Understanding the Universe: An Introduction to Astronomy


96 lectures – 30 min each

Who has not gazed with wonder at the night sky? The great canopy of stars stretching overhead suggests that our world is part of a vastly larger cosmos. But how large is it? Where do we fit in? And how did it all begin? These questions have puzzled stargazers for thousands of years, and the search for answers helped spark the great advances of the Scientific Revolution in the 16th and 17th centuries. But only in our own time has the full picture of the true immensity, variety, and surpassing strangeness of the Universe come into focus.

Explore Everything There Is in 96 Lectures

Understanding the Universe: An Introduction to Astronomy, 2nd Edition is a nontechnical description of where that picture stands today. In 96 richly illustrated half-hour lectures, you survey the main concepts, methods, and discoveries in astronomy—in depth—from the constellations drawn by the ancients, to the latest reports from planetary probes in our Solar System, to the most recent images offered by telescopes probing the farthest frontiers of space and time.

These lectures fully update Professor Alex Filippenko’s 1998 edition of this course and his companion course from 2003. All of the material in this course is integrated so that one topic builds on another as you develop the conceptual tools that allow you to explore the Universe. For example, the study of the Solar System leads naturally to the investigation of planets around other stars and the possibility of life elsewhere in the cosmos. Likewise, rainbows and similar atmospheric phenomena introduce the subject of light, and light is the key to unraveling the mysteries of stars and galaxies.

Dr. Filippenko uses thousands of diagrams and photographs. There are almost 300 short movies and computer animations that make astronomical phenomena easier to understand, and they put planets, stars, and galaxies into context as you zoom through the cosmos. A showman in the classroom, Dr. Filippenko delights in simple, easily reproducible demonstrations that use tennis balls, apples, paper plates, and other objects to explain scientific concepts. Furthermore, he has a gift for analogies: At one point, he makes the energy content of one erg vivid by comparing it to one fly doing one push-up!

Altogether, this course is an unrivaled opportunity to experience a full-year introductory college course on astronomy, delivered by a five-time winner of “Best Professor” on campus at the University of California, Berkeley, who himself is a leading participant in some of the groundbreaking discoveries at the forefront of the field. Dr. Filippenko is both a world-class teacher and researcher: In 2006 he was named one of four national Professors of the Year by The Carnegie Foundation for the Advancement of Teaching, and in 1998 his international team of astronomers was credited with the top “science breakthrough of the year” for their amazing discovery that the expansion of the Universe is speeding up—a finding that is now shaking the foundations of physics.

The Universe Is More Exciting Than Ever

Much has happened in astronomy in a few short years. Many of these new discoveries are scientifically sophisticated, but the comprehensive scope of this course allows you to absorb the background you need to grasp such exciting recent developments as these:

  • Martian “blueberries”: The Spirit and Opportunity rovers on Mars have found clues in tiny, blue spherical stones nicknamed “blueberries” that indicate liquid water was once abundant on the red planet.
  • Water on Enceladus: The Cassini spacecraft now orbiting Saturn has turned up evidence of liquid water just beneath the frozen surface of Saturn’s moon Enceladus. Where there’s water, there may be life.
  • Is Pluto a planet? In August 2006, Pluto was demoted from the planetary status it had enjoyed since its discovery in 1930. Professor Filippenko discusses the persuasive reasons for this change—and his misgivings about how it was done.
  • Exoplanets: Astronomers continue to find new planets orbiting other stars at a rate of about 20 per year. They are rapidly approaching the Holy Grail of planet hunters: extra-solar planets the size of Earth.
  • Gamma-ray bursts: Thanks to NASA’s Swift satellite, astronomers know more than ever about the most luminous and baffling events in the cosmos: gamma-ray bursts. They have found strong indications that many such bursts probably represent the birth cries of black holes.
  • Dark energy: A mysterious force is causing the Universe to expand at an accelerating rate. The nature of this “dark energy” is now better understood thanks to recent observations that have narrowed exactly when this speed-up began.
  • Supermassive black holes: Once considered highly speculative, gigantic black holes are now known to be at the centers of most galaxies. These objects, from millions to billions of times more massive than the Sun, can produce vast jets of material moving outward at nearly the speed of light.


