 km.

The amplitude of oscillations in red giants increase dramatically with age.

We’ve never observed the helium flash event in a red giant star, though models predict that it must occur. It is very brief and would be difficult to detect observationally.

Brad Schaefer, Professor Emeritus at Louisiana State University, gave a talk on “Predictions for Upcoming Recurrent Nova Eruptions”. Typically, recurrent novae have about a 30% variation in eruptive timescales, so predicting the next eruption is not trivial. Due to the solar gap (when the object is too close to the Sun to observe on or near the Earth), we are obviously missing some eruptions. However, orbital period changes (O-C curve) can tell us about an eruption we missed. U Sco and T CrB are well-known examples of recurrent novae. Better monitoring of recurrent novae is needed during the pre-eruption plateau. Monitoring in the blue band is important for prediction.

I had the good fortune to talk with Brad on several occasions during the conference, and found him to be enthusiastic, knowledgeable, and engaging. Perhaps you have seen The Remarkable Science of Ancient Astronomy (The Great Courses), and he is just as articulate and energetic in real life. Among other things, we discussed how the internet is filled with misinformation, and even after an idea has been convincingly debunked, the misinformation continues to survive and multiply in cyberspace. This is a huge problem in the field of archaeoastronomy and, indeed, all fields of study. People tend to believe what they want to believe, never mind the facts.

Astrobites is a daily astrophysical-literature blog written by graduate students in astronomy around the world. The goal of Astrobites is to present one interesting paper from astro-ph per day in a brief format accessible to its target audience: undergraduate students in the physical sciences who are interested in active research.

Helioseismology can be done both from space (all) and the ground (some). Active regions on the far side of the Sun can be detected with helioseismology.

All HMI (Helioseismic and Magnetic Imager) data from the Solar Dynamics Observatory is available online.

A good approach to studying solar data is to subtract the average differential rotation at each point/region on the Sun and look at the residuals.

The Wilcox Solar Observatory has been making sun-as-a-star mean magnetic field measurements since 1975.

It is possible to infer electric currents on the Sun, but this is much more difficult than measuring magnetic fields.

Future directions in solar studies: moving from zonal averages to localized regions in our modeling, and the ability through future space missions to continuously monitor the entire surface of the Sun at every moment.

Systematic errors are nearly always larger than statistical uncertainty.

Day 2 – Tuesday, June 11, 2019

It is probably not hyperbole to state that every star in our galaxy has planets. About 1/5 of G-type stars have terrestrial planets within the habitable zone. Life is widespread throughout the universe.

Gas-grain interaction is at the core of interstellar chemistry. Interstellar ices, charged ices, surface chemistry – there is more time for interactions to occur on a dust grain than in a gas. Grain collisions are important, too.

Hot cores are transient regions surrounding massive protostars very early in their evolution. Similar regions are identified around low-mass protostars and are called corinos.

Methanol (CH3OH) is key to making simple organic molecules (SOM). Evaporating ice molecules drive rich chemistry. Dust plays a key role in the chemistry and in transporting material from the interstellar medium (ISM) to planetary systems.

The Rosetta mission detected amino acids on comet 67P/Churyumov–Gerasimenko.

JUICE (JUpiter ICy moons Explorer) is an ESA mission scheduled to launch in 2022, will enter orbit around Jupiter in October 2029 and Ganymede in 2032. It will study Europa, Ganymede, and Callisto in great detail.

The gravitational wave event GW170817 (two infalling and colliding neutron stars) was also detected as a gamma-ray burst (GRB) by the Fermi gamma-ray space telescope, which has a gamma-ray burst detector that at all times monitors the 60% of the sky that is not blocked by the Earth.

The time interval between the GW and GRB can range between tens of milliseconds up to 10 seconds.

The Milky Way galaxy circumnuclear disk is best seen at infrared wavelengths around 50 microns. Linear polarization tells us the direction of rotation. The star cluster near the MW center energizes and illuminates gas structures. Gravity dominates in this region. The role of magnetic fields in this region has been a mystery.

Pitch angle – how tightly wound the spiral arms are in a spiral galaxy.

Are spiral arms transient or long lived? They are probably long lived. There may be different mechanisms of spiral arm formation in grand design spirals compared with other types of spiral galaxies.

In studying spiral galaxies, we often deproject to face-on orientation.

The co-rotation radius is the distance from the center of a spiral galaxy beyond which the stars orbit slower than the spiral arms. Inside this radius, the stars move faster than the spiral arms.

The Sun is located near the corotation circle of the Milky Way.

The origins of supermassive black holes (SMBH) at the centers of galaxies are unclear. Were they seeded from large gas clouds, or were they built up from smaller black holes?

The black holes at the centers of spiral galaxies tend to be more massive when the spiral arm winding is tight, and less massive when the spiral arm winding is loose.

Spiral Graph is in review as a Zooniverse project and has not yet launched. Citizen scientists will trace the spiral arms of 6,000 deprojected spiral galaxies, and 15 traces will be needed for each galaxy. Spiral arm tracings will provide astronomers with intermediate mass black hole candidate galaxies.

Barred spiral galaxies are very common. 66% to 75% of spiral galaxies show evidence of a bar at near-infrared wavelengths.

Magnetic fields in the inner regions of spiral galaxies are scrambling radio emissions to some extent, but radio astronomers have ways to deal with this.

For me, the plenary lecture given by Suvrath Mahadevan, Pennsylvania State University, was the first truly outstanding presentation. His topic was “The Tools of Precision Measurement in Exoplanet Discovery: Peeking Under the Hood of the Instruments”. His discussion of the advance in radial velocity instrumentation was revelatory to me, as his starting point was Roger F. Griffin’s radial velocity spectrometer we used at Iowa State University in the 1970s and 1980s, giving us a precision of about 1 km/s. My, we have come a long way since then!

To discover our Earth from another star system in the ecliptic plane would require detecting an 8.9 cm/s velocity shift in the Sun’s motion over the course of a year.

Precision radial velocity measurement requires we look at the displacement of thousands of spectral lines using high resolution spectroscopy.

The two main techniques are 1) Simultaneous reference and 2) Self reference (iodine cell). Also, externally dispersed interferometry and heterodyne spectroscopy can be used.

