Wednesday, March 17, 2021

An Online Class in Astrophysics

I recently completed a survey in astrophysics offered by the École polytechnique fédérale de Lausanne through edX, a program created by Harvard and MIT and now involving many other institutions. I recommend this class with serious reservations. If you want a structured experience in learning astrophysics on your own, this can help. I found the content informative, challenging, and edifying. However, the presentation was often marred by careless transcription and poor translation of the lectures. 

 I do grant that as listed, Physics 209, this is about what I would expect from an American university for a sophomore class in physics, though for non-majors. Calculus is at a minimum here. Once a week or so, I did spend an hour with one math problem among three or four multiple choice. If you did well at “Conceptual Physics” and have a head for algebra, this is a solid survey of topics in astrophysics. 


Nominally, the course takes seven weeks to work through seven chapters which are presented as 27 lectures and quizzes. Each lecture is about 20 to 35 minutes. So, this is about a fourth of a semester of effort, all in all. There is a thin textbook that you can download as a PDF. I found it helpful and referred to it often. 


I took the course for certification. So, I paid the registration fee of USD 139. Personally, I need that kind of motivation. If I had not been financially invested in the outcome, I would have walked away from it—which I also considered. More than once, I almost cut my losses and left the money on the table. Instead, I toughed it out and actually earned the certificate of completion a little more than halfway through the class because at that point, it was arithmetically impossible for me to fail. I viewed all of the lectures and completed all of the quizzes. They drop your three lowest scores. With that, I averaged 90%.


The grading system is simple. Each video is followed by several quizz, either multiple choice questions or questions requesting a numerical answer. In most cases it is possible to try 2-3 answers before giving the final answer. You qualify for the certificate with at least 50% of correct answers for at least 19 out of the 22 quizz that we propose. 


When a numerical answer is required, usually, a 10% error bar is included in the calculation, more if it's an order of magnitude estimate.


Week 1: General introduction - Kepler's Laws - Virial theorem.

Week 2: Radiation processes - Line radiation - Black body - Measuring radiation.

Week 3: Doppler-Fizeau effect and astrophysical applications - Interstellar and intergalactic radiation - Strömgroen sphere - Absorption/emission - Color index - Tidal forces. 

Week 4: Roche limit - Comets - Planetary energy balance - Planetary atmospheres - 

Week 5: Stellar formation - Stellar classification - Stellar evolution. 

Week 6: The galaxies - Rotation of the Milky Way and Oort constants - Dark matter.

Week 7: Fundamentals of cosmology - Distance ladder - Gravitational lensing.


Length: 7 Weeks

Effort: 3–4 hours per week

I kept two notebooks, in fact. The first is set of Word files made from the lecture notes provided. The other is a spiral bound that I also have for working astronomy problems from books that I buy or borrow from the library. I captured the solutions to all of the answers that I missed and some that I guessed right and put those in either or both as needed.


First, and foremost, I do not have the mindset of a physicist or I would have become one a long, long time ago. So, for some problems, I had to see how it was done, what approach was needed, which contexts were relevant, where the equations of solution had to come from. So, that was learning. I missed a couple of others just because I did not understand what was being asked. 


Just below halfway on the left
find "stronger in radius"
for "Strömgren radius."

I also invested a lot of time into correcting the transcripts of the lectures. The English language speaker did not understand the material he was reading. Often, he spoke “v” for the Greek letter nu and “p” for rho, and so on. Sometimes he left symbols out entirely. Once, he spoke “proton” for “photon.” The course was replete with such problems. In one way, the careless transcription of text gave me the opportunity to read and review the lecture in detail. I formatted paragraphs and formatted equations. The fact remains that some lecture notes lacked any punctuation.


A more subtle difficulty was in the differences between sentence structure in French and English. The English speaker paused when the professor did, even though the thoughts presented as subordinate clauses, parenthetical comments, noted asides, or dependent clauses were strung differently in the two languages. 


Despite the fact that the course was supposedly monitored by a professor and three assistants, in point of fact, no one monitored the course. When I finally tried to send an email the EPFL coordinator, the message bounced as undeliverable. So, you are on your own here. 


Through a YouTube channel created by a maths boffin named Tibees. (Tibees

An MIT final exam in astrophysics”), I found MIT's open courseware. She misidentified this as a final examine in astrophysics from MIT. I followed the links and found that it was merely plain old astronomy, which, apparently, at MIT is astrophysics. (See here.  And they have a lot more if you want to work on your own through a structured course.) Anyway, most of these topics were touched on by at least one quiz question in the EPFL course though in much less depth.




Steven Weinberg on Gravity Waves 

Measuring Your Universe: Alan Hirshfeld’s Astronomy Activity Manual 

Newton and Leibniz 

The Solstice Seasons 

Seeing in the Dark: Your Front Row Seat to the Universe 


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