A Science for Everyone

As befits its subject matter, astronomy is perhaps the most diverse science there is. It teaches you about natural phenomena that we all experience, such as the seasons, rainbows, and phases of the Moon. It teaches you about tools of the trade, such as telescopes, spectrographs, and space probes. It teaches you about different areas of science, such as optics, physics, and chemistry. It also poses one of the most profound questions in the field of biology, namely, is there life elsewhere in the Universe?

Because astronomy has played a leading role in the development of science since antiquity, it is filled with key discoveries in the history of science, such as Copernicus’s heliocentric model of the Solar System, Newton’s universal law of gravitation, and Einstein’s theory of relativity.

And, of course, astronomy serves as a field guide for understanding the night sky:

  • The prominent star Betelgeuse in the constellation Orion is a red supergiant, hundreds of times the diameter of the Sun.
  • The brightest star in the sky, Sirius, is orbited by a stellar corpse called a white dwarf, which points to the ultimate fate of the Sun.
  • The beautiful Crab Nebula is the remnant of a supernova, whose explosion was observed by Chinese astronomers on July 4, 1054, and that at its center is an immensely dense, rapidly spinning neutron star.

At Home in the Universe

There is no doubt that we live in a golden age of astronomy. At this moment, space probes are on their way to new destinations, innovative new telescopes are on the drawing board, experiments are underway to test bold ideas about the cosmos, and powerful new tools for probing the nature of matter are about to come online. Astronomers everywhere are watching the skies with instruments of every size and type, searching for clues that will extend our knowledge of the Universe.

Armed with the wide-ranging and unified view of astronomy that this course offers, you can truly appreciate these future findings as they are announced. Moreover, you can feel at home in our wondrous Universe in a deeply satisfying way.