Griffin 1967 ~ km/s → CORAVEL 1979 ~300 m/s → CORALIE/ELODIE 1990 ~ 5-10 m/s → HARPS 2000 ~ 1 m/s → ESPRESSO/VLT, EXPRES/DCT, NEID/KPNO, HPF/HET.

We cannot build instruments that are stable over time at 10 cm/s resolution or less.

You can track the relative change in velocity much better than absolute velocity because of the “noise” generated by stellar internal motions.

Measuring the radial velocity at red or infrared wavelengths is best for M dwarfs, and cooler stars.

High radial velocity precision will require long-term observations, and a better understanding of and mitigation for stellar activity. Many things need to be considered: telescope, atmosphere, barycentric correction (chromatic effects can lead to 1/2 m/s error), fibers, modal noise, instrument decoupled from the telescope, calibrators, optics, stability, pipeline, etc. Interdisciplinary expertise is required.

NEID will measure wavelengths of 380 – 930 nm, and have a spectral resolution of R ~ 90,000.

Pushing towards 10 cm/s requires sub-milli-Kelvin instrument stability high-quality vacuum chambers, octagonal fibers, scrambling, and excellent guiding of the stellar image on the fiber to better than 0.05 arcseconds.

Precision radial velocity instruments such as NEID and HPF weigh two tons, so at present they can only be used with ground-based telescopes.

Charge Transfer Efficiency (CTE): need CCDs with CTE > 0.999999. Other CCD issues that don’t flat field out accurately: CCD stitch boundaries, cross hatching in NIR detectors, crystalline defects, sub-pixel quantum efficiency differences. Even the act of reading out the detector introduces a noise source.

10 cm/s is within reach from a purely instrumental perspective, but almost everything has to be just right. But we need to understand stellar activity better: granulation, supergranulation, flares, oscillations, etc. We may not be able to isolate these components of stellar activity, but we will certainly learn a lot in the process.

1s time resolution is required to properly apply barycentric corrections.

NASA’s Universe of Learning : Connecting Learners to the Subject-Matter Experts of NASA Astrophysics: https://www.universe-of-learning.org/

The OpenStax Astronomy Text: https://openstax.org/details/astronomy

Andrew Fraknoi gave an update on the OpenStax Astronomy text.

• about 70 people have been involved in its development and vetting
• each chapter includes collaborative group activities
• math examples are in separate boxes
• it is estimated that 500+ institutions have adopted this online and free introductory astronomy textbook, and ~200,000 students have used it, including ~30,000 amateur astronomers
• multiple choice question bank for registered instructors
• short videos with each chapter
• available to everyone

Open Educational Resources (OER): https://www.oercommons.org/

International Lunar Observatory Association (ILOA); http://www.spaceagepub.com/

The surface of the Moon has a thinner atmosphere than low-Earth orbit.

Kenneth Gayley, University of Iowa, gave an interesting short talk, “The Real Explanation for Type Ia Supernovae and the Helium Flash”. Here’s the abstract: https://ui.adsabs.harvard.edu/abs/2019AAS…23422404G/abstract . I’m looking forward to reading the entire paper.

Gene Byrd, University of Alabama, gave an interesting short presentation, “Two Astronomy Demos”. The first was “Stars Like Grains of Sugar”, reminiscent of Archimedes’ The Sand Reckoner. And “Phases with the Sun, Moon, and Ball”. He uses a push pin in a golf ball (the golf ball even has craters!). Morning works best for this activity. The Sun lights the golf ball and the Moon and they have the same phase—nice! Touching as well as seeing the golf ball helps students understand the phases of the Moon. Here’s a link to his paper on these two activities.

Daniel Kennefick, University of Arkansas, gave a short presentation on the 1919 eclipse expedition that provided experimental evidence (besides the correct magnitude of the perihelion precession of Mercury) that validated Einstein’s General Relativity. Stephen Hawking in his famous book A Brief History of Time mis-remembered that the 1979 re-analysis of the Eddington’s 1919 eclipse data showed that he may “fudged” the results to prove General Relativity to be correct. He did not! See Daniel Kennefick’s new book on the subject, No Shadow of a Doubt: The 1919 Eclipse That Confirmed Einstein’s Theory of Relativity.

Brad Schaefer, Louisiana State University, gave another engaging talk, presenting evidence that the Australian aborigines may have discovered the variability of the star Betelgeuse, much earlier than the oft-stated discovery by John Herschel in 1836. Betelgeuse varies in brightness between magnitude 0.0 and +1.3 quasi-periodically over a period of about 423 days. It has been shown that laypeople can detect differences in brightness as small as 0.3 magnitude with the unaided eye, and with good comparison stars (like Capella, Rigel, Procyon, Pollux, Adhara, and Bellatrix—not all of which are visible from Australia—for Betelgeuse). It is plausible that the variability of Betelgeuse may have been discovered by many peoples at many different times. The Australian aborigines passed an oral tradition through many generations that described the variability of Betelgeuse. https://ui.adsabs.harvard.edu/abs/2019AAS…23422407S/abstract.

As a longtime astronomical observer myself, I have actually never noticed the variability of Betelgeuse, but Brad has. After his presentation, I mentioned to Brad that it would be interesting to speculate what would lead early peoples to look for variability in stars in the first place, which seems to me to be a prerequisite for anyone discovering the variability of Betelgeuse. His response pointed out that all it would take is one observant individual in any society who would notice/record the variability and then point it out to others.

During the last plenary session of the day, it was announced that the Large Synoptic Survey Telescope (LSST), which is expected to see first light in 2020, is expected to be renamed the Vera Rubin Survey Telescope. Tremendous applause followed! https://aas.org/posts/news/2019/06/lsst-may-be-renamed-vera-rubin-survey-telescope .

If you haven’t looked at the NASA/IPAC Extragalactic Database (NED) lately, you will find new content and functionality. It has been expanded a great deal, and now includes many stellar objects, because we don’t always know what is really a star and what is not. There is now a single input field where you can enter names, coordinates with search radius, etc. NED is “Google for Galaxies”.

I noticed during the 10-minute iPoster Plus sessions that there is a countdown timer displayed unobtrusively in the upper right hand corner that helps the presenter know how much time they have remaining. I think this would be a great device for anyone giving a short presentation in any venue.

