Introduction to Astrophysics

Course No. 1360
Professor Joshua Winn, Ph.D.
Princeton University
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Course No. 1360
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What Will You Learn?

  • The chain of reasoning that led Isaac Newton to his revolutionary theory of universal gravitation.
  • How to predict the fate of stars.
  • The formulas behind our current view of the universe.
  • How to calculate the event horizon of a black hole.
  • Shortcuts for complex calculations.

Course Overview

Everyone loves to observe the beauty of the star-studded night sky, to say nothing of the dazzling images from the Hubble Space Telescope. But how many of us truly understand how stars shine, where Saturn’s rings come from, or why galaxies have their distinctive shapes? Observational astronomy excels at imaging and cataloging celestial objects, but it takes a more rigorous discipline to come up with physical explanations for them. That field is astrophysics.

Astrophysics uses the laws of physics to investigate everything beyond Earth. In the process, it has made breakthroughs such as these:

  • Celestial motion: Astrophysics got its start with Isaac Newton’s laws of motion and universal gravitation, proposed in the late 1600s. Together, these cornerstones of classical physics explain why everything in the universe moves the way it does.
  • A universe of galaxies: In the 1920s, Edwin Hubble’s study of an enigmatic nebula proved that it was, in fact, another galaxy—at a time when the Milky Way was thought to be the only galaxy there was. Since then, we have amassed evidence for hundreds of billions of galaxies.
  • Exoplanets: The hunt for planets orbiting other stars was considered impractical, until a range of clever techniques were developed in the 1990s. Thousands of “exoplanets” have since been discovered, many in planetary orbits unlike anything seen in our solar system.

Introduction to Astrophysics plunges you into this exciting quest, taking you step by step through the calculations that show how planets, stars, and galaxies work. In these 24 illuminating half-hour lectures, taught by noted astrophysicist Professor Joshua Winn of Princeton University, you tour the universe of exploding stars, colliding black holes, dark matter, and other wonders, just as in a traditional astronomy course. But Professor Winn takes you beyond the images and descriptions to teach you how to understand and solve the physics problems at the heart of the field.

Throughout the course, Dr. Winn uses custom-designed graphics and animations to help you visualize what’s happening. As he makes clear from the first lecture, this course relies heavily on high school and first-year college math. As Dr Winn says: “I’ll also rely on first-year physics. Sometimes I’ll go deeper—I’ll use vectors, or take derivatives, which I hope will be helpful to those of you who’ve studied more math. But if not, don’t worry, I promise to do my very best to help you understand the results, even if you can’t follow every step.”

Even if you feel like math and physics are not your strongest subjects, you will find his presentation enthralling, as you witness how astrophysicists frame, analyze, and solve problems that have led to astonishing discoveries throughout the universe.

Experience the Thrill of Discovery

Dr. Winn has wide experience using Earth- and space-based telescopes in his research, and in Introduction to Astrophysics he puts you in the driver’s seat—showing you how to gather data, pick formulas, simplify mathematical expressions, and come up with results that give you the thrill of discovering something concrete and often unexpected about the universe. For example:

  • How dense is the Sun: From observations of total solar eclipses and ocean tides, you can deduce that the Sun is, on average, half as dense as the Moon, or about 1.5 grams per cubic centimeter. This is an important clue about how the Sun and other stars work.
  • The Milky Way’s core: At the center of our galaxy, stars orbit a seemingly empty point in space. It can only be a black hole. But how big is it? Given the orbital period and semimajor axis of one of the stars, the calculation is simple: four million solar masses.
  • Faster than light: The spectra of galaxies reveal that all except the closest are speeding away from us; the farther, the faster—with the most distant seeming to exceed light speed! But this apparent violation of the cosmic speed limit is an illusion, caused by the expansion of space itself.