A Grand Tour of the Cosmos
You embark on a fantastic voyage throughout the Universe, starting in this lecture with a whirlwind tour of the course, which extends from phenomena in Earth’s atmosphere to events at the farthest reaches of space and time.
The Rainbow Connection
This lecture explains how Newton’s theories were received by leading thinkers in France and Germany and describes the events that led to the eventual creation of a worldview that claimed Newton as its hero.
Sunrise, Sunset
Continuing the study of the daytime sky, this lecture examines phenomena associated with sunrise and sunset, including the elusive green flash, Buddha’s rays, and Earth’s own shadow at sunset.
Bright Objects in the Night Sky
Many objects in the night sky can be enjoyed with the naked eye, even from the city. These include several famous constellations, bright stars, planets, and satellites such as the International Space Station.
Fainter Phenomena in the Night Sky
Far from city lights, the night sky becomes spectacular and includes such sights as the Milky Way, the Magellanic Clouds, zodiacal light, and comets. Though not technically “celestial,” auroras are also wondrous spectacles.
Our Sky through Binoculars and Telescopes
A relatively inexpensive telescope and even a simple pair of binoculars greatly increase the number of celestial objects you can study, such as the craters on the Moon, the Orion Nebula, and the Andromeda Galaxy.
The Celestial Sphere
As Earth rotates on its axis and orbits the Sun, the night sky changes over a 24-hour period, as well as throughout the year. This lecture explains how to locate celestial bodies and why the sky appears different from place to place.
The Reason for the Seasons
Contrary to popular belief, the seasons are not caused by changes in the distance between Earth and the Sun over the course of a year. Instead, the tilt of Earth’s axis of spin relative to the axis of its orbital plane produces seasons.
Lunar Phases and Eerie Lunar Eclipses
Lunar phases occur as the Moon orbits Earth, changing the viewing perspective of the Moon’s illuminated hemisphere. Lunar eclipses take place when Earth, the Sun, and Moon are almost exactly aligned.
Glorious Total Solar Eclipses
Total solar eclipses are stunning celestial sights, which occur when the Moon comes between Earth and the Sun, totally blocking the Sun for a small portion of the Earth. These rare events reveal beautiful and thrilling phenomena.
More Eclipse Tales
Solar eclipses also come in annular and hybrid varieties, reflecting the varying distance of the Moon from Earth. A famous total solar eclipse in 1919 provided observational evidence for Einstein’s general theory of relativity.
Early Studies of the Solar System
Astronomy has its roots in the ancient civilizations of Mesopotamia, Babylon, India, Egypt, and China. The Greeks in particular developed sophisticated and sometimes surprisingly accurate theories of the nature of the heavens.
The Geocentric Universe
The Greek philosopher Aristotle concluded that Earth is stationary at the center of the Universe, surrounded by 55 nested spheres. Ptolemy elaborated this geocentric model into a system that held sway for nearly 1,500 years.
Galileo and the Copernican Revolution
In 1543, Copernicus proposed a heliocentric system, in which Earth and other planets orbit the Sun, which is at the center of the Universe. In the early 1600s Galileo provided strong evidence for this model with the newly invented telescope
Refinements to the Heliocentric Model
This lecture explores the refinements in the Copernican model made possible by Tycho’s extremely accurate measurements of planetary positions, which were analyzed by Kepler to produce his laws of planetary motion.
On the Shoulders of Giants
According to legend, Newton saw a falling apple and realized that the force that pulled it toward Earth also pulled on the Moon, keeping it in its orbit. Building on the work of Kepler and Galileo, he revolutionized astronomy with his law of universal gravitation and laws of motion.
Surveying Space and Time
Observations of the transit of Venus across the face of the Sun in the 18th century helped determine the scale of the Solar System. In the wider Universe, distances are so vast that the finite speed of light means we are literally looking back in time.
Scale Models of the Universe
The best way to understand the size of the Universe is to investigate it in a series of steps, each 10 times larger or smaller than the one preceding. In this way, you explore the cosmos from the smallest to the largest scale.
Light—The Supreme Informant
Astronomers obtain most of their information through the analysis of light. This lecture introduces the electromagnetic spectrum and the technique of spectroscopy, in which light is dispersed into its component colors, as in a rainbow.
The Wave-Particle Duality of Light
Delving deeper into the nature of light, you explore the strange duality between electromagnetic waves (light waves) and particles (photons), which is a fundamental feature of quantum theory..
The Colors of Stars
The surface temperature of a star determines its apparent color. The hottest stars are bluish in color, and the coldest stars are reddish; stars at intermediate temperatures appear white. The Sun is a white star.