Galactic archaeology is the study of the oldest stars and other structures in our galaxy to better understand how our galaxy evolved.

Day 2 ended with an evening presentation of “Cielo”, a documentary film by Alison McAlpine. Highly recommended!

I noted that “Cielo” was presented on the Documentary Channel in Canada. Too bad we do not have a channel like that here in the U.S.!

Day 3 – Wednesday, June 12, 2019

Day 3 began with what for me was the finest presentation of the entire conference: Joshua Winn, Princeton University, speaking on “Transiting Exoplanets: Past, Present, and Future”. I first became familiar with Josh Winn through watching his outstanding video course, The Search for Exoplanets: What Astronomers Know, from The Great Courses. I am currently watching his second course, Introduction to Astrophysics, also from The Great Courses. Josh is an excellent teacher, public speaker, and presenter, and it was a great pleasure to meet him at this conference.

Transits provide the richest source of information we have about exoplanets. For example, we can measure the obliquity of the star’s equator relative to the planet’s orbital plane by measuring the apparent Doppler shift of the star’s light throughout transit.

Who was the first to observe a planetary transit? Pierre Gassendi (1592-1655) was the first to observe a transit of Mercury across the Sun in November 1631. Jeremiah Horrocks (1618-1641) was the first to observe a transit of Venus across the Sun in November 1639. Christoph Scheiner (1573-1650) claimed in January 1612 that spots seen moving across the Sun were planets inside Mercury’s orbit transiting the Sun, but we know know of course that sunspots are magnetically cooled regions in the Sun’s photosphere and not orbiting objects at all. Though Scheiner was wrong about the nature of sunspots, his careful observations of them led him to become the first to measure the Sun’s equatorial rotation rate, the first to notice that the Sun rotated more slowly at higher latitudes, and the first to notice that the Sun’s equator is tilted with respect to the ecliptic, and to measure its inclination.

An exoplanet can be seen to transit its host star if the exoplanet’s orbit lies within the transit cone, an angle of 2R*/a centered on our line of sight to the star. R* is the star’s radius, and a is the semi-major axis of the planet’s orbit around the star.

Because of the geometry, we are only able to see transits of 1 out of every 215 Earth-Sun analogs.

Space is by far the best place to study transiting exoplanets.

If an exoplanet crosses a starspot, or a bright spot, on the star, you will see a “blip” in the transit light curve that looks like this:

Are solar systems like our own rare? Not at all! There are powerful selection effects at work in exoplanet transit statistics. We have discovered a lot of “hot Jupiters” because large, close-in planets are much easier to detect with their short orbital periods and larger transit cones. In actuality, only 1 out of every 200 sun-like stars have hot Jupiters.

Planet statistical properties was the main goal of the Kepler mission. Here are some noteworthy discoveries:

Kepler 89 – two planets transiting at the same time (only known example)

Kepler 36 – chaotic three-body system

Kepler 16 – first known transiting exoplanet in a circumbinary orbit

Transiting Exoplanet Survey Satellite (TESS) – Unlike Kepler, which is in an Earth-trailing heliocentric orbit, TESS is in a highly-elliptical orbit around the Earth with an apogee approximately at the distance of the Moon and a perigee of 108,000 km. TESS orbits the Earth twice during the time the Moon orbits once, a 2:1 orbital resonance with the Moon.

TESS has four 10.5 cm (4-inch) telescopes, each with a 24˚ field of view. Each TESS telescope is constantly monitoring 2300 square degrees of sky.

TESS is fundamentally about short period planets. Data is posted publicly as soon as it is calibrated. TESS has already discovered 700 planet candidates. About 1/2 to 2/3 will be true exoplanets. On average, TESS is observing stars that are about 4 magnitudes brighter than stars observed by Kepler.

The TESS Follow-Up Observing Program (TFOP) is a large working group of astronomical observers brought together to provide follow-up observations to support the TESS Mission’s primary goal of measuring the masses for 50 planets smaller than 4 Earth radii, in addition to organizing and carrying out follow-up of TESS Objects of Interest (TOIs).

HD 21749 – we already had radial velocity data going back several years for this star that hosts an exoplanet that TESS discovered

Gliese 357 – the second closest transiting exoplanet around an M dwarf, after HD 219134

TESS will tell us more about planetary systems around early-type stars.

TESS will discover other transient events, such as supernovae, novae, variable stars, etc. TESS will also make asteroseismology measurements and make photometric measurements of asteroids.

The James Webb Space Telescope (JWST) will be able to do follow-up spectroscopy of planetary atmospheres.

Upcoming exoplanet space missions: CHEOPS, PLATO, and WFIRST.

Hot Jupiter orbits should often be decaying, so this is an important area of study.

Sonification is the process of turning data into sound. For example, you could “listen” to a light curve (with harmonics, e.g. helioseismology and asteroseismology) of a year’s worth a data in just a minute or so.

Solar cycles have different lengths (11-ish years…).

Some predictions: 2019 will be the warmest year on record, 2020 will be less hot. Solar cycle 24 terminate in April 2020. Solar cycle 25 will be weaker than cycle 24. Cycle 25 will start in 2020 and will be the weakest in 300 years, the maximum (such as it is) occurring in 2025. Another informed opinion was that Cycle 25 will be comparable to Cycle 24.

Maunder minimum: 1645 – 1715

Dalton minimum: 1790 – 1820

We are currently in the midst of a modern Gleissberg minimum. It remains to be seen if it will be like the Dalton minimum or a longer “grand minimum” like the Maunder minimum.

Citizen scientists scanning Spitzer Space Telescope images in the Zooniverse Milky Way Project, have discovered over 6,000 “yellow balls”. The round features are not actually yellow, they just appear that way in the infrared Spitzer image color mapping.

Yellow balls (YBs) are sites of 8 solar mass or more star formation, surrounded by ionized hydrogen (H II) gas. YBs thus reveal massive young stars and their birth clouds.

Antlia 2 is a low-surface-brightness (“dark”) dwarf galaxy that crashed into our Milky Way galaxy. Evidence for this collision comes from “galactoseismology” which is the study of ripples in the Milky Way’s disk.

The Large Magellanic Cloud (LMC), Small Magellanic Cloud (SMC), and the Sagittarius Dwarf Galaxy have all affected our Milky Way Galaxy, but galactoseismology has shown that there must be another perturber that has affected the Milky Way. Antlia 2, discovered in November 2018 from data collected by the Gaia spacecraft, appears to be that perturber.

