<https://zh-tw.coursera.org/learn/black-holes-astro-101#mooc>
<http://www.w3.org/1999/02/22-rdf-syntax-ns#type>
<http://schema.org/Course>
<https://zh-tw.coursera.org/lecture/black-holes-astro-101/04-01-introduction-sizing-up-black-holes-r1jZ7>