Cover the Fundamentals

One of the intellectual delights of this course is retracing the steps that led to some of the great ideas in astrophysics. In a typical physics class, Newton’s laws are memorized with little appreciation for where they came from. But Professor Winn shows how Newton was inspired by Johannes Kepler’s three laws of planetary motion. Analyzing the laws with calculus (which he invented for the purpose), Newton discovered principles such as the inverse square law of gravity. His astonishing achievement was to prove that physical laws that apply on Earth also operate throughout the universe—something we take for granted today but which was a revelation to thinkers at the time.

Similarly, most astronomy lectures on the electromagnetic spectrum—the range of wavelengths from radio to visible light to gamma ray—don’t normally explain the ultimate origin of radiation. James Clerk Maxwell’s equations, published in the 1860s, show that electrically charged particles, when accelerated, generate a pattern of electric fields accompanied by magnetic fields—an electromagnetic wave—that travels at the speed of light. Quantum theory later modified this description to handle the case of electrons orbiting around a nucleus. Dr. Winn believes that ideas so fundamental to the physics of stars and telescopes deserve to be treated in detail.

In other cases, detail is exactly what you want to avoid. One of the tricks for dealing with the vast range of concepts and spatial scales in astrophysics is using shortcuts whenever possible, such as:

  • Order of magnitude: Dispense with constants, extra significant figures, and other unnecessary marks of precision, to zero in on the order of magnitude—the nearest factor of ten—of the answer you seek.
  • Dimensional analysis: Normally used to keep units straight in a calculation, this technique sometimes lets you guess the right equation you need, by reverse-engineering it from the known units of the solution.
  • Scaling relation: This trick is a shortcut for creating a streamlined version of an equation, based on a benchmark case with known values. Analogous cases can then be scaled up or down by the appropriate factor.

The Most Amazing Subject in the Universe

An award-winning teacher at both the undergraduate and graduate levels, Professor Winn has a fresh, exciting approach to astrophysics. For example, while many traditional astrophysics classes often don’t touch on black holes until the end of the course, Professor Winn believes it is “a terrible idea—it’s educational malpractice! Black holes are some of the most fascinating things in the universe.” His infectious joy in the subject—even when it is at its most complex—is palpable in every lecture.

He then explains these intriguing objects in depth, walking you through the calculations for the theoretical boundaries of black holes with the mass of Earth (9 millimeters), the Sun (3 kilometers), and the supermassive black hole at the core of the Milky Way Galaxy (12 million kilometers). His treatment of black holes flows seamlessly from his discussion of gravity and tidal forces in the preceding lectures.

In the same enthusiastic spirit, Dr. Winn analyzes supernovas; gravitational waves; the Big Ban; dark energy; and much, much more. “Astrophysics is the concatenation of all of physics,” he says proudly. That makes Introduction to Astrophysics an exploration of the most amazing subject in the universe.