Gaia Data Release 2 (DR2) indicates that the Antlia 2 dwarf galaxy is about 420,000 ly distant, and it is similar in extent to the LMC. It is an ultra-diffuse “giant” dwarf galaxy whose stars average two magnitudes fainter than the LMC. Antlia 2 is located 11˚ from the galactic plane and has a mass around 1010 solar masses.

A question that is outstanding is what is the density of dark matter in Antlia 2? In the future, Antlia 2 may well be an excellent place to probe the nature of dark matter.

Gravity drives the formation of cosmic structure, dark energy slows it down.

Stars are “noise” for observational cosmologists.

“Precision” cosmology needs accuracy also.

The Vera Rubin telescope (Large Synoptic Survey Telescope) in Chile will begin full operations in 2022, collecting 20 TB of data each night!

We have a “galaxy bias” – we need to learn much more about the relation between galaxy populations and matter distribution.

Might there be an irregular asymmetric cycle underlying the regular 22-year sunspot cycle? The dominant period associated with this asymmetry is around 35 to 50 years.

The relationship between differential rotation and constant effective temperature of the Sun: the Sun has strong differential rotation along radial lines, and there is little variation of solar intensity with latitude.

Solar filaments (solar prominences) lie between positive and negative magnetic polarity regions.

Alfvén’s theorem: in a fluid with infinite electric conductivity, the magnetic field is frozen into the fluid and has to move along with it.

Some additional solar terms and concepts to look up and study: field line helicity, filament channels, kinetic energy equation, Lorentz force, magnetic energy equation, magnetic flux, magnetic helicity, magnetohydrodynamics (MHD), meridional flow, polarity inversion lines, relative helicity, sheared arcade, solar dynamo.

Filamentary structures: barbs, Hα, dextral, sinistral.

We would like to be able to predict solar eruptions before they happen.

1. Magnetic helicity is injected by surface motions.
2. It accumulates at polarity inversion lines.
3. It is removed by coronal mass ejections.
Day 4 – Thursday, June 13, 2019

Cahokia (our name for it today) was the largest city north of Mexico 1,000 years ago. It was located at the confluence of the Mississippi, Missouri, and Illinois Rivers. At its height from 1050 – 1200 A.D., Cahokia city covered 6 square miles and had 10,000 to 20,000 people. Cahokia was a walled city. Some lived inside the walls, and others lived outside the walls.

Around 120 mounds were built at greater Cahokia; 70 are currently protected. Platform mounds had buildings on top, and some mounds were used for burial and other uses.

Monks mound is the largest prehistoric earthwork in the Americas. Mound 72 has an appalling history.

Woodhenge – controversial claim that it had an astronomical purpose. Look up Brad Schaefer’s discussion, “Case studies of three of the most famous claimed archaeoastronomical alignments in North America”.

Cahokia’s demise was probably caused by many factors, including depletion of resources and prolonged drought. We do not know who the descendents of the Cahokia people are. It is possible that they died out completely.

The Greeks borrowed many constellations from the Babylonians.

One Sky, Many Astronomies

The neutron skin of a lead nucleus (208Pb) is a useful miniature analog for a neutron star.

Infalling binary neutron stars, such as GW 170817, undergo tidal deformation.

SmallSats

• Minisatellite: 100-180 kg
• Microsatellite: 10-100 kg
• Nanosatellite: 1-10 kg
• Picosatellite: 0.01-1 kg
• Femtosatellite: 0.001-0.01 kg

CubeSats are a class of nanosatellites that use a standard size and form factor. The standard CubeSat size uses a “one unit” or “1U” measuring 10 × 10 × 10 cm and is extendable to larger sizes, e.g. 1.5, 2, 3, 6, and even 12U.

The final plenary lecture and the final session of the conference was a truly outstanding presentation by James W. Head III, Brown University, “The Apollo Lunar Exploration Program: Scientific Impact and the Road Ahead”. Head is a geologist who trained the Apollo astronauts for their Moon missions between 1969 and 1972.

During the early years of the space program, the United States was behind the Soviet Union in space technology and accomplishments. The N1 rocket was even going to deliver one or two Soviet cosmonauts to lunar orbit so they could land on the Moon.

Early in his presidency, John F. Kennedy attempted to engage the Soviet Union in space cooperation.

Chris Kraft’s book, Flight: My Life in Mission Control is recommended.

The Apollo astronauts (test pilots) were highly motivated students.

The United States flew 21 robotic precursor missions to the Moon in the eight years before Apollo 11. Rangers 1-9 were the first attempts, but 1 through 6 were failures and we couldn’t even hit the Moon.

Head recommends the recent documentary, Apollo 11, but called First Man Hollywood fiction, saying, “That is not the Neil Armstrong I knew.”

The Apollo 11 lunar samples showed us that the lunar maria (Mare Tranquillitatis) has an age of 3.7 Gyr and has a high titanium abundance.

The Apollo 12 lunar excursion module (LEM) landed about 600 ft. from the Surveyor 3 probe in Oceanus Procellarum, and samples from that mission were used to determine the age of that lunar maria as 3.2 Gyr.

Scientists worked shoulder to shoulder with the engineers during the Apollo program, contributing greatly to its success.

Apollo 11 landed at lunar latitude 0.6˚N, Apollo 12 at 3.0˚S, Apollo 14 at 3.6˚S, and Apollo 15 at 26.1˚N. Higher latitude landings required a plane change and a more complex operation to return the LEM to the Command Module.

The lunar rover was first used on Apollo 15, and allowed the astronauts to travel up to 7 km from the LEM. Head said that Dave Scott did remarkable geological investigations on this mission. He discovered and returned green glass samples, and in 2011 it was determined that there is water inside those beads. Scott also told a little fib to Mission Control to buy him enough time to pick up a rock that turned out to be very important, the “seat belt basalt”.

In speaking about Apollo 16, Head called John Young “one of the smartest astronauts in the Apollo program”.

Harrison Schmitt, Apollo 17, was the only professional geologist to go to the Moon, and he discovered the famous “orange soil”. This is the mission where the astronauts repaired a damaged fender on the lunar rover using duct tape and geological maps to keep them from getting covered in dust while traveling in the rover.

