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Understanding the Universe: An Introduction to Astronomy, 2nd Edition

Get a comprehensive overview of the grandest of all subjects-the universe-in this brilliant and fascinating course taught by one of America's foremost astronomers.
Understanding the Universe: An Introduction to Astronomy, 2nd Edition is rated 4.8 out of 5 by 336.
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Rated 5 out of 5 by from I bought this course a decade ago. Bought the current for my grandchildren.
Date published: 2021-11-13
Rated 5 out of 5 by from A magical trip with a wonderful guide. Dr. Filippenko is outstanding in his explanation of where we are and how we got here.
Date published: 2021-07-26
Rated 5 out of 5 by from Excellent survey course. As a retired scientist I was obviously drawn to this course. I took an astronomy class back in the 1970s and found this course equivalent to a university level class. I was also amazed how much new knowledge has been attained since then. I especially liked the many illustrations of concepts presented. A very professional presentation and while some may think the 96 lectures excessive, it takes this many to adequately understand this topic.
Date published: 2021-07-20
Rated 5 out of 5 by from Excellent course. I bought this course about a month ago as a present for a relative. I had already bought this for myself and found it to be very informative and fascinating. The presenter was very knowledgeable and easy to listen to. The images and diagrams shown during the lectures were very helpful and enhanced ones understanding of the material and of the universe. I was so pleased and impressed with the course hence buying another copy for a relative.
Date published: 2021-06-25
Rated 5 out of 5 by from Astonishingly well done... A+ Very well put together 96 lectures flew by... If you have interest in the sky, you can't go wrong. Alex is great at explaining the complicated I'm sure I'll rewatch someday again
Date published: 2021-06-11
Rated 5 out of 5 by from Completeness, Ease of Understanding Topics I gave this 5 stars because of what it is, basically more complete than a standard college course in Introduction to Astronomy similar to one I took some years ago (got an A). That was 15 weeks, 4 hours per week or 60 total hours which besides lectures included question and answer time, tests, mid-term and final. Most 1st year classes are usually 3 hrs/week or 45 hours. This course being 96 half hour lectures or 48 hours exceeds what you will get in a 3 hour course and is comparable to the actual lecture time in a 4 hour course. The drawback, of course, is that this course was recorded around 2008 which means a lot of discoveries since then won't be covered. However in his 1st lecture Professor Filippenko mentions his college text - The Cosmos - then in its 3rd or 4th edition that coincides in some ways with these lectures. That text is now in its 5th printing as of mid 2019 and so now includes a lot of the discoveries since these lectures. If you're interested I'd recommend that text as a companion if you want more up to date information (every chapter concludes with a series of 'test' questions). However, even in the past year or so there have many more discoveries. With new telescopes constantly coming on line, more satellites being sent up, and more probes being sent out, discoveries will continue to easily outstrip any publication - lectures or texts. If you want to get a start in understanding the past history of astronomy, how physics and chemistry work in space, discoveries up to 2008, so you can understand new discoveries being made almost daily, in context, these lectures are a good place to start. .
Date published: 2021-05-01
Rated 5 out of 5 by from A force to rule them all An excellent introduction to Astronomy, no doubt Professor Filippenko is a great teacher. However, nobody is perfect, in lecture 40 he made a big mistake by saying that nucleic acids are constituted by aminoacids (as they were proteins), although this mistake is recognized in the guidebook of the course. I hope this will be corrected in a future edition of the course. If there is a third edition, I would like to suggest to have at least two separate courses: on for classical Astronomy and the other one for Cosmology. I felt the course too long with 96 lectures.
Date published: 2021-04-21
Rated 5 out of 5 by from Long course but worth it . This course is a very long one but it is chock full of information. The 96 lectures present basically everything you need to know about the universe. From our own sun to black holes, Alex Filippenko presents a wide array of topics. The last lecture reflects on the course as a whole, and it was the best one. I liked the animations and demonstrations that he showed us. All in all, this course was the best one I've watched so far. I highly recommend it.
Date published: 2021-04-12
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In 96 richly illustrated half-hour lectures you will survey the main concepts, methods, and discoveries in astronomy&;amp;-from the constellations drawn by the ancients, to the latest reports from planetary probes in our Solar System and the most recent images offered by telescopes probing the farthest frontiers of space and time. While updating this course, Professor Alex Filippenko added hundreds of images, including nearly 300 short movies and computer animations.


Alex Filippenko
Alex Filippenko

Perhaps the next time you go out to the countryside, you will ponder the magnificence of the Universe and its contents-and the fact that, through careful experiments, observations, and thought, humans are coming to a good understanding of what makes it all tick.