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24 lectures
 |  Average 32 minutes each
  • 1
    Zooming Out to Distant Galaxies
    Begin by defining the difference between astrophysics and astronomy. Then study the vast range of scales in astrophysics—from nanometers to gigaparsecs, from individual photons to the radiation of trillions of suns. Get the big picture in a breathtaking series of exponential jumps—zooming from Earth, past the planets, stars, galaxies, and finally taking in countless clusters of galaxies. x
  • 2
    Zooming In to Fundamental Particles
    After touring the universe on a macro scale in the previous lecture, now zoom in on the microcosmos—advancing by powers of ten into the realm of molecules, atoms, and nuclei. Learn why elementary particles are just as central to astrophysics as stars and galaxies. Then review the four fundamental forces of nature and perform a calculation that explains why atoms have to be the size they are. x
  • 3
    Making Maps of the Cosmos
    Discover how astrophysicists map the universe. Focus on the tricky problem of calculating distances, seeing how a collection of overlapping techniques provide a “cosmic distance ladder” that works from nearby planets (by means of radar) to stars and galaxies (using parallax and Cepheid variable stars) to far distant galaxies (by observing a type of supernova with a standard intrinsic brightness). x
  • 4
    The Physics Demonstration in the Sky
    In the first of two lectures on motion in the heavens, investigate the connection between Isaac Newton's laws of motion and the earlier laws of planetary motion discovered empirically by Johannes Kepler. Find that Kepler's third law is the ideal method for measuring the mass of practically any phenomenon in astrophysics. Also, study the mathematics behind Kepler's second law. x
  • 5
    Newton's Hardest Problem
    Continue your exploration of motion by discovering the law of gravity just as Newton might have—by analyzing Kepler’s laws with the aid of calculus (which Newton invented for the purpose). Look at a graphical method for understanding orbits, and consider the conservation laws of angular momentum and energy in light of Emmy Noether’s theory that links conservation laws and symmetry. x
  • 6
    Tidal Forces
    Why are the rings around Saturn and the much fainter rings around Jupiter, Uranus, and Neptune at roughly the same relative distances from the planet? Why are large moons spherical? And why are large moons only found in wide orbits (i.e., not close to the planets they orbit)? These problems lead to an analysis of tidal forces and the Roche limit. Close by calculating the density of the Sun based on Earth's ocean tides. x
  • 7
    Black Holes
    Use your analytical skill and knowledge of gravity to probe the strange properties of black holes. Learn to calculate the Schwarzschild radius (also known as the event horizon), which is the boundary beyond which no light can escape. Determine the size of the giant black hole at the center of our galaxy and learn about an effort to image its event horizon with a network of radio telescopes. x
  • 8
    Photons and Particles
    Investigate our prime source of information about the universe: electromagnetic waves, which consist of photons from gamma ray to radio wavelengths. Discover that a dense collection of photons is comparable to a gas obeying the ideal gas law. This law, together with the Stefan-Boltzmann law, Wien's law, and Kepler's third law, help you make sense of the cosmos as the course proceeds. x
  • 9
    Comparative Planetology
    Survey representative planets in our solar system with an astrophysicist's eyes, asking what makes Mercury, Venus, Earth, and Jupiter so different. Why doesn't Mercury have an atmosphere? Why is Venus so much hotter than Earth? Why is Jupiter so huge? Analyze these and other riddles with the help of physical principles such as the Stefan-Boltzmann law. x
  • 10
    Optical Telescopes
    Consider the problem of gleaning information from the severely limited number of optical photons originating from astronomical sources. Our eyes can only do it so well, and telescopes have several major advantages: increased light-gathering power, greater sensitivity of telescopic cameras and sensors such as charge-coupled devices (CCDs), and enhanced angular and spectral resolution. x
  • 11
    Radio and X-Ray Telescopes
    Non-visible wavelengths compose by far the largest part of the electromagnetic spectrum. Even so, many astronomers assumed there was nothing to see in these bands. The invention of radio and X-ray telescopes proved them spectacularly wrong. Examine the challenges of detecting and focusing radio and X-ray light, and the dazzling astronomical phenomena that radiate in these wavelengths. x
  • 12
    The Message in a Spectrum
    Starting with the spectrum of sunlight, notice that thin dark lines are present at certain wavelengths. These absorption lines reveal the composition and temperature of the Sun's outer atmosphere, and similar lines characterize other stars. More diffuse phenomena such as nebulae produce bright emission lines against a dark spectrum. Probe the quantum and thermodynamic events implied by these clues. x
