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Understanding the Universe: An Introduction to Astronomy

YEAR: 2007 | LENGTH: 96 parts (31 minutes each)  |  SOURCE: TGC

description:

This visually rich course is designed to provide a nontechnical description of modern astronomy, including the structure and evolution of planets, stars, galaxies, and the Universe as a whole. It includes almost all of the material in my first two astronomy courses for The Teaching Company, produced in 1998 and 2003, but with a large number of new images, diagrams, and animations. The discoveries reported in the 2003 course are integrated throughout these new lectures, and more recent findings (through mid-2006) are included, as well. Much has happened in astronomy during the past few years; we will discuss the most exciting and important advances.

episodes:

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 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.

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.

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.

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.

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.

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.

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 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.

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.

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.

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 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.

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.

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.

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.

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.

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.

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.

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 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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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 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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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 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.

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.

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 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.

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.

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.

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.

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.

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.

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 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.

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.

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.

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.

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.

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.

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.

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!

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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."

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.

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 dark energy that is causing the expansion of the Universe to accelerate makes up about 75 percent of the cosmos. Ordinary matter glowing at any wavelength, optical or otherwise, accounts for less than 5 percent. The remainder is dark matter, most of which may consist of exotic subatomic particles.

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.

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.

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 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.

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?

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.

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.

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.

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.

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.