The Fingerprints of Atoms
Electrons jumping between different energy levels in atoms emit and absorb photons in a characteristic way for each element. Thus, astronomers can use the light from distant objects to deduce their chemical compositions.
Modern Telescopes
Today’s telescopes are designed to provide huge light-gathering mirrors at relatively low cost. The mirrors focus light from distant objects onto sensitive electronic detectors that are far more efficient than traditional photographic film.
A Better Set of Eyes
This lecture looks at radio telescopes, adaptive optics for ground-based infrared telescopes, and NASA’s Great Observatories, which include the Hubble Space Telescope, Chandra X-Ray Observatory, and Spitzer Space Telescope
Our Sun, the Nearest Star
Beginning a sequence of lectures on the Solar System, you start with the Sun, which you explore from the interior to the surface. Sunspots are cooler regions associated with strong magnetic fields, and violent eruptions blast solar material into space.
The Earth, Third Rock from the Sun
Earth is one of the four innermost, or terrestrial, planets; the others are Mercury, Venus, and Mars. All are relatively small, rocky, and dense. This lecture examines Earth’s structure, properties, and the forces that affect it.
Our Moon, Earth’s Nearest Neighbor
This lecture covers the wealth of knowledge about the Moon, a heavily cratered world with extensive lava-filled basins on the Earth-facing side and yet few such features on the far side—which is not perpetually dark.
Mercury and Venus
Though broadly similar to Earth, Mercury and Venus differ in detail. Mercury has a negligible atmosphere and is heavily cratered. Venus has an incredibly thick atmosphere and suffers from an extreme greenhouse effect that makes it intensely hot.
Of Mars and Martians
Recent missions to Mars provide evidence for an early water-rich era that may have fostered primitive life. Today, Mars is a cold, apparently lifeless world. Evidence for fossil life in a Martian meteorite remains controversial.
Jupiter and Its Amazing Moons
Beyond Mars lie the four gas giants: Jupiter, Saturn, Uranus, and Neptune. Jupiter is the Solar System’s largest planet by far. Its moons include Io, which is volcanically active, and Europa, which may have an ocean of liquid water below its frozen surface.
Magnificent Saturn
Best known for its extensive ring system, Saturn has come into focus recently thanks to the Cassini spacecraft, which landed a probe on Saturn’s largest moon, Titan; and also discovered evidence of liquid water on the moon Enceladus.
Uranus and Neptune, the Small Giants
Though less massive than Jupiter and Saturn, Uranus and Neptune are similar in that they consist mostly of hydrogen and helium. Both have bizarre magnetic fields that are highly tilted relative to the planet’s rotation axis and offset from the planet’s center.
Pluto and Its Cousins
Discovered in 1930, Pluto was long considered a planet. However, the detection of more and more objects in the same region strongly suggests that it is a member of a reservoir of comet-like bodies in the Kuiper belt.
Asteroids and Dwarf Planets
Under a new definition adopted in 2006, planets are more narrowly defined and a new class called dwarf planets has been created, which includes Pluto, Eris (a Kuiper-belt object larger than Pluto), and Ceres (the largest asteroid).
Comets—Gorgeous Primordial Snowballs
Comets are “dirty snowballs” from beyond the orbit of Neptune. As they approach the Sun, they begin to evaporate and produce gaseous tails. In 2005, a space probe impacted Comet Tempel 1, revealing the nature of its interior.
Catastrophic Collisions
Comets and asteroids have struck Earth throughout its history. One such collision 65 million years ago probably caused the extinction of the dinosaurs. Astronomers now search for asteroids that could be a potential threat to Earth.
The Formation of Planetary Systems
Planets form inside a rotating cloud of dust and gas, which flattens as it contracts. At the center, the temperature is high enough to create a star; farther from the center, leftover material collects into planets.
The Quest for Other Planetary Systems
In 1995, the first extra-solar planet was discovered around a Sun-like star. Since then, about 200 have been found. The easiest to detect are those with large masses, close to their host stars, and with brief orbital periods.
Extra-Solar Planets Galore!
Because extra-solar planets are too dim to be seen directly, they are detected using a variety of ingenious techniques. Two examples: Minute variations in a star’s spectrum and tiny changes in its brightness can signal the presence of planets.
Life Beyond the Earth
The recent discovery of extra-solar planets rekindles the age-old question of whether there is life beyond Earth. This lecture examines the possibility of rudimentary life on other planets and moons in the Solar System.
The Search for Extraterrestrials
Are there intelligent extraterrestrials elsewhere in our Galaxy? One way to search is to scan the radio spectrum for evidence of their electronic signals. TheDrake equation suggests ways to estimate their prevalence.