  • 13
    The Properties of Stars
    Take stock of the wide range of stellar luminosities, temperatures, masses, and radii using spectra and other data. In the process, construct the celebrated Hertzsprung–Russell diagram, with its main sequence of stars in the prime of life, including the Sun. Note that two out of three stars have companions. Investigate the orbital dynamics of these binary systems. x
  • 14
    Planets around Other Stars
    Embark on Professor Winn's specialty: extrasolar planets, also known as exoplanets. Calculate the extreme difficulty of observing an Earth-like planet orbiting a Sun-like star in our stellar neighborhood. Then look at the clever techniques that can now overcome this obstacle. Review the surprising characteristics of many exoplanets and focus on five that are especially noteworthy. x
  • 15
    Why Stars Shine
    Get a crash course in nuclear physics as you explore what makes stars shine. Zero in on the Sun, working out the mass it has consumed through nuclear fusion during its 4.5-billion-year history. While it's natural to picture the Sun as a giant furnace of nuclear bombs going off non-stop, calculations show it's more like a collection of toasters; the Sun is luminous simply because it's so big. x
  • 16
    Simple Stellar Models
    Learn how stars work by delving into stellar structure, using the Sun as a model. Relying on several physical principles and sticking to order-of-magnitude calculations, determine the pressure and temperature at the center of the Sun, and the time it takes for energy generated in the interior to reach the surface, which amounts to thousands of years. Apply your conclusions to other stars. x
  • 17
    White Dwarfs
    Discover the fate of solar mass stars after they exhaust their nuclear fuel. The galaxies are teeming with these dim “white dwarfs” that pack the mass of the Sun into a sphere roughly the size of Earth. Venture into quantum theory to understand what keeps these exotic stars from collapsing into black holes, and learn about the Chandrasekhar limit, which determines a white dwarf’s maximum mass. x
  • 18
    When Stars Grow Old
    Trace stellar evolution from two points of view. First, dive into a protostar and witness events unfold as the star begins to contract and fuse hydrogen. Exhausting that, it fuses heavier elements and eventually collapses into a white dwarf—or something even denser. Next, view this story from the outside, seeing how stellar evolution looks to observers studying stars with telescopes. x
  • 19
    Supernovas and Neutron Stars
    Look inside a star that weighs several solar masses to chart its demise after fusing all possible nuclear fuel. Such stars end in a gigantic explosion called a supernova, blowing off outer material and producing a super-compact neutron star, a billion times denser than a white dwarf. Study the rapid spin of neutron stars and the energy they send beaming across the cosmos. x
  • 20
    Gravitational Waves
    Investigate the physics of gravitational waves, a phenomenon predicted by Einstein and long thought to be undetectable. It took one of the most violent events in the universe—colliding black holes—to generate gravitational waves that could be picked up by an experiment called LIGO on Earth, a billion light years away. This remarkable achievement won LIGO scientists the 2017 Nobel Prize in Physics. x
  • 21
    The Milky Way and Other Galaxies
    Take in our entire galaxy, called the Milky Way. Locate Earth’s position; then survey other galaxies, classifying their structure. Use the virial theorem to analyze a typical galaxy, which can be thought of as a “collisionless gas” of stars. Note that galaxies themselves often collide with each other, as the nearby Andromeda Galaxy is destined to do with the Milky Way billions of years from now. x
  • 22
    Dark Matter
    Begin with active galaxies that have supermassive black holes gobbling up nearby stars. Then consider clusters of galaxies and the clues they give for missing mass—dubbed “dark matter.” Chart the distribution of dark matter around galaxies and speculate what it might be. Close with the Big Bang, deduced from evidence that most galaxies are speeding away from us; the farther away, the faster. x
  • 23
    The First Atoms and the First Nuclei
    The Big Bang theory is one pillar of modern cosmology. Another is the cosmic microwave background radiation, which is the faint “echo” of the Big Bang, permeating all of space and discovered in 1965. The third pillar is the cosmic abundances of the lightest elements, which tell the story of the earliest moment of nucleosynthesis taking place in the first few minutes of the Big Bang. x
  • 24
    The History of the Universe
    In this last lecture, follow the trail of the greatest unsolved problem in astrophysics. Along the way, get a grip on the past, present, and future of the universe. Discovered in the 1990s, the problem is “dark energy,” which is causing the expansion of the universe to accelerate. Trace this mysterious force to the lambda term in the celebrated Friedmann equation, proposed in the 1920s. x

