I highly recommend doing the full
edX/ANUx Astrophysics series. It's well worth it. The only one I haven't done yet is Exoplanets. But I can vouch for
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Astrophysics: The Violent Universe; See
Violent Universe intro video
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Greatest Unsolved Mysteries of the Universe; See
Greatest Mysteries intro video
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Astrophysics: Cosmology; See
Cosmology intro video
The Cosmology one is a ripper, but you'll get more out of it if you do the other two first (or at least The Violent Universe), which are also fascinating in their own right. (Expoplanets is more of a specialised area and you could skip it.) Violent Universe just went live again on 1st of March, and it's not too late to join in.
These courses are fairly gentle on the maths compared with other uni physics courses (designed so that year 10+ high school students could follow them). Whatever you need to know is explained clearly as you go by Prof Paul Francis and Physics Nobel laureate Brian Schmidt (of dark energy discovery fame). And even if your high school maths/physics is rusty, there is plenty of help on the forums. Paul Francis goes out of his way to answer everyone's questions, especially in relation to the lessons, exercises and homework problem sets. There will also be other students who already have solid maths and physics backgrounds and are willing to help.
The courses are very well constructed and entertainingly presented. I don't believe there is anything else out there that comes close or takes you as deep at this introductory level. The pace is leisurely: you need to invest only a few hours a week.
(FYI: I am giving you an educated opinion as a physicist who teaches uni level physics, including first year astronomy and honours level quantum mechanics. I keep a close eye on what's happening in the world of online learning; something that I also participate in on all levels: as student, tutor, instructor and course coordinator. I have no affiliation with ANU though or any reason to promote their online courses other than the fact that they are excellent.)