This is the best series of astronomical courses on the Internet. The teaching style, the mysteries (Yes, the mysteries in which you're investigating a made-up universe and play around with it! Or your job is to try to save the world!), the homework sets - everything was perfect. The easiest thing is to take these courses in order, so that Cosmology doesn't surprise you.
After these courses, I gained so much interest, so I switched a bit from my theoretical mathematics background to astrophysics, and I'm loving it in here. So, it can really change your life. This is the best thing.