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Video DVD
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  • Downloadable PDF of the course guidebook
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DVD Includes:
  • 24 lectures on 4 DVDs
  • 360-page printed course guidebook
  • Downloadable PDF of the course guidebook
  • FREE video streaming of the course from our website and mobile apps
  • Closed captioning available

What Does The Course Guidebook Include?

Video DVD
Course Guidebook Details:
  • 360-page printed course guidebook
  • Photos and illustrations
  • Quiz & Quiz Solutions
  • Important Numerical Values

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Your professor

Joshua Winn

About Your Professor

Joshua Winn, Ph.D.
Princeton University
Dr. Joshua N. Winn is the Professor of Astrophysical Sciences at Princeton University. After earning his Ph.D. in Physics from MIT, he held fellowships from the National Science Foundation and NASA at the Harvard-Smithsonian Center for Astrophysics. Dr. Winn’s research goals are to explore the properties of planets around other stars, understand how planets form and evolve, and make progress on the age-old question of...
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Reviews

Introduction to Astrophysics is rated 4.6 out of 5 by 23.
Rated 5 out of 5 by from Wide Ranging I have browsed quantum physics and cosmology for many years, and have wondered about the mathematical underpinnings of Astronomy. This course was perfect for me. There is lots of straightforward math, including a splash of elementary calculus, so much math that I’ve let some of it slide. But it brings the viewer to some deeper understanding, which is what I’m really after, and what I got. At my level the coverage is comprehensive and dense ( I got even more by going through the course a second time). However it now appears that I’m going to have to dive deeper into general relativity. Sigh, does it never end? Oh and Professor Winn is a great lecturer. I highly recommend this course!
Date published: 2019-01-14
Rated 5 out of 5 by from Excellently presented The professor presented the subject excellently. An advanced understanding of physics and mathematics is not required to learn an overview of this complex subject. The graphics and video presentation is excellent also. I will watch again. Very interesting and informative.
Date published: 2019-01-13
Rated 4 out of 5 by from Challenging This course--especially the first few lectures--will appeal to the more mathematically-literate, since it involves math up to and including calculus. The latter part of the course, though still relying heavily on math (mainly algebra), is a more conventional tour of the universe. Non-mathematical viewers will be puzzled many times, but the tour is still worthwhile.
Date published: 2019-01-13
Rated 5 out of 5 by from Loved it! Great astroPHYSICS course This is an introductory astrophysics course, not just an astronomy course. As such, you'll get more out of it if you think back to what your physics and math courses were like, and understand that it's going to be similar. That is, Dr. Winn presents intriguing how and why questions about how the universe works, then goes through the logic of how to answer these questions using physical principles. The transcript book even has a few practise problems that helped keep me thinking after the lectures. I thought the structure of the courses was great, and it was kind of like a "Cliffs notes" version of the Carroll and Ostlie classic text ("Bob") in the subject. If you're not quite ready to jump into physics and all the math that it requires, the Great Courses has some really wonderful introductory astronomy courses. I'd suggest those if you're more interested in a broad survey. I'd suggest this course if you want to begin understanding the fundamental basis for those beautiful patterns we see in the universe.
Date published: 2019-01-13
Rated 5 out of 5 by from Excellent teacher ! Great subject. For a university level student with a solid Science background
Date published: 2019-01-12
Rated 5 out of 5 by from WOW!! I viewed this course on the Plus app. I was VERY happy to see a more current course available. The math did indeed get heavy from time to time but served to help explain the physics. The progress in cosmology is exciting!
Date published: 2019-01-11
Rated 4 out of 5 by from Very interesting material, challenges the brain Professor Winn comes with a killer resume - he graduated from MIT in 1994 with a bachelor's and master's degree in physics (meaning that he likely did those two at the same time), then was a Fulbright scholar followed by stints at MIT before landing at Princeton. However, he approaches the material with a general simplicity that I appreciate. Candidly, I'm not very good at math and I didn't try to solve the formulas Dr. Winn derives in the material, but I got the general point. I had flashbacks to college math ("find x"..."it's right there"), but if you stick through it, this is a good challenge for the brain. And the best part is there is no test at the end with half a page a question for you to map out the calculus. My only recommendation to Dr. Winn is to ditch the pleated pants in the next lecture. Those haven't been a thing since you graduated from MIT with your seven degrees.
Date published: 2019-01-08
Rated 4 out of 5 by from Great Introduction Interesting subject matter with just enough math. Professor Winn's excitement with the subject is evident and contagious.
Date published: 2019-01-08
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