When asked about the newly discovered large mass under the lunar surface, Head replied that it is probably uplifted mantle material rather than an impactor mass underneath the surface.

Radiometric dating of the Apollo lunar samples have errors of about ± 5%.

One of the reasons the Moon’s albedo is low is that space weather has darkened the surface.

The South Pole-Aitken basin is a key landing site for future exploration. In general, both polar regions are of great interest.

Smaller objects like the Moon and Mars cooled efficiently after their formation because of their high surface area to volume ratio.

We do not yet know if early Mars was warm and wet, or cold and icy with warming episodes. The latter is more likely if our solar system had a faint young sun.

Venus has been resurfaced in the past 0.5 Gyr, and there is no evidence of plate tectonics. The first ~80% of the history of Venus is unknown. Venus probably had an ocean and tectonic activity early on, perhaps even plate tectonics. Venus may have undergone a density inversion which exchanged massive amounts of material between the crust and mantle. 80% of the surface of Venus today is covered by lava flows.

A mention was made that a new journal of Planetary Science (in addition to Icarus, presumably) will be coming from the AAS soon.

I attend a lot of meetings and lectures (both for astronomy and SAS), and I find that I am one of the few people in attendance who write down any notes. Granted, a few are typing at their devices, but one never knows if they are multitasking instead. For those that don’t take any notes, I wonder, how do they really remember much of what they heard days or weeks later without having written down a few keywords and phrases and then reviewing them soon after? I did see a writer from Astronomy Magazine at one of the press conferences writing notes in a notebook as I do. I believe it was Jake Parks.

Anyone who knows me very well knows that I love traveling by train. To attend the AAS meeting, I took a Van Galder bus from Madison to Chicago, and then Amtrak from Chicago to St. Louis. Pretty convenient that the AAS meeting was held at the Union Station Hotel, just a few blocks from Amtrak’s Gateway Station. It is a fine hotel with a lot of history, and has an excellent on-site restaurant. I highly recommend this hotel as a place to stay and as a conference venue.

The bus and train ride to and fro afforded me a great opportunity to catch up on some reading. Here are a few things worth sharing.

astrometry.net – you can upload your astronomical image and get back an image with all the objects in the image astrometrically annotated. Wow!

16 Psyche, the most massive metal-rich asteroid, is the destination for a NASA orbiter mission that is scheduled to launch in 2022 and arrive at Psyche in 2026. See my note about 16 Psyche in the AAS notes above.

The lowest hourly meteor rate for the northern hemisphere occurs at the end of March right after the vernal equinox.

A tremendous, dynamic web-based lunar map is the Lunar Reconnaissance Orbiter Camera (LROC) Quickmap, quickmap.lroc.asu.edu.

I read with great interest Dr. Ken Wishaw’s article on pp. 34-38 in the July 2019 issue of Sky & Telescope, “Red Light Field Test”. Orange or amber light is probably better that red light for seeing well in the dark while preserving your night vision. You can read his full report here. Also, see my article “Yellow LED Astronomy Flashlights” here.

Jupiter and Saturn will have a spectacular conjunction next year. As evening twilight fades on Monday, December 21, 2020, the two planets will be just 1/10th of a degree apart, low in the southwestern sky.

An oblate spheroid with axes a = b > c is called a Maclaurin spheroid. If all three axes have different lengths a > b > c, then you have a Jacobi ellipsoid.

The light curve of a stellar occultation by a minor planet (asteroid or TNO) resembles a square well if the object has no atmosphere (or one so thin that it cannot be detected, given the sampling rate and S/N), and the effects of Fresnel diffraction and the star’s angular diameter are negligible.

Astronomer Margaret Burbidge, who turns 100 on August 12, 2019, refused the AAS Annie Jump Cannon Award in 1972, stating in her rejection letter that “it is high time that discrimination in favor of, as well as against, women in professional life be removed, and a prize restricted to women is in this category.” In 1976, Margaret Burbidge became the first woman president of the AAS, and in 1978 she announced that the AAS would no longer hold meetings in the states that had not ratified the Equal Rights Amendment (ERA).

During the days following the conference when I was writing this report, I received the happy news from both the AAS and Sky & Telescope that AAS was the winning bidder of S&T during a bankruptcy auction of its parent company, F+W Media. I believe that this partnership between the AAS and Sky & Telescope will benefit both AAS members and S&T readers immensely. Peter Tyson, Editor in Chief of Sky & Telescope, stated in the mutual press release, “It feels like S&T is finally landing where it belongs.” I couldn’t agree more!

Direct Imaging of Exoplanets Through Occultations

Planetary orbits are randomly oriented throughout our galaxy. The probability that an exoplanet’s orbit will be fortuitously aligned to allow that exoplanet to transit across the face of its parent star depends upon the radius of the star, the radius of the planet, and the distance of the planet from the star. In general, planets orbiting close-in are more likely to be seen transiting their star then planets orbiting further out.

The equation for the probability of observing a exoplanet transit event is

$p_{tra} = \left (\frac{R_{\bigstar}+R_{p}}{a} \right )\left (\frac{1}{1-e^{2}} \right )$

where ptra is the transit probability, R* is the radius of the star, Rp is the radius of the planet, a is the semi-major axis of the planetary orbit, and e is the eccentricity of the planetary orbit

Utilizing the data in the NASA Exoplanet Archive for the 1,463 confirmed exoplanets where the above data is available (and assuming e = 0 when eccentricity is unavailable), we find that the median exoplanet transit probability is 0.0542. This means that, on average, 1 out of every 18 planetary systems will be favorably aligned to allow us to observe transits. However, keep in mind that our present sample of exoplanets is heavily biased towards large exoplanets orbiting close to their parent star. Considering a hypothetical sample of Earth-sized planets orbiting 1 AU from a Sun-sized star, the transit probability drops to 0.00469, which means that we would be able to detect only about 1 out of every 213 Earth-Sun analogs using the transit method.

How might we detect some of the other 99.5%? My admired colleague in England, Abdul Ahad, has written a paper about his intriguing idea: “Detecting Habitable Exoplanets During Asteroidal Occultations”. Abdul’s idea in a nutshell is to image the immediate environment around nearby stars while they are being occulted by asteroids or trans-Neptunian objects (TNOs) in order to detect planets orbiting around them. While there are many challenges (infrequency of observable events, narrow shadow path on the Earth’s surface, necessarily short exposure times, and extremely faint planetary magnitudes), I believe that his idea has merit and will one day soon be used to discover and characterize exoplanets orbiting nearby stars.

Ahad notes that the apparent visual magnitude of any given exoplanet will be directly proportional to the apparent visual magnitude of its parent star, since exoplanets shine by reflected light. Not only that, Earth-sized and Earth-like planets orbiting in the habitable zone of any star would shine by reflected light of the same intrinsic brightness, regardless of the brightness of the parent star. He also notes that the nearer the star is to us, the greater will be a given exoplanet’s angular distance from the occulted star. Thus, given both of these considerations (bright parent star + nearby parent star = increased likelihood of detection), nearby bright stars such as Alpha Centauri A & B, Sirius A, Procyon A, Altair, Vega, and Fomalhaut offer the best chance of exoplanet detection using this technique.

Since an exoplanet will be easiest to detect when it is at its greatest angular distance from its parent star, we will be seeing only about 50% of its total reflected light. An Earth analog orbiting Alpha Centauri A would thus shine at visual magnitude +23.7 at 0.94″ angular distance, and for Alpha Centauri B the values would be +24.9 and 0.55″.

Other considerations include the advantage of an extremely faint occulting solar system object (making it easier to detect faint exoplanets during the occultation event), and the signal boost offered by observing in the infrared, since exoplanets will be brightest at these wavelengths.

A distant (and therefore slow-moving) TNO would be ideal, but the angular size of the TNO needs to be larger than the angular size of the occulted star. However, slow-moving objects mean that occultation events will be rare.

The best chance of making this a usable technique for exoplanet discovery would be a space-based observatory that could be positioned at the center of the predicted shadow and would be able to move along with the shadow to increase exposure times (Ahad, personal communication). It would be an interesting challenge in orbital mechanics to design the optimal base orbit for such a spacecraft. The spacecraft orbit would be adjusted to match the position and velocity of the occultation shadow for each event using an ion drive or some other electric propulsion system.

One final thought on the imaging necessary to detect exoplanets using this technique. With a traditional CCD you would need to begin and end the exoplanet imaging exposure(s) only while the parent star is being occulted. This would not be easy to do, and would require two telescopes – one for the occultation event detection and one for the exoplanet imaging. A better approach would be to use a Geiger-mode avalanche photodiode (APD). Here’s a description of the device captured in 2016 on the MIT Lincoln Labs Advanced Imager Technology website:

A Geiger-mode avalanche photodiode (APD), on the other hand, can be used to build an all-digital pixel in which the arrival of each photon triggers a discrete electrical pulse. The photons are counted digitally within the pixel circuit, and the readout process is therefore noise-free. At low light levels, there is still noise in the image because photons arrive at random times so that the number of photon detection events during an exposure time has statistical variation. This noise is known as shot noise. One advantage of a pixel that can digitally count photons is that if shot noise is the only noise source, the image quality will be the best allowed by the laws of physics. Another advantage of an array of photon counting pixels is that, because of its noiseless readout, there is no penalty associated with reading the imager out frequently. If one reads out a thousand 1-ms exposures of a static scene and digitally adds them, one gets the same image quality as a single 1-s exposure. This would not be the case with a conventional imager that adds noise each time it is read out.

References
Ahad, A., “Detecting Habitable Exoplanets During Asteroidal Occultations”, International Journal of Scientific and Innovative Mathematical Research, Vol. 6(9), 25-30 (2018).
MIT Lincoln Labs, Advanced Imager Technology, https://www.ll.mit.edu/mission/electronics/ait/single-photon-sensitive-imagers/passive-photon-counting.html. Retrieved March 17, 2016.
NASA Exoplanet Archive https://exoplanetarchive.ipac.caltech.edu.
Winn, J.N., “Exoplanet Transits and Occultations,” in Exoplanets, ed. Seager, S., University of Arizona Press, Tucson (2011).

Exoplanets with Deep Transits

The list above shows the 35 stars presently known to dip in brightness by 0.02 magnitudes or more due to a transiting exoplanet.

The change in the star’s magnitude during transit is given by

$\Delta m = 2.5\log_{10}\left ( 1+\delta \right )$

where Δm is the drop in magnitude, and δ is the transit depth

The time between transits for these exoplanets ranges between 0.79 and 5.72 days, with a median period of 2.24 days.  You can generate your own ephemeris for any of these transiting exoplanets at:

https://exoplanetarchive.ipac.caltech.edu/cgi-bin/TransitView/nph-visibletbls?dataset=transits

The transit duration for these exoplanets ranges between 1.08 and 3.11 hours, with a median duration of 2.11 hours.

The exoplanets with the deepest transits, HATS-6 b at 0.035 magnitudes and Kepler-45 b at 0.034 magnitudes, cross stars that are 15.2 and 16.9 magnitude, respectively, so these events might be out of reach for most amateur photometrists.  The only other star hosting a transiting exoplanet with a Δm ≥ 0.03m is Tycho 5165-481-1 in Aquila (WASP-80 b) which at visual magnitude 11.9 is a better candidate for smaller instruments.  The brightest star on our list (by far) is HD 189733 in Vulpecula, magnitude 7.7, with a drop in brightness that is almost as good at 0.026 magnitudes.

References
Fakhouri, O. (2018). Exoplanet Orbit Database | Exoplanet Data Explorer. [online] Exoplanets.org. Available at: http://exoplanets.org/ [Accessed 11 Dec. 2018].

Einstein, Brahms, and Exoplanets

What do Albert Einstein, Johannes Brahms, and exoplanets have in common?  They are all great courses provided by The Great Courses.

Call me old fashioned, but I love a great lecture presented by an expert in the field.  What a wonderful way to get introduced to a new subject, or refamiliarize yourself with an old subject, or deepen your knowledge about a subject with which you are already familiar.

I recently finished watching the magnificent course “Albert Einstein: Physicist, Philosopher, Humanitarian” by Don Howard, Professor of Philosophy at the University of Notre Dame, former Director of Notre Dame’s Graduate Program in History and Philosophy of Science, and a Fellow of the University of Notre Dame’s Reilly Center for Science, Technology, and Values.

