Celebrated astronomer Carl Sagan, who took millions of television viewers on a tour of the universe in his "Cosmos" series, kicked off National Science Week in the D.C. public schools yesterday with a lecture at Cardozo High School.

Sagan, the son of a Brooklyn garment industry laborer and a housewife, said his journey toward becoming a famous astronomer and author was aided by two essential things -- he was interested in science at a young age and "my parents never discouraged me."

Often, Sagan said, children with an aptitude for scientific knowledge are criticized by their peers and parents and eventually pursue other subjects.

"My general sense is that kids start out being naturally interested in science. In the first grade, everybody is a scientist. Kids want to know, 'Why is the grass green? Why is the earth round?' These are deep scientific questions, but when you grow up, people start telling you, 'Don't ask stupid questions.' "

Talking about topics ranging from the formation of planets to the threat of nuclear war, Sagan seemed to highlight the essential purpose of National Science Week, to "increase public awareness and understanding of science and technology," according to a spokesman for the National Science Foundation, which sponsors the annual, nationwide event. Corporate sponsors are DuPont, Kodak, General Electric and IBM, the spokesman said.

The National Academy of Sciences, which adopted Cardozo as part of the White House Partnership in Education Program, sponsored Sagan's lecture.

Monica Fields, a 17-year-old senior at Cardozo, said she was "excited" and "intrigued" by Sagan. "I had a lot of questions about the planets, the solar system and black holes, and he seemed to answer a lot of them.

"I didn't know that Saturn was made of rocks and ice and that the rings around it were made of millions of snowballs . . . that's fascinating," she said.

Cardozo Principal James Williams said: "This is probably one of the most important events we've had all year. Sagan's lecture was nonemotional and highly intellectual. It's the kind of thing that our students are going to have to get used to when they go to college."