Life in the Universe - Southwestern Collegedept.swccd.edu/jveal/lect/180.pdfJ. M. Veal, Life in the...

Life in the Universe

J. M. Veal, Ph. D.

version 18.01.10

Contents

1 Introduction 21.1 Cosmic Perspective . . . . . . . . . . . . . . . . . . 21.2 Nature of Science . . . . . . . . . . . . . . . . . . . 2

2 Light & Other Basics 32.1 Electromagnetic Radiation (a.k.a. Light) . . . . . . 32.2 Temperature . . . . . . . . . . . . . . . . . . . . . 32.3 Thermal Radiation . . . . . . . . . . . . . . . . . . 42.4 Scattering . . . . . . . . . . . . . . . . . . . . . . . 42.5 The Classical Atom . . . . . . . . . . . . . . . . . . 42.6 Quanta, Absorption, & Emission . . . . . . . . . . 52.7 Spectral Lines . . . . . . . . . . . . . . . . . . . . . 52.8 Ionization, Recombination, & Cascade . . . . . . . 62.9 Collisional vs. Radiative Processes . . . . . . . . . 62.10 Example: Shooting Stars . . . . . . . . . . . . . . . 62.11 Magnetic Fields . . . . . . . . . . . . . . . . . . . . 62.12 Doppler Shift . . . . . . . . . . . . . . . . . . . . . 62.13 Newton’s Universal Law of Gravitation . . . . . . . 72.14 Exam I . . . . . . . . . . . . . . . . . . . . . . . . 7

3 From Subatomic Particles to Intelligence 73.1 Early Universe . . . . . . . . . . . . . . . . . . . . 73.2 Overview of Galaxies . . . . . . . . . . . . . . . . . 83.3 Star Formation . . . . . . . . . . . . . . . . . . . . 83.4 Nuclear Fusion . . . . . . . . . . . . . . . . . . . . 93.5 Stellar Evolution & the H-R Diagram . . . . . . . 103.6 Life of Our Sun . . . . . . . . . . . . . . . . . . . . 103.7 Exam II . . . . . . . . . . . . . . . . . . . . . . . . 113.8 Supergiants & Supernovae . . . . . . . . . . . . . . 123.9 Cosmic Recycling . . . . . . . . . . . . . . . . . . . 123.10 Interstellar Chemistry . . . . . . . . . . . . . . . . 133.11 Comets . . . . . . . . . . . . . . . . . . . . . . . . 133.12 Origin of Life . . . . . . . . . . . . . . . . . . . . . 143.13 Nature of Life . . . . . . . . . . . . . . . . . . . . . 153.14 Mass Extinction . . . . . . . . . . . . . . . