End-of-Term Projects In class, May 14th for both presentations and reports Reports: ~12 pp on material which is relevant to course Presentations: ~20min, hand in a ~1p summary Worth 10 pts (5 assignments) Grades due May 17th; NOTHING ACCEPTED after class time on 14th. Can pick up marked copies from my mailbox
Search for Life In the Universe: `Best Of' Edition. Clip Show #1: The Science behind the Search
Science in a Nutshell Observations (reality) Consequences(predictions) Explanation(theory) Tests
Science in a Nutshell The universe is understandable Scientific knowledge is forever Scientific ideas are subject to change Science demands evidence Science explains and predicts
Careful Observation Careful observation is the beginnings of all science. Observations can be of the world as it is or of carefully set up situations to see what happens (experiments) In some sciences experimentation isn't possible (astronomy) or is limited (human behavior), and only observations are feasible Making careful observations isn't as easy as it may seem.
Overthrow-ability of Theories Theories can be disproven by finding evidence which contrdicts them. (Evidence itself must be verified; data might be wrong) Any new theory must explain everything that previous theory did, plus the new evidence.
Complexity in Life Even the simplest life form has a lot more going on than even fairly complex non-alive things
Chemistry of Life Chemistry has as building blocks the elements around us. Big bang: produced mainly Hydrogen, Helium Sun: Everything's Hydrogen, Helium, Oxygen, Carbon, small traces of other stuff – Pop I star Earth: Richer in heavier stuff (Iron, Silicon,...) Other stars, planets likely to be similar Relative abundances of (Hydrogen, Helium) and (Carbon, Oxygen, Silicon, Iron...) may vary Of these building blocks, what chemicals can build complex chemistry?
Chemistry of life: Carbon Of plentiful stuff, Carbon (alone) can build very complex molecules
Chemistry of life: Water? Water is essential to life on earth Thought that life began in the water (more on that in later lectures) For chemical life, need a way to get chemicals to different part of the body Water is a very powerful solvent. Dissolve chemical Allow transport through body in liquid form Very few liquid solvents as powerful, or as common.
Limitations of water If life depends on water, then very strict limit on where life can be Can't be anywhere colder than freezing Can't be anywhere hotter than boiling Other liquid solvents have different freezing/boiling points, but problem remains Hard to see how chemicals can be efficiently transported through a body otherwise
Water On Mars Current Mars rover saw `spherules' (`blueberries') in many places Could indicate water Form as `concretions' Speck of something in water Other sediments build up Same process as pearls, snowflakes, raindrops But other possibilities
Water On Mars Other evidence: rock formations Suggestive of water erosion In particular, spherule formed inside crack in rock Evidence that spherule did form through concretion Cracks seem to be from water rivulets
Water On Mars More evidence; build up of sulfates, other salts on surface Very strong evidence that water laced with minerals was flowing When evaporated, left these minerals behind
Water On Mars No evidence yet of life Water would seem to be a necessary ingredient No evidence for oceans, or that Mars was warm enough to have liquid water for long time
What sort of life are we interested in? In our own solar system, even bacteria fossils would be enormous find. Have some chance of looking at Mars, other nearby planets For life outside our solar system, won't be able to visit for foreseeable future Only way to recognize life is to receive signals Life must be intelligent enough to communicate with us in a way we can recognize
The Distance Ladder • Four `realms' of distance in exploring Universe • Solar system • Nearby stars • Galactic distances • Intergalactic distance • So vastly different that each needs different techniques, units to measure distances
Solar System • Can use direct observation, simple geometry to measure solar system distances to reasonable accuracy • These were available to the ancients • More modern techniques (radar, spacecraft..) allow increased accuracy
New length unit: Astronomical Unit (AU): • Earth-Sun distance so handy for measuring solar system distances that new unit created: • 1 AU = mean distance between Earth and Sun • 1 AU = 92,955,807 miles • Can use this information to work way up to next realm: nearby stars
The displacement is measured as an angle on the sky • 0.5 degree: about your thumb at arms length • 1 arc minute: 1/60th of a degree • 1 arc second: 1/60th of an arc minute • A distance at which the parallax (from Jan to Mar) is 1 arc second is a parsec (PARallax SECond) • Can find it from 1 AU with some trig • 1 pc = 206265 AU
Distances to distant clusters of stars • Clusters also contain stars such as RR Lyrae or Cephieds • Variable stars • `Pulse’ over days • Pulsation period tells you their brightness • Bright enough to be seen in quite distant clusters
Distances to nearby galaxies • Cepheids can even be seen in our galactic neighbors, so can measure distances to galaxies directly!
