Certain professors at the University of Illinois now have classes of nearly 32,000 students from around the world.

The students are taking the classes via an online learning platform called Coursera. The university is offering seven free online classes this fall. The first class is "Introduction to Sustainability" in the university's School of Earth, Society and Environment.

Professor Jonathan Tomkin says sending an email to thousands of students can be nerve-wracking. Tomkin says he is volunteering to teach the class "as an experiment."

The Urbana-Champaign campus joins Princeton, Stanford, the University of Michigan and other schools offering online classes through Coursera.

The courses won't count toward a degree. But universities may get some revenue from charging students for a certificate showing they completed a course.