I have taken an interest in Einstein since I first encountered relativity in my early teens, and of course being a physics major in college I became much more familiar with Einstein’s remarkable scientific contributions.  But this course surprised and delighted me with many details about Einstein himself.  Howard obviously has a much deeper understanding of Einstein the person than most physicists do, and his enthusiasm for his subject comes through in every lecture.  I doubt you will find a more thorough treatment of Einstein anywhere short of reading a biography.  Recommended!

As luck would have it, while I was nearing the end of this course, Time came out with an updated reissue of its special edition, “Albert Einstein: The Enduring Legacy of a Modern Genius”.  Great photographs, great text.  Well worth every penny!

Robert Greenberg is music historian-in-residence with San Francisco Performances and has produced a lot of high-quality music courses for The Great Courses.  I am in the process of watching all of them (yes, really, they’re that good!).  Recently, I finished his course on Johannes Brahms, who is probably my all-time favorite composer.

The music of Brahms is well known by many, but how much do you know about Johannes Brahms the person, and the events of his life?  This course is the perfect introduction to those subjects, as well as his extraordinary compositions.

It is amazing to me that no one has yet made a feature-length film about the life of Johannes Brahms (1833-1897).  A historically accurate dramatic portrayal could easily become one of the most significant musical film biographies ever made.  Brahms was one of the greatest composers who ever lived, and he had an interesting life—there is much material to draw upon for the making of this movie.  Greenberg’s course is a great place to begin, and I would also recommend the definitive biography, “Brahms: His Life and Work” by Karl Geiringer.

You’ve just got to love The Great Courses.  This is what television could have been.  PBS is the only thing that even comes close.  I recently completed “The Search for Exoplanets: What Astronomers Know” presented by Joshua Winn, now Professor of Astrophysical Sciences at Princeton University.  Not since Carl Sagan or Neil deGrasse Tyson have I been this excited about an astronomy presenter.  Josh Winn presents his exoplanets course with enthusiasm, precision, and a delivery that really draws you in to the subject.  I hope we see much more of him in the future.

Separating Observer from Observed

One of the most difficult things to do in observational science is to separate the observer from the observed.  For example, in CCD astronomy, we apply bias, dark, and flat-field corrections as well as utilize median combines of shifted images to yield an image that is, ideally, free of any CCD chip defects including differences in pixel sensitivity and zero-point.

We as observers are constrained by other limitations.  For example, when we look at a particular galaxy, we observe it from a single vantage point in space and time, a vantage point we cannot change due to our great distance from the object and our existence within an exceedingly short interval of time.

Yet another limitation is a phenomenon that astronomers often call “observational selection”.  Put simply, we are most likely to see what is easiest to see.  For example, many of the exoplanets we have discovered thus far are “hot Jupiters”.  Is this because massive planets that orbit very close to a star are common?  Not necessarily.  The radial velocity technique we use to detect many exoplanets is biased towards finding massive planets with short-period orbits because such planets cause the biggest radial velocity fluctuations in their parent star over the shortest period of time.  Planets like the Earth with its relatively small mass and long orbital period (1 year) are much more difficult to detect using the radial velocity technique.  The same holds true for the transit method.  Planets orbiting close to a star will transit more often—and are more likely to transit—than comparable planets further out.  Larger planets will exhibit a larger Δm than smaller planets, regardless of their location.  It may be that Earthlike planets are much more prevalent than hot Jupiters, but we can’t really conclude that looking at the data collected so far (though Kepler has helped recently to make a stronger case for abundant terrestrial planets).

Here’s another important observational selection effect to consider in astronomy: the farther away a celestial object is the brighter that object must be for us to even see it.  In other words, many far-away objects cannot be observed because they are too dim.  This means that when we look at a given volume of space, intrinsically bright objects are over-represented.  The average luminosity of objects seems to increase with increasing distance.  This is called the Malmquist bias, named after the Swedish astronomer Gunnar Malmquist (1893-1982).

Stars Like Our Sun

The spectral type of our Sun is G2V, that is to say, a G2 main-sequence star.

Sun
Zodiacal Constellations
mv = -26.75, mb = -26.10, B-V = 0.65
Ecliptic
0.0000158 ly
Single star

Here are the brightest stars visible in the nighttime sky that have the same spectral type and therefore are, arguably, most like our Sun.  All have an apparent visual magnitude brighter than +6.00.

Rigil Kentaurus A, Alpha Centauri A (α Cen A)
Centaurus
mv = 0.01, mb = 0.72, B-V = 0.71
α2000 = 14h 39m 36s, δ2000 = -60° 50′ 02″
4.30 – 4.34 ly
Triple star system

Alula Australis B, Xi Ursae Majoris B (ξ UMa B)
Ursa Major
mv = 4.73, mb = 5.38, B-V = 0.65
α2000 = 11h 18m 11s, δ2000 = +31° 31′ 46″
28 – 30 ly
Quintuple star system

HR 4523 A
Centaurus
mv = 4.88, mb = 5.55, B-V = 0.67
α2000 = 11h 46m 31s, δ2000 = -40° 30′ 01″
30.0 – 30.1 ly
Binary star system; exoplanet

Eta Coronae Borealis A & B (η CrB A & B)
Corona Borealis
A: mv = 5.577, mb = 6.123, B-V = 0.546
B: mv = 5.95, mb = 6.48, B-V = 0.53
α2000 = 15h 23m 12s, δ2000 = +30° 17′ 18″
57 – 59 ly
Triple star system

HR 8323
Grus
mv = 5.58, mb = 6.18, B-V = 0.60
α2000 = 21h 48m 16s, δ2000 = -47° 18′ 13″
51.9 – 52.5 ly
Single star

Mu Velorum B (μ Vel B)
Vela
mv = 5.59, mb = 6.10, B-V = 0.51
α2000 = 10h 46m 46s, δ2000 = -49° 25′ 12″
116 – 119 ly
Binary star system

HR 7845 A
Capricornus
mv = 5.65, mb = 6.34, B-V = 0.69
α2000 = 20h 32m 24s, δ2000 = -09° 51′ 12″
79 – 80 ly
Binary star system

HR 3578
Hydra
mv = 5.86, mb = 6.39, B-V = 0.53
α2000 = 8h 58m 44s, δ2000 = -16° 07′ 58″
68 – 69 ly
Single star

HR 2007
Orion
mv = 5.97, mb = 6.61, B-V = 0.64
α2000 = 5h 48m 35s, δ2000 = -4° 05′ 41″
49.2 – 49.8 ly
Single star with exoplanet

The Eta Coronae Borealis system is noteworthy in that its two primary components are both G2V stars orbiting each other every 41.6 years.  The third component of this system is a distant infrared dwarf, spectral type L8V.