University of California, Berkeley

Dr. Alex Filippenko is Professor of Astronomy and the Richard and Rhoda Goldman Distinguished Professor in the Physical Sciences at the University of California, Berkeley. He earned his B.A. in Physics from the University of California, Santa Barbara, and his Ph.D. in Astronomy from the California Institute of Technology. Dr. Filippenko's research accomplishments, documented in more than 500 scientific publications and 600 abstracts and astronomical circulars, are among the most highly cited in the world. Science magazine credited two international teams of astronomers (on which he was the only coauthor contributing to both teams) with the top "Science Breakthrough of 1998" for research on exploding stars (supernovae), which shows that the universe is expanding at an accelerating rate, propelled by mysterious "dark energy." Professor Filippenko received a share of the 2007 Gruber Cosmology Prize for this discovery, work that went on to receive the 2011 Nobel Prize in Physics. Professor Filippenko also leads the world's most successful robotic search for exploding stars. Dr. Filippenko was elected in 2009 to the National Academy of Sciences, one of the highest honors accorded to a U.S. scientist. He has also been recognized with several major awards, including the 2010 Richard H. Emmons Award for excellence in the teaching of college-level introductory astronomy for non-science majors from the Astronomical Society of the Pacific, the 2007 Richtmyer Memorial Award of the American Association of Physics Teachers, the 1997 Robert M. Petrie Prize of the Canadian Astronomical Society, and the 1992 Newton Lacy Pierce Prize of the American Astronomical Society. He was a Guggenheim Fellow in 2001 and a Phi Beta Kappa Visiting Scholar in 2002. In 2006, he was honored nationally as the "Outstanding Doctoral and Research Universities Professor of the Year" by the Carnegie Foundation for the Advancement of Teaching and the Council for Advancement and Support of Education. At UC Berkeley, Dr. Filippenko's teaching awards include the Donald S. Noyce Prize for Excellence in Undergraduate Teaching in the Physical Sciences and the Distinguished Teaching Award. He was also voted the Best Professor on Campus nine times in student polls.\r\n\r\nDr. Filippenko is coauthor of The Cosmos: Astronomy in the New Millennium, now in its 4th edition (2013), and winner of the 2001 Texty Excellence Award for best new textbook in the physical sciences. He has played a prominent role in numerous television documentaries, including about 40 episodes spanning six seasons of The Universe on The History Channel.

By This Professor

Understanding the Universe: An Introduction to Astronomy, 2nd Edition
Skywatching: Seeing and Understanding Cosmic Wonders
Understanding the Universe: An Introduction to Astronomy, 2nd Edition


A Grand Tour of the Cosmos

01: 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....

34 min
The Rainbow Connection

02: The Rainbow Connection

The daytime sky contains intriguing natural sights that offer a colorful introduction to astronomy. One such phenomenon is the rainbow. This lecture examines how a rainbow is created and how it appears under different circumstances....

30 min
Sunrise, Sunset

03: 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....

32 min
Bright Objects in the Night Sky

04: 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....

31 min
Fainter Phenomena in the Night Sky

05: 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....

31 min
Our Sky through Binoculars and Telescopes

06: 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....

30 min
The Celestial Sphere

07: 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....

33 min
The Reason for the Seasons

08: 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....

31 min
Lunar Phases and Eerie Lunar Eclipses

09: 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....

30 min
Glorious Total Solar Eclipses

10: 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....

31 min
More Eclipse Tales

11: 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....

31 min
Early Studies of the Solar System

12: 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....

29 min
The Geocentric Universe

13: 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....

30 min
Galileo and the Copernican Revolution

14: 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....

31 min
Refinements to the Heliocentric Model

15: 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....

31 min
On the Shoulders of Giants

16: 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....

32 min
Surveying Space and Time

17: 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....

31 min
Scale Models of the Universe

18: 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....

31 min
Light-The Supreme Informant

19: 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....

31 min
The Wave-Particle Duality of Light

20: 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....

30 min
The Colors of Stars

21: 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....

31 min
The Fingerprints of Atoms

22: 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....

31 min
Modern Telescopes

23: 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....

31 min
A Better Set of Eyes

24: 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....

31 min
Our Sun, the Nearest Star

25: 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....

30 min
The Earth, Third Rock from the Sun

26: 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....

32 min
Our Moon, Earth's Nearest Neighbor

27: 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....

31 min
Mercury and Venus

28: 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....

33 min
Of Mars and Martians

29: 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....

31 min
Jupiter and Its Amazing Moons

30: 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....

31 min
Magnificent Saturn

31: 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....

30 min
Uranus and Neptune, the Small Giants

32: 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....

30 min
Pluto and Its Cousins

33: 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....

30 min
Asteroids and Dwarf Planets

34: 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)....

33 min
Comets-Gorgeous Primordial Snowballs

35: 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....

31 min
Catastrophic Collisions

36: 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....

31 min
The Formation of Planetary Systems

37: 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....

32 min
The Quest for Other Planetary Systems

38: 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....

