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An environmental superstar and a physicist who reaches for the stars are scheduled to make presentations here next week. Both programs are free and open to the public.

• Bill McKibben, author and activist, and scholar in residence in environmental studies, Middlebury College educator — With The End of Nature (1989), he was one of the first people to warn a general audience about climate change. In Eaarth (2011), he says we need to acknowledge that we've waited too long, and that massive change is not only unavoidable but already under way.

He is a founder of the grass-roots climate campaign, 350.org, which has coordinated 15,000 rallies since 2009. He organization stepitup07.com, founded in 2007, issued a demand to Congress: Enact curbs on carbon emissions to cut global warming pollution 80 percent by 2050.

With six college students, he organized 1,400 global warming demonstrations across the country on April 15, 2007 — considered the largest day of protest about climate change in U.S. history.

Hosted here by the and . 9:30-10:30 a.m. Monday (April 16), Conference Center Ballroom. RSVPs are required, and can be arranged .

• Lisa Randall, professor of physics, Harvard University — Presenting a lecture based on her book Knocking on Heaven's Door: How Physics and Scientific Thinking Illuminate the Universe and the Modern World (2011), described as an exhilarating and accessible overview of recent developments in physics, and an impassioned argument for the significance of science.

Randall, best-selling author of Warped Passages (2006), is an expert in both particle physics and cosmology. In her new book, she explores how we decide which scientific questions to study and how we go about answering them. She examines the role of risk, creativity, uncertainty, beauty and truth in scientific thinking, and explains with wit and clarity the latest ideas in physics and cosmology.

Hosted here by the and the . 8 p.m. Tuesday (April 17), Activities and Recreation Center Ballroom. .