Two of these G2V stars host at least one exoplanet: HR 4523A in Centaurus and HR 2007 in Orion.

HR 4523A has a planet midway in mass between Uranus and Neptune orbiting every 122 days between 0.30 and 0.62 AU from the star (similar to orbital distance of the planet Mercury in our own solar system).  The other stellar component of this system. HR 4523B, is a distant M4V star currently orbiting about 211 AU from HR 4523A.

HR 2007, a single star like the Sun, has a planet about 78% more massive than Neptune, orbiting every 407 days, more or less.  If this planet were in our own solar system, it would range between the orbits of Venus and Mars, roughly.

Metallicity

No, it’s not the name of a rock band. Astronomers (unlike everybody else) consider all elements besides hydrogen and helium to be metals. For example, our Sun has a metallicity of at least 2% by mass (Vagnozzi 2016). That means as much as 98% of the mass of the Sun is hydrogen (~73%) and helium (~25%), with 2% being everything else.

Traditionally, elemental abundances in the Sun have been measured using spectroscopy of the Sun’s photosphere.  In principle, stronger spectral lines (usually absorption) of an element indicate a greater abundance of that element, but deriving the correct proportions from the cacophony of spectral lines is challenging.

A more direct approach to measuring the Sun’s elemental abundances is analyzing the composition of the solar wind, though the material blown away from the surface of the Sun that we measure near Earth’s orbit may be somewhat different from the actual photospheric composition.  The solar wind appears to best reflect the composition of the Sun’s photosphere in the solar polar regions near solar minimum.  The Ulysses spacecraft made solar wind measurements above both the Sun’s north and south polar regions during the 1994-1995 solar minimum.  Analysis of these Ulysses data indicate the most abundant elements are (after hydrogen and helium, in order of abundance): oxygen, carbon, nitrogen, magnesium, silicon, neon, iron, and sulfur—though one analysis of the data shows that neon is the third most abundant element (after carbon).

The elephant in the room is, of course, are the photospheric abundances we measure using spectroscopy or the collection of solar wind particles indicative of the Sun’s composition as a whole?  As it turns out, we do have ways to probe the interior of the Sun.  Both helioseismology and the flux of neutrinos emanating from the Sun are sensitive to metal abundances within the Sun.  Helioseismology is the study of the propagation of acoustic pressure waves (p-waves) within the Sun.  Neutrino flux is devilishly hard to measure since neutrinos so seldom interact with the matter in our instruments.  Our studies of the interior of the Sun (except for sophisticated computer models) are still in their infancy.

You might imagine that if measuring the metallicity of the Sun in our own front yard is this difficult, then measuring it for other stars presents an even more formidable challenge.

In practice, metallicity is usually expressed as the abundance of iron relative to hydrogen.  Even though iron is only the seventh most abundant metal (in the Sun, at least), it has 26 electrons, leading to the formation of many spectral lines corresponding to the various ionization states within a wide range of temperature and pressure regimes.  Of the metals having a higher abundance than iron, silicon has the largest number of electrons, only 14, and it does not form nearly as many spectral lines in the visible part of the spectrum as does iron.  Thus defined, the metallicity of the Sun [Fe/H] = 0.00 by definition.  It is a logarithmic scale: [Fe/H] = -1.0 indicates an abundance of iron relative to hydrogen just 1/10 that of the Sun.  [Fe/H] = +1.0 indicates an abundance of iron relative to hydrogen 10 times that of the Sun.

The relationship between stellar metallicity and the existence and nature of exoplanets is an active topic of research.  It is complicated by the fact that we can never say for certain that a star does not have planets, since our observational techniques are strongly biased towards detecting planets with an orbital plane near our line of sight to the star.

References
Vagnozzi, S. 2016, 51st Recontres de Moriond, Cosmology, At La Thuile

Many times each week, all manner of asteroids and trans-Neptunian objects pass in front of stars, casting shadows a few miles wide all over the Earth.  There are several potential events each week at any particular location.  I use the word “potential” because there is still significant uncertainty in the paths for many of these events.  The orbits of most small solar system objects are not yet precisely known, and, to a lesser extent, there is some uncertainty in the position of the occulted (obscured) star.

On Sunday evening, November 20, I got lucky.  Not only did I record a 1.02 second occultation event, but I was lucky to see it at all as I was significantly south of the predicted path.

The star affected was Tycho 5182-758-1 (also known as BD -3° 5037) in Aquarius and the object that moved in front of it was the asteroid 430 Hybris, a space rock about 20 miles across that orbits once around the Sun every 4.8 years.  Many asteroids have interesting names, and Hybris is no exception.  In Greek mythology, Hybris is a spirit of insolence, violence, and outrageous behavior.  It is also an alternative form of the word hubris.  All quite appropriate given the outcome of the U.S. presidential election less than two weeks earlier.

Here is the video I recorded of the event:

Occultation of the star Tycho 5182-758-1 in Aquarius by the asteroid 430 Hybris

And here is the light curve I derived from the video which clearly shows the event:

Steve Messner (near Northfield, Minnesota) and I were the only ones to observe this event.  It was a miss for Steve, and he was much closer to the predicted path!

Why do we do it? Even a single positive observation can greatly improve our knowledge of the orbit of the asteroid or trans-Neptunian object.  More than one positive observation gives us valuable information about its size and shape.  We can discover asteroid/TNO satellites and even rings!  But that’s not all.  These occultation events can also give us valuable information about the star.  Its size, position, and the separation and position angle of new or known companion stars.  Someday, we may even be able to use these events to discover exoplanets!

If you love observational astronomy and would like to contribute scientifically valuable observations by observing occultation events, contact me and I will help you get started.  The more observers we have, the more valuable our scientific contribution will be.