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Announcements • Turn in Homework 14 (5 questions) • Today is the deadline for the Monty Python challenge! • Observatory session tonight, weather permitting • Review session on Friday • Final Exam next Monday or Tuesday: 2 hours, 100 points, comprehensive, with an emphasis on galaxies and cosmology
Book Recommendation 365 Starry Nights, by Chet Raymo My favorite guide to the constellations: hand-drawn, grouped by season, with a miniature lesson on mythology or astronomy for each night of the year.
Course Outline ü Naked-eye astronomy ü Crash course in physics ü Our solar system ü The stars ü Structure and history of the universe
The Observable Universe Galaxy Dark era Quasar Background radiation (3000 -degree ionized gas)
We can use redshifts to map the universe… You are here!
The Outer Solar System Neptune Uranus On this scale: 1 A. U. = 7 pixels Saturn Jupiter Orbital radii: Jupiter 5. 2 A. U. Saturn 9. 5 A. U. Uranus 19 A. U. Neptune 30 A. U.
The Inner Solar System Mars Earth Venus Mercury On this scale: 1 A. U. = 120 pixels Diam. of Sun = 1 pixel Sun Diam. of moon’s orbit = 1/2 pixel Diam. of earth = 1/100 pixel
Earth and Moon to scale
Life in the Universe 6 December 2006
Today: • History of life on earth • Search for life in the solar system • Life in other solar systems? How to tell? • Why aren’t they here (or are they)?
History of Life on Earth • Bacteria (simple cells) > 3 billion years ago • Complex cells about 1 billion years ago • Multicellular life 700 million years ago • Humans < 1 million years ago • Agriculture 10, 000 years ago • Radio communication 100 years ago • Space travel 45 years ago
The planets, to scale
The Terrestrial Worlds
Life on Mars? • Past robotic visits found no clear sign of life • Allen Hills meteorite inconclusive • Robotic exploration has discovered pretty good evidence for past surface water
Europa: Water beneath ice Water is probably kept warm by tidal friction. Could this be a place to look for life?
Titan (Saturn’s largest moon) Opaque atmosphere of nitrogen, methane, smog. Surface (cold!) could have liquid methane, other hydrocarbons. Huygens probe landed in Jan. 2005.
What about other solar systems? • Planets seem to be common around other stars • Hot, massive stars probably don’t live long enough for life (as we know it) to develop • Cool, low-mass stars are unlikely to have earth-like planets with liquid water • Best bet: sun-like stars, which are still fairly common (perhaps 1010 in our galaxy). • But we can’t go there (in the foreseeable future), so how can we learn if there’s life?
3 ways to look for life in other solar systems: • Look for clues in spectra. For example, an atmosphere with abundant oxygen would probably indicate the presence of life, and may be detectable with future instruments… • Look/listen for artificial signals from extraterrestrial civilizations… • Speculate…
Search for Extraterrestrial Intelligence Jodi Foster in “Contact” (heard a message from an alien civilization and got to go for a visit!) Dr. Jill Tarter, SETI Institute (hasn’t heard any messages yet!)
Speculations are easy… • Simple life may be relatively common in the universe… • Multicellular life is probably much less common, since it may require rather rare conditions (stable environment for billions of years)… • Intelligent life is probably still less common… • Technological civilizations comparable to ours would be rarer still, but may develop far superior technologies in a cosmic nanosecond… • What are the chances that we would discover such a civilization, or that they would discover us?
Or have they? (And if you believe everything you see on Fox…)
Seriously, why aren’t they here? • Maybe they aren’t there! • Or maybe interstellar travel is just too difficult… • Or maybe they just aren’t interested.
The bottom line? We really have no idea whether life exists elsewhere, let alone intelligent life or advanced civilizations. Everyone seems to have an opinion, but by and large, these opinions are based upon personal wishes, not facts. Part of being “scientific” is setting personal wishes aside and admitting when you just don’t know.