Special Relativity and Interstellar Travel
Are interstellar voyages possible within a single human lifetime? According to Einstein’s special theory of relativity, the answer is “yes” in principle but “no” in practice, given current technology. You explore the reasons for both answers.
Stars—Distant Suns
A voyage to another star would require exact information on distances and properties of the stars. This lecture shows how astronomers measure the distance to nearby stars and how they determine their surface temperatures, which are the basis for stellar classification.
The Intrinsic Brightnesses of Stars
Apparent brightness is the observed brightness of a star. Because stars are at different distances, astronomers need a standard reference by which to compare stars in absolute terms, as if they’re all at the same distance: This standard is called intrinsic brightness, orluminosity.
The Diverse Sizes of Stars
This lecture discusses the Hertzsprung-Russell diagram, a plot of stellar surface temperature versus luminosity that contains a wealth of information. Stars spend most of their existence on the diagram’s well-defined main sequence; outliers include supergiants and white dwarfs.
Binary Stars and Stellar Masses
Many stars are members of binary systems, in which two stars orbit a common center of mass. Our best estimates of how much mass stars have come from observations of binaries. We find that massive stars have far shorter lives than low-mass stars.
Star Clusters, Ages, and Remote Distances
Star clusters are gravitationally bound groups of stars that are all about the same age and the same distance from Earth. Astronomers can determine the approximate ages of clusters. This lecture also explains how the distance of extremely remote stars is found.
How Stars Shine—Nature’s Nuclear Reactors
When the central temperature of a contracting cloud of gas grows sufficiently high, it becomes a star—a gigantic, controlled, thermonuclear reactor, fusing hydrogen into helium and maintaining a constant luminosity and size.
Solar Neutrinos—Probes of the Sun’s Core
Physicists had long assumed that nuclear fusion occurred in the Sun’s core, though without a way to physically measure or observe this. Ghostly particles called neutrinos provide direct evidence of events in the Sun’s nuclear furnace.
Brown Dwarfs and Free-Floating Planets
Brown dwarfs are low-mass objects whose dim glow is caused by slow gravitational contraction and the fusion of deuterium, a heavier but far less abundant isotope of hydrogen. Free-floating planets have even less mass than brown dwarfs and are not associated with any star.
Our Sun’s Brilliant Future
As it gradually uses up the hydrogen in its core, fusing it to helium, the Sun will expand to form a red giant star. Through a series of relatively nonviolent eruptions, it will expel its outer layers of gas, producing a beautiful, glowing nebula around the dying core.
White Dwarfs and Nova Eruptions
The burned out Sun will eventually contract into a white dwarf. This is the fate of all stars between roughly 0.08 and 8 solar masses. A white dwarf in a binary system may steal matter from its companion star and flare up in an eruption called a nova.
Exploding Stars—Celestial Fireworks!
A few stars end their lives with cataclysmic explosions, expelling gas at huge speeds. At its peak, such asupernova can rival the brightness of an entire galaxy, and its remnants can be seen for centuries. The Crab Nebula is one such remnant.
White Dwarf Supernovae—Stealing to Explode
Supernovae come in several types, based primarily on their spectra. This lecture focuses on the important, hydrogen-deficient subclass called Type Ia, in which a white dwarf robs gas from its companion star and then becomes violently unstable.
Core-Collapse Supernovae—Gravity Wins
Type II supernovae, whose spectra exhibit hydrogen, come from massive supergiant stars whose core suddenly collapses, ejecting the rest of the star. This mechanism also applies to “stripped” stars that had previously lost their outermost layers through winds and transfer to companions.
The Brightest Supernova in Nearly 400 Years
In 1987 a Type II supernova only 170,000 light years away became visible. Earlier photos of the region showed that the exploded star was a blue supergiant, a previously unsuspected candidate for this fate. Ghostly neutrinos were detected from this supernova.
The Corpses of Massive Stars
The imploding core of a Type II supernova typically forms a bizarre, enormously compact object called aneutron star, consisting entirely of tightly packed neutrons, a teaspoon of which would weigh about a billion tons.
Einstein’s General Theory of Relativity
Understanding the enormous gravitational fields of neutron stars requires Einstein’s general theory of relativity, which postulates that gravity is a manifestation of the warping of space and time produced by matter and energy.
Warping of Space and Time
This lecture explores observational tests of general relativity. Astronomers exploit its effects by searching for distant objects that are gravitationally lensed, which occurs when an object’s light is bent and focused by foreground masses such as galaxy clusters.
