"It does not harm the romance of a sunset to know a little bit about it."
Before you ask, he wasn't involved with a speakeasy explosion in 1931. Now, moving on...
Carl Sagan (Born November 9th, 1934 – December 20th, 1996) was an astronomer, astrophysicist, astrobiologist, philosopher, and author who through his various books, including one screenplay which would become the film Contact, and the TV Series Cosmos, became widely known as "The People's Scientist". Apart from his popularizations of science, he also contributed to research in the fields of planetary science, spacecraft exploration of the solar system, and the radio Search for Extraterrestrial Intelligence (SETI). He also developed theories on topics such as the nature of extraterrestrial lifeforms, the environmental effects of nuclear war (nuclear winter), and more.
In mainstream, he's somewhat of a star on Youtube; he's the centerpiece of the Symphony of Science videos, which auto-tunes Sagan's Cosmos along with other science programs to create
unexpectedly awesome music videos, which total at about 8 million views. His Pale Blue Dot speech is also a big hit, the two most watched versions having 1.7 Million views combined. His work, although the bulk of it was done 30+ years ago, still stands tall today within science, an incredible accomplishment.
Apart from his science-related work, he was also an activist to legalize marijuana, of which he was a regular user, and was very strongly opposed the proliferation of nuclear weapons, even getting arrested in 1986 for interfering in a nuclear test. He was also a founder of the Committee for Scientific Investigation of Claims of the Paranormal (CSICOP), which sought (and continues to seek, as the Committee for Skeptical Inquiry) to debunk most forms of paranormal claims (enlisting the assistance of James Randi and others). He was thrice married, first to biologist Lynn Margulis, then to artist Linda Salzman, and finally to author Ann Druyan, with whom he co-wrote Cosmos and several of his books.
Works he has been involved in include:
- Cosmos (Co-writer, host)
- Contact, both the novel and the movie (Writer)
- Music from a Small Planet (with Ann Druyan), a Radio documentary about the Voyager Interstellar Record 
- Threads (Scientific adviser)
- Symphony of Science (He's credited as artist on a released single)
Books by Sagan include:
- The Dragons of Eden: Speculations of the Evolution of Human Intelligence - 1978
- Broca's Brain: Reflections on the Romance of Science - 1979
- Cosmos - Book form, 1980
- Comet - With Ann Druyan, 1985
- Contact - Book form, 1985
- Shadows of Forgotten Ancestors: A Search for Who We Are - With Ann Druyan, 1993
- Pale Blue Dot: A Vision of the Human Future in Space - 1994. Video excerpts are pretty common online.
- The Demon-Haunted World: Science as a Candle in the Dark - 1996
- Billions & Billions: Thoughts on Life and Death at the Brink of the Millennium - With Ann Druyan, 1997
His major scientific contributions include:
- Providing research and evidence crucial to our modern-day knowledge of conditions on Venus.
- Initiating groundbreaking research on extraterrestrial life, including contributions to SETI (Search for ExtraTerrestrial Intelligence).
- Contributing to the Galileo (Jupiter), Viking (Mars), Voyager (Outer Solar System), and Mariner (Venus) probe missions.
- Advocacy for humankind's expansion into space. Was one of the founders of The Planetary Society, a space advocacy group known for, among other things, its work on solar sail propulsion.
- Warning about the dangers of anthropogenic climate change and nuclear war.
Tropes often invoked by Sagan include:
- Arc Words: Coined and repeatedly used some phrases, such as "technological adolescence", throughout his works.
- Beam Me Up, Scotty: "Billions and billions!"
I never said it. Honest. ... I said "billion" many times on the Cosmos television series, which was seen by a great many people. But I never said "billions and billions". [A page later.] For a while, out of childish pique, I wouldn't utter or write the phrase, even when asked to. But I've gotten over that. So, for the record, here goes: "Billions and billions."
- Sagan claimed he always pronounced "billions" with a strong, plosive B on the front so as to emphasize the fact that he wasn't just talking about mere millions. He also got rather annoyed when he did this at public speaking events and heard giggles from the audience.
- Cool Teacher: Sagan tried to be this to the world, and for the most part he succeeded. More specifically, many of his own students are now celebrity scientists in their own right. (Neil De Grasse Tyson might be the best example.)
- Deadpan Snarker: From time to time.
We associate radio waves with intelligent life... or at least semi-intelligent life; we have radio waves...
- Emotional Torque: With regards to the real world, that too.
- Erudite Stoner: Had toked many a joint in his lifetime. His wife Ann Druyan carries out his legacy as a popularizer of science and as president of the NORML Foundation Board of Directors.
- First Contact
- Humans Are Special: In a certain sense.
- Insignificant Little Blue Planet: Played with. On the one hand, he got a spacecraft, out almost 4 billion miles, to turn around and actually take a picture of the Earth as the eponymous Pale Blue Dot, and repeatedly argued that we shouldn't consider ourselves 'privileged' or 'special' over any other group of humans, or any other species, merely by birth. On the other hand, he spoke of humanity's potential to spread out into space and seek out its bearings in the cosmos, and that, while our homeworld Earth may be a tiny blue dot, it is the only place known to have evolved and sustained life, and there are no 'better places', so far at least.
- Layman's Terms
- Large Ham: Sometimes comes across as this in his documentaries.
- The Obi-Wan: Inspired Neil De Grasse Tyson and taught Bill Nye at Cornell. And inspired and taught countless others as well.
- Inspired the Bit.Trip series as well.
- Measuring the Marigolds: Also repeatedly debunked and strongly inverted.
- Patrick Stewart Speech: Several times, and eloquently delivered. May have even been the inspiration for the Trope Namer's speeches in Star Trek: The Next Generation, which aired 7 years after Cosmos.
- Romanticism Versus Enlightenment: Related to the above, a strong subversion. As indicated by the page quote, he believed that understanding the Universe (Enlightenment) did not detract from and even enhanced one's appreciation for nature (Romanticism).
- Science Hero: Himself.
- The Scientific Method
- Space Is an Ocean: Although the trope itself is averted, Sagan did make poetic analogies between the oceanic voyages in the Age of Exploration to the modern day exploration of the space - he called Earth the 'shore of the cosmic ocean', and repeatedly referred to spacecraft, of all kinds - from small robotic probes like the Mariners and Voyagers, to grand hypothetical Bussard Ramjet interstellar designs - as "ships". Also, the Planetary Society's logo - a sailing ship against the backdrop of a ringed planet.
- The episode of Cosmos about the Voyager space probes ended with an image of the Voyager morphing into a wooden sailing ship.
- Speculative Documentary
- Spiritual Successor: Neil De Grasse Tyson.
- Take That: Directed against Sagan by Apple Computer. Near the height of Sagan's popularity in the mid 1990s, Apple had three tiers of the new Power Macintosh design they were working on, which would eventually be marketed as the 6100, 7100, and 8100. During development, as is common practice before the marketing names are settled upon, they had code names. The 6100 was "Cold Fusion," the 7100 was "Carl Sagan," and the 8100 was "Piltdown Man" -- in other words, Sagan's name was grouped together with a known scientific hoax and a suspected (at the time) scientific hoax. When Sagan complained, they changed the code name for the 7100 to "Butt-head astronomer." (Sagan later sued them for this second slight; the judge threw the case out, saying "One does not seriously attack the expertise of a scientist using the undefined phrase 'butt-head'.")
- The World Is Just Awesome: The former Trope Namer, through Contact
- Voice Clip Song: Symphony of Science