Electromagnetic Radiation • All these observations are made with light, or some other form of electromagnetic radiation • Electromagnetic radiation from a source is in the form of waves • Both Electric and Magnetic components • Wave travels at speed of light
Inverse Square Law • Electromagnetic (and most other kinds) of radiation obey the Inverse-Square Law • Intensity of radiation (brightness) falls off with the square of the distance • Doubling the distance to something makes it appear four times as dim (¼ as bright) • Tripling the distance makes it appear nine times as dim (1/9 as bright) • etc.
Electromagnetic Waves 15” ~9' TV Antenna VHF: ~200 MHz; wavelength~60” UHF: ~575 MHz; wavelength~20” ~4.5” CB Radio Antenna ~27 MHz; wavelength~ 36 ft Satellite TV dish ~12 GHz; wavelength ~9”
Electromagnetic Waves • Light is one facet of the entire electromagnetic spectrum • Our eyes have dedicated cells which are sensitive to electromagnetic radiation in this range • `Antenna' sensitive to light • Eyes most sensitive to yellow light – this is where the sun emits the peak amount of energy
What Generates Electromagnetic Waves? • Thermal radiation: Hot things glow. • Heat causes atoms to rattle about in an object • Atoms contain charged particles (electrons, protons) • Accelerating charged particles emit electromagnetic radiation. • Other processes • Nuclear reactions • Magnetic fields interacting with charged particles
Thermal Radiation • If material is dense enough to be opaque, hot body emits radiation in a characteristic `blackbody' spectrum High frequency Low frequency Short wavelength Long wavelength • Hot objects emit more and at shorter wavelengths (higher frequencies)
Line Spectra • For non-opaque materials, spectra can look quite different. • Atoms/molecules can emit or absorb photons only of particular energies. • If dense enough, these lines get blended out into blackbody spectrum • If not (like gas in flame) the spectrum is composed of lines
Solar Spectrum • Central region of sun fairly dense • Emits as blackbody • Outer layers progressively less dense • Line effects start becoming noticeable • We see continuum blackbody spectrum from the inner star with absorption features from the outer layers hot core Wispier outer layers
Solar Spectrum Solar Spectrum Calcium Oxygen Molecules Sodium Hydrogen
Doppler Shift in Light • Sound or light from a source moving towards you is shifted to higher frequencies (light is bluer) • From a source moving away from you, shifted to lower frequencies (redder)
Doppler Shift in Light • Effect is fairly modest, but spectra can be measured very accurately • Astronomers can measure velocities towards/away very precisely
The Drake Equation • Drake Equation structured the class until now • Astronomy • Number of stars in galaxy • Number of suitable stars • Number of stars that form planets • Geophysics • Number of planets suitable for life • Biology • Where and low life forms on those planets
Spiral Galaxies • Flat, disk-shaped galaxies with spiral arms • Rotate (our part of our galaxy rotates around the center every ~200 million years) • Gas clouds, dust, stars
Elliptical Galaxies • Spheroidal • Featureless • Much brighter in core than in outer regions • Often the brightest galaxies in clusters are ellipticals • Less active in star formtion / young stars than spirals
Galaxies moving away from us! • Once `spiral nebulae’ were established as galaxies, Hubble examined their redshifts, and distances • Found that galaxies were all moving away from us; faster
Expanding Universe • Either we are very special and everything is moving away from us, or Universe as a whole is expanding • But if universe is steadily increasing in size, implies that at some time in the past, Universe was a single point. • `Start of the Universe’ • Big Bang
The Microwave background • Accidentally discovered by radio astronomers (thought it was noise) • 1980s, COBE satellite went up to take careful measurements • Blackbody temperature agrees with predictions • Slight fluctuations; hot spots which eventually gave rise to galaxies!
`Big Bang’ Nucleosynthesis • Can also predict what nuclei are formed at such temperatures • Too cold: can’t form nuclei • Too hot: large nuclei are torn apart • Prediction: Universe should be mostly Hydrogen, Helium, some Lithium: Prediction agrees with observation
Gas Clouds • Two broad types of clouds: • Gas clouds • Warm • Very wispy • Molecular clouds • Colder • Much denser • Gas has condensed enough that complex molecules have formed
Molecular Clouds • Because molecular clouds are cooler and denser, atoms collide more often • Can form complex molecules • Greatly helped by presence of grains • Provides sites for atoms to latch onto • Region of high atom density; atoms more easily find other atoms to interact with
Gas Clouds • All of these gas clouds are turbulent • Random motions, eddies • Where fluid comes together, dense regions • Fluid is moving fast enough that can compress very dense spots