Black Holes—Abandon Hope, Ye Who Enter
If a neutron star exceeds two to three solar masses, it becomes unstable and collapses. The resulting object is called ablack hole—a region of such extreme space-time curvature that nothing, not even light, can escape.
The Quest for Black Holes
Because they don’t emit any light, black holes can’t be seen directly. But they reveal their presence through their gravitational influence on other objects. This lecture looks at the methods astronomers use to track them down.
Imagining the Journey to a Black Hole
What’s a black hole really like? Without taking the fatal journey into one, astronomers can calculate the bizarre experiences that would ensue, including dramatic distortions in visual phenomena as a traveler approached the event horizon.
Wormholes—Gateways to Other Universes?
Rotating black holes appear to connect our Universe to others or provide shortcuts—or wormholes—within our Universe. This raises the theoretical possibility of time travel, although several factors seem to rule it out.
Quantum Physics and Black-Hole Evaporation
Originally, astronomers thought that black holes were truly black, but physicist Stephen Hawking has calculated that black holes slowly evaporate through a quantum mechanical process. Very small black holes should literally explode as a burst of gamma rays!
Enigmatic Gamma-Ray Bursts
Roughly once per day, somewhere in the sky, there is a short, intense burst of gamma rays. Most of these events originate in very distant galaxies, making them among the most powerful explosions in the Universe—but they are not evaporating black holes.
Birth Cries of Black Holes
Until recently, astronomers had no smoking gun to identify the precise location and cause of gamma-ray bursts. Now they have assembled an abundance of clues pointing to two separate mechanisms: the collapse of a massive star, and the merging of two neutron stars—in each case creating a black hole.
Our Home—The Milky Way Galaxy
Starting a series of lectures on galaxies, you begin with our home galaxy, the Milky Way. The band of light by that name is simply the Galaxy seen edge-on from our place within it. You also explore the nebulae in our Galaxy, many of which are regions of stellar birth.
Structure of the Milky Way Galaxy
How do you map the structure of a galaxy when you live inside it? Astronomers have used various clues to infer the spiral structure of the Milky Way, the orbital speed of its stars, and the existence of a supermassive black hole at its center.
Other Galaxies—”Island Universes”
The discovery of other galaxies beyond the Milky Way was one of the great scientific detective stories of the early 20th century. Astronomers now know that there are hundreds of billions of galaxies, spanning billions of light years of space.
The Dark Side of Matter
Until a few decades ago, astronomers thought that galaxies were composed primarily of stars. There is now strong evidence that most of the mass of galaxies may be invisible dark matter. Clusters of galaxies are also dominated by dark matter.
Cosmology—The Really Big Picture
This lecture starts the study of the Universe as a whole—or cosmology. A key finding made by Edwin Hubble in 1929 was that the spectra of distant galaxies are redshifted more than those of nearby galaxies, suggesting that the Universe is expanding.
Expansion of the Universe and the Big Bang
The Universe can be thought of as expanding into a mathematical dimension to which we have no physical access. Even an infinite Universe can expand, becoming less dense. The expansion suggests that there was a hot, dense beginning long ago—a Big Bang.
Searching for Distant Galaxies
The finite speed of light allows observers to look back in time and see the unfolding history of the Universe. This lecture shows how astronomers search for distant galaxies to compare with better understood, nearby galaxies.
The Evolution of Galaxies
How do galaxies form and evolve over time? Is it possible to determine what nearby galaxies, or even the Milky Way, once looked like? The answers can be found by examining distant galaxies that formed when the Universe was young.
Active Galaxies and Quasars
The central regions of many galaxies go through an active, very luminous phase early in their development. The most powerful of these active galaxies, calledquasars, shine like beacons across billions of light years of space.
Cosmic Powerhouses of the Distant Past
The high luminosity of quasars puzzled astronomers in the 1960s. How could these peculiar, star-like objects be so bright and yet so far away? Only a few light years across, they are in fact even more powerful than entire galaxies.
Supermassive Black Holes
Astronomers now have strong evidence that quasars and other active galactic nuclei are powered by supermassive black holes, voraciously swallowing surrounding material. Less active galaxies also appear to harbor these monsters.
Feeding the Monster
This lecture explores the disks of gas around supermassive black holes. Material escaping from the vicinity of these objects often follows a highly focused jet along the rotation axis of the disk, sometimes approaching or even appearing to surpass the speed of light.
