Roughly two minutes later, before his drink was ready, he was recognized anyway. Two awed young women approached to ask if he was really Bill Nye the Science Guy. Like more than a dozen other college students who would approach him over the next several hours, they asked if they could take a picture with him. He smiled, took a proffered iPhone, scooched the students in and, in a practiced gesture, stretched out his arm to take a shot of the three of them that you just knew was totally going on Facebook.

Mr. Nye had come to talk to them, and a few thousand of their friends, at Iowa State University. If he were a politician, college students would be his base. Instead, he is something more: a figure from their early days in front of the family TV, a beloved teacher and, more and more these days, a warrior for science. They, in turn, are his fans, his students and his army.

They have gone from watching him explain magnetism and electricity to defending the scientific evidence for climate change, the age of the earth and other issues they have seen polemicized for religious, political and even economic reasons.

He takes on those who would demand that the public schools teach alternative theories of evolution and the origins of the earth — most famously, in a video clip from the site BigThink.com that has been viewed some five million times. In it, he flatly tells adult viewers that “if you want to deny evolution and live in your world — in your world that’s completely inconsistent with everything we observe in the universe — that’s fine. But don’t make your kids do it, because we need them. We need scientifically literate voters and taxpayers for the future.”