31 min
Extra-Solar Planets Galore!

39: 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....

31 min
Life Beyond the Earth

40: 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....

31 min
The Search for Extraterrestrials

41: 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. The Drake equation suggests ways to estimate their prevalence....

31 min
Special Relativity and Interstellar Travel

42: 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....

31 min
Stars-Distant Suns

43: 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....

32 min
The Intrinsic Brightnesses of Stars

44: 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, or luminosity....

30 min
The Diverse Sizes of Stars

45: 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....

31 min
Binary Stars and Stellar Masses

46: 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....

31 min
Star Clusters, Ages, and Remote Distances

47: 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....

31 min
How Stars Shine-Nature's Nuclear Reactors

48: 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....

30 min
Solar Neutrinos-Probes of the Sun's Core

49: 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....

31 min
Brown Dwarfs and Free-Floating Planets

50: 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....

31 min
Our Sun's Brilliant Future

51: 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....

31 min
White Dwarfs and Nova Eruptions

52: 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....

31 min
Exploding Stars-Celestial Fireworks!

53: Exploding Stars-Celestial Fireworks!

A few stars end their lives with cataclysmic explosions, expelling gas at huge speeds. At its peak, such a supernova can rival the brightness of an entire galaxy, and its remnants can be seen for centuries. The Crab Nebula is one such remnant....

31 min
White Dwarf Supernovae-Stealing to Explode

54: 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....

31 min
Core-Collapse Supernovae-Gravity Wins

55: 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....

31 min
The Brightest Supernova in Nearly 400 Years

56: 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....

31 min
The Corpses of Massive Stars

57: The Corpses of Massive Stars

The imploding core of a Type II supernova typically forms a bizarre, enormously compact object called a neutron star, consisting entirely of tightly packed neutrons, a teaspoon of which would weigh about a billion tons....

31 min
Einstein's General Theory of Relativity

58: 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....

32 min
Warping of Space and Time

59: 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....

31 min
Black Holes-Abandon Hope, Ye Who Enter

60: 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 a black hole-a region of such extreme space-time curvature that nothing, not even light, can escape....

32 min
The Quest for Black Holes

61: 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....

31 min
Imagining the Journey to a Black Hole

62: 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....

31 min
Wormholes-Gateways to Other Universes?

63: 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....

32 min
Quantum Physics and Black-Hole Evaporation

64: 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!...

31 min
Enigmatic Gamma-Ray Bursts

65: 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....

31 min
Birth Cries of Black Holes

66: 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....

31 min
Our Home-The Milky Way Galaxy

67: 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....

30 min
Structure of the Milky Way Galaxy

68: 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....

30 min
Other Galaxies-

69: 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....

31 min
The Dark Side of Matter

70: 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....

31 min
Cosmology-The Really Big Picture

71: 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....

32 min
Expansion of the Universe and the Big Bang

72: 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....

32 min
Searching for Distant Galaxies

73: 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....

31 min
The Evolution of Galaxies

74: 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....

31 min
Active Galaxies and Quasars

75: 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, called quasars, shine like beacons across billions of light years of space....

32 min
Cosmic Powerhouses of the Distant Past

76: 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....

31 min
Supermassive Black Holes

77: 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....

32 min
Feeding the Monster

78: 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....

32 min
The Paradox of the Dark Night Sky

79: 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....

32 min
The Age of the Universe

80: 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....

31 min
When Geometry Is Destiny

81: 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....

31 min
The Mass Density of the Universe

82: 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....

32 min
Einstein's Biggest Blunder?

83: 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."...

31 min
The Afterglow of the Big Bang

84: 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....

31 min
Ripples in the Cosmic Background Radiation

85: 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....

31 min
The Stuff of the Cosmos

86: 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....

32 min
Dark Energy-Quantum Fluctuations?

87: 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....

31 min
Dark Energy-Quintessence?

88: Dark Energy-Quintessence?

This lecture looks at problems with the quantum fluctuations explanation for dark energy. One alternative is called quintessence?-a class of models that postulate repulsive energy that may be associated with unified forces or fields....

32 min
Grand Unification & Theories of Everything

89: Grand Unification & Theories of Everything

A major effort is underway to unify the mutually inconsistent theories of general relativity and quantum mechanics into a theory of everything. Successfully explaining dark energy might serve as an observational test for such a theory....

32 min
Searching for Hidden Dimensions

90: 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....

32 min
The Shape, Size, and Fate of the Universe

91: 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?...

32 min
In the Beginning

92: 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 foam winking in and out of existence, to conditions conducive for star and galaxy formation....

32 min
The Inflationary Universe

93: 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, called inflation....

32 min
The Ultimate Free Lunch?

94: 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....

32 min
A Universe of Universes

95: 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....

32 min
Reflections on Life and the Cosmos

96: 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....

38 min