Unssolved mysteries of the Universe

I think the teachers here in this free online course won a Nobel prize in astrophysics. The course has just started if any wish to
try it. It's easy to sign up and you do it all free, or if you want a verification of having passed it, it will cost $50 - I just do the freebie thing myself. I have a good job in RF electromagnetic calibration and electronics repair, so I don't care about looking for work, but I do really like learning new things.

https://www.edx.org/course/greatest-unsolved-mysteries-universe-anux-anu-astro1x-0#!

You can also explore on youtube if preferred. https://www.youtube.com/watch?v=LPtXpTB5uao

That sucked me in like the force of gravity.

It starts out a little simple and a lot interesting, but it gets complex very fast. If you join, start out by going to courseware. Then on to https://courses.edx.org/courses/course-v1:ANUx+ANU-ASTRO1x+2T2015/courseware/13c05bb6f1e6422a9b57b06a6c8a4b6a/ea37bc82c3cb4ffba37c0b091b1aa1c6/

Wow, just came across the first practice question. It was kind of tough! To do it, we have to know the formula to calculate Luminosity which we have to derive from the formula for flux.

f=L/(4*PI*D^2) - where f=flux, L=Luminosity, and D =Distance.

PQ1.3 FLUX TO LUMINOSITY  (1/1 point)
Imagine that you are an astronomer studying a star which is 4 parsecs away (a parsec is 3.09x1016 m). You measure the flux coming from this star with your telescope, getting a value of 2.3x10-12 W m-2.

What is the luminosity of this star?

correct
4.42×10^23
4.4e23
EXPLANATION

Flux is equal to luminosity divided by 4 Pi D2. Rearranging, we find that luminosity is equal to 4 Pi D2 times the flux. D=4 times 3.09x1016 m - square this, multiply by four Pi and then by the flux. The answer you should type in is 4.42e23.

Took me a while to get this correct. Actually I got 4.41545E+23

In Excel 3.141592654 Pi
12.56637061 4Pi
3.09E+16=D                     3.88301E+17= 4PiD
1.236E+17= 4D                 2.3E-12= f
1.5277E+34= D^2             4.41545E+23 derived from formula f4PiD^2= L= (answer for L)
1.91976E+35= 4PiD^2

The way they keep changing the parameters is unnecessarily complex.
If you are going to work with angles, then keep it in radians the whole way through.
If you are going to work with distances, then keep it in meters the whole way through.
Its like they've got nothing better to do than deliberately make it more complex than it needs to be.
The basic idea of keeping it as simple as possible has been lost.
Its a bit of a time-waster.

WOW!! This is great! I didn't know about edX. Looking forward to getting started.  Thanks for the heads-up.