The Paradox of the Dark Night Sky
Why is the sky dark at night? In an infinitely old and large Universe the sky should be ablaze with light at all times. There are several possible answers to this paradox, each of which has profound cosmological implications. The relative youth of the Universe is now known to be the main explanation.
The Age of the Universe
How old is the Universe? The Hubble Space Telescope was designed, in part, to answer this question. You follow the chain of reasoning that has led astronomers to conclude that the Universe began 13.7 billion years ago.
When Geometry Is Destiny
According to general relativity, the fate of the Universe is tied to its global geometry. If the Universe has positive curvature, like a sphere, it must eventually collapse in a “Big Crunch.” If it is flat or has negative curvature, however, it will expand forever.
The Mass Density of the Universe
This lecture explores methods used by astronomers to determine the mass density and expansion history of the Universe. To make this measurement, a race developed between two teams of astronomers searching for Type Ia supernovae in distant galaxies.
Einstein’s Biggest Blunder?
The unexpected finding that the Universe is expanding at an ever-faster rate lends support for the existence of a non-zero cosmological constant, a formerly discredited idea of Einstein’s, which he once called his “biggest blunder.”
The Afterglow of the Big Bang
An accidental discovery in 1965 overturned the “steady-state theory” of the Universe, an alternative to the Big Bang theory. The detection of a uniform microwave “glow” in all directions was exactly what was expected if the Universe was hot and dense long ago.
Ripples in the Cosmic Background Radiation
The cosmic microwave background radiation preserves intriguing details about the Universe around 380,000 years after the Big Bang, when the temperature had cooled enough so that neutral atoms formed, allowing photons to pass freely through space.
The Stuff of the Cosmos
The dark energy that is causing the expansion of the Universe to accelerate makes up about 75 percent of the cosmos. Visible matter accounts for less than 1 percent. The bulk of the remainder is dark matter, most of which may consist of exotic subatomic particles.
Dark Energy—Quantum Fluctuations?
According to one idea, repulsive dark energy having a negative pressure might be the result of a non-perfect cancellation of quantum fluctuations in space—virtual particles created literally out of nothing, as predicted by quantum physics.
Dark Energy—Quintessence?
This lecture looks at problems with the quantum fluctuations explanation for dark energy. One alternative is calledquintessence?—a class of models that postulate repulsive energy that may be associated with unified forces or fields.
Grand Unification & Theories of Everything
A major effort is underway to unify the mutually inconsistent theories of general relativity and quantum mechanics into atheory of everything. Successfully explaining dark energy might serve as an observational test for such a theory.
Searching for Hidden Dimensions
The leading contenders for a theory of everything are string theories, which postulate that fundamental particles act like tiny, vibrating strings of energy. This approach requires at least 10 dimensions, most of which are curled up on minuscule size scales.
The Shape, Size, and Fate of the Universe
Is the Universe a finite bubble in a higher-dimensional space? Or, is it infinite regardless of whether it’s imbedded in extra dimensions? Will it expand forever or ultimately recollapse? If it does expand forever, how will this limitless future unfold?
In the Beginning
This lecture turns back the clock to almost the moment of creation—a fraction of a second after the Big Bang—and follows events as they sort themselves out, from what may have been packages of space-time foamwinking in and out of existence, to conditions conducive for star and galaxy formation.
The Inflationary Universe
The remarkable large-scale uniformity and “flatness” of the Universe pose a problem for the standard Big Bang theory. A startling but powerful suggested explanation is that the Universe went through an initial period of exponential expansion, calledinflation.
The Ultimate Free Lunch?
Why should inflation have occurred? Theorists have proposed several intriguing ideas, including that the Universe, whose total energy is quite possibly equal to zero, formed from a random quantum fluctuation that grew to gargantuan proportions.
A Universe of Universes
If a quantum fluctuation gave rise to our Universe, must ours be the only one? Are others possible, perhaps even with different rules? This lecture examines reasons for suspecting the existence of other universes, though we do not know how to test for their presence.
Reflections on Life and the Cosmos
The course ends on a philosophical note, with reflections on intelligent life in the cosmos and of our place in its grand structure. Perhaps the most astonishing thing about the Universe is that we are able to contemplate and understand it through systematic studies.


  • 01. A Grand Tour of the Cosmos

  • 02. The Rainbow Connection

  • 03. Sunrise, Sunset

  • 04. Bright Objects in the Night Sky

  • 05. Fainter Phenomena in the Night Sky

  • 06. Our Sky through Binoculars and Telescopes

  • 07. The Celestial Sphere

  • 08. The Reason for the Seasons

  • 09. Lunar Phases and Eerie Lunar Eclipses

  • 10. Glorious Total Solar Eclipses

  • 11. More Eclipse Tales

  • 12. Early Studies of the Solar System

  • 13. The Geocentric Universe

  • 14. Galileo and the Copernican Revolution

  • 15. Refinements to the Heliocentric Model

  • 16. On the Shoulders of Giants

  • 17. Surveying Space and Time

  • 18. Scale Models of the Universe

  • 19. Light—The Supreme Informant

  • 20. The Wave-Particle Duality of Light

  • 21. The Colors of Stars

  • 22. The Fingerprints of Atoms

  • 23. Modern Telescopes

  • 24. A Better Set of Eyes

  • 25. Our Sun, the Nearest Star

  • 26. The Earth, Third Rock from the Sun

  • 27. Our Moon, Earth's Nearest Neighbor

  • 28. Mercury and Venus

  • 29. Of Mars and Martians

  • 30. Jupiter and Its Amazing Moons

  • 31. Magnificent Saturn

  • 32. Uranus and Neptune, the Small Giants

  • 33. Pluto and Its Cousins

  • 34. Asteroids and Dwarf Planets

  • 35. Comets—Gorgeous Primordial Snowballs

  • 36. Catastrophic Collisions

  • 37. The Formation of Planetary Systems

  • 38. The Quest for Other Planetary Systems

  • 39. Extra-Solar Planets Galore!

  • 40. Life Beyond the Earth

  • 41. The Search for Extraterrestrials

  • 42. Special Relativity and Interstellar Travel

  • 43. Stars—Distant Suns

  • 44. The Intrinsic Brightnesses of Stars

  • 45. The Diverse Sizes of Stars

  • 46. Binary Stars and Stellar Masses

  • 47. Star Clusters, Ages, and Remote Distances

  • 48. How Stars Shine—Nature's Nuclear Reactors

  • 49. Solar Neutrinos—Probes of the Sun's Core

  • 50. Brown Dwarfs and Free-Floating Planets

  • 51. Our Sun's Brilliant Future

  • 52. White Dwarfs and Nova Eruptions

  • 53. Exploding Stars—Celestial Fireworks!

  • 54. White Dwarf Supernovae—Stealing to Explode

  • 55. Core-Collapse Supernovae—Gravity Wins

  • 56. The Brightest Supernova in Nearly 400 Years

  • 57. The Corpses of Massive Stars

  • 58. Einstein's General Theory of Relativity

  • 59. Warping of Space and Time

  • 60. Black Holes—Abandon Hope, Ye Who Enter

  • 61. The Quest for Black Holes

  • 62. Imagining the Journey to a Black Hole

  • 63. Wormholes—Gateways to Other Universes?

  • 64. Quantum Physics and Black-Hole Evaporation

  • 65. Enigmatic Gamma-Ray Bursts

  • 66. Birth Cries of Black Holes

  • 67. Our Home—The Milky Way Galaxy

  • 68. Structure of the Milky Way Galaxy

  • 69. Other Galaxies—"Island Universes"

  • 70. The Dark Side of Matter

  • 71. Cosmology—The Really Big Picture

  • 72. Expansion of the Universe and the Big Bang

  • 73. Searching for Distant Galaxies

  • 74. The Evolution of Galaxies

  • 75. Active Galaxies and Quasars

  • 76. Cosmic Powerhouses of the Distant Past

  • 77. Supermassive Black Holes

  • 78. Feeding the Monster

  • 79. The Paradox of the Dark Night Sky

  • 80. The Age of the Universe

  • 81. When Geometry Is Destiny

  • 82. The Mass Density of the Universe

  • 83. Einstein's Biggest Blunder?

  • 84. The Afterglow of the Big Bang

  • 85. Ripples in the Cosmic Background Radiation

  • 86. The Stuff of the Cosmos

  • 87. Dark Energy—Quantum Fluctuations?

  • 88. Dark Energy—Quintessence?

  • 89. Grand Unification & Theories of Everything

  • 90. Searching for Hidden Dimensions

  • 91. The Shape, Size, and Fate of the Universe

  • 92. In the Beginning

  • 93. The Inflationary Universe

  • 94. The Ultimate Free Lunch?

  • 95. A Universe of Universes

  • 96. Reflections on Life and the Cosmos