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A respected astronomer and dogged critic of pseudoscience, Carl Sagan is best known for his enthusiastic efforts at popularizing science and famous for allegedly saying billions and billions of stars...

A Sagan is a humorous unit of measurement equal to at least 4,000,000,000.

The unit is derived from the phrase billions and billions (of stars), attributed to the American astronomer Carl Sagan. The minimum lower bound of a number conforming to the constraint of billions and billions must be 2,000,000,000 plus 2,000,000,000 or 4,000,000,000, assuming, of course, that we attibute Sagan to using the American English definition for billion.

Note that though many believe Sagan was famous for using the phrase billions and billions, he claims he never actually did, having used instead the simpler phrase billions of stars.... It was actually Johnny Carson, impersonating Sagan, who popularized the phrase attributed to Sagan. (ref. Billions & Billions, Carl Sagan, Random House, 1997).