As of September 12, 2016, Google Hangouts on Air will no longer be available through Google+. Go to YouTube Live to schedule a live event.
How does this differ from the previous version of Google Hangouts on Air? The Q & A, Showcase, and Applause apps are discontinued.
Hangouts On Air moving from Google+ to YouTube Live – announcement and instructions for scheduling a live event in YouTube Live.
How to Use YouTube Live Streaming for Free Lecture Capture – from the Chronicle of Higher Education.