Carl Sagan's passion was simple to define: it included the entire universe.

But that passion would make an impression on one influential television viewer: Johnny Carson.

By the early 1970s, Sagan (1934-96) was an accomplished scientist and author. With his book "The Cosmic Connection" a few weeks from release, his publisher, Doubleday, had booked Sagan on a Dick Cavett TV special. Carson was watching and immediately told his staff he wanted Sagan as a guest on his "Tonight Show."

"(Sagan) was passionate about astronomy and science in general — and was able to convey that passion to the general public without sounding condescending," Carson said, as quoted by William Poundstone in "Carl Sagan: A Life in the Cosmos."

The exposure from "The Tonight Show" and its 15 million viewers at that time helped make Sagan a household name. More importantly, it gave him the platform to help educate and excite the general public about the importance of understanding the universe.

After Sagan's first, short appearance on "The Tonight Show" in November 1973, Carson had him back three weeks later for what turned out to be a defining moment. Sagan explained the connection between the history of the universe and the rise of life.

"Sagan launched into a cosmological crash course for adults. When Sagan finished and settled back into the eye of the hush that he had generated, one was willing to bet that if a million teenagers had been watching, at least 100,000 vowed on the spot to become full-time astronomers," wrote Stuart Baur of New York Magazine.

At the time of his death, Sagan was the David Duncan Professor of Astronomy and Space Sciences and director of the Laboratory for Planetary Studies at Cornell University. An authority on planetary systems, he played a leading role at NASA from the 1950s until his death, and was a prime mover in the foundation of the Search for Extraterrestrial Intelligence (SETI).

Sagan published more than 600 scientific papers and more than 20 books, including "Cosmos," which became the best-selling book about science in English and became an Emmy Award-winning series. Sagan's studies of Venus led to the discovery that the plant suffered from a massive greenhouse effect, and he was involved in the Mariner, Viking, Voyager and Galileo unmanned expeditions.

His many awards included the Public Welfare Medal, the National Academy of Sciences' highest award for "distinguished contributions in the application of science to the public welfare." Sagan also received the NASA medals for Exceptional Scientific Achievement and Distinguished Public Service, in addition to the NASA Apollo Achievement Award.

A Fascinating Story

Yet Sagan's big thrill lay in showing people the importance of the science of astronomy while making it interesting and understandable.

"My only secret in being able to talk to others about science is to remember what it was like when I didn't understand whatever it was we were talking about," Sagan said in "Carl Sagan," by Ellen R. Butts and Joyce R. Schwartz.

That was Sagan's primary aim with the 13-part series "Cosmos." He took what some might consider "dry" scientific facts and turned them into a fascinating story about the origins of the universe. Since its first airing in 1980, "Cosmos" has been seen by more than 500 million people in 60 countries, making it the most-watched series in the history of public television.

Young Carl was the child of parents who had little financial means but put a premium on education even though neither graduated from college. They filled their Brooklyn home with books and taught Carl and his sister to appreciate reading, music and art.

They also tried to fire his imagination. In 1939, when the World's Fair and its theme of the future came to Long Island, his parents took 5-year-old Carl. Sagan remembered it as "a day that powerfully influenced my thinking — a perfect future made possible by science," he said.

"Carl was extremely well-read, and as a result he was able to discuss almost any subject. He had this huge reservoir of memories of interesting events in the history of science, pithy quotations from writers and such that he could call on and make his talks and books interesting," said Dr. Frank Drake, director of the SETI Institute's Center for the Study of Life in the Universe and SETI's chairman emeritus.

"He didn't talk down to people. Carl used a vocabulary they could understand, and he always made it interesting," Drake said. "He always found some interesting twist in whatever it was that he was describing, and so people felt comfortable listening to him."

Sagan also took criticism of his work in stride: he believed good science included arguing for and against theories.

In fact, Sagan was extremely gracious to many of his harshest critics. When Nobel Prize winner Harold Urey wrote a letter apologizing to Sagan for past unkind comments directed at him, and feeling especially bad because Sagan had recently praised him in a speech, Sagan wrote back to him:

"I've thought as carefully as I can, and I can't recall a single instance of unkindness from you. If you are thinking of some anonymous activity — such as refereeing a paper or giving you opinion on promotion — it is no unkindness to express a candid view under such circumstances. But whatever the incident was, I cannot believe that your motive was anything other than the best interest of science."

'Ask Courageous Questions'

Sagan stood up for what he believed in. A strong supporter of women's rights and civil rights, he refused to back down even when some of his superiors warned him that it could cost him professionally.

He often pursued unpopular opinions. Some fellow scientists scoffed at his research on nuclear winter, but Sagan kept at it, backing up his theories with hard data. It later became apparent that his presentation to both U.S. and Soviet officials on the subject and its catastrophic consequences was significant.

Mikhail Gorbachev told Sagan personally that he'd read all of his nuclear winter research, and that it had strengthened the Soviet resolve to ensure the nightmare scenario never played out.

"Sagan's greatest coup was his 1986 briefing of the Soviet Central Committee on nuclear winter," Poundstone wrote. "Several who were present have said that the effect of Sagan's talk on Soviet military thinking was profound."

As a scientist and person, intellectual curiosity ran through Sagan's veins. He even set up a system he called his "Baloney Detection Kit," which includes insisting on independent confirmation of facts, hearing all points of view, and being careful not to became attached to a theory simply because it's yours.

"Ask courageous questions. Do not be satisfied with superficial answers," Sagan said. "Be open to wonder and at the same time subject all claims to knowledge, without exception, to intense skeptical scrutiny. Be aware of human fallibility. Cherish your species and your planet."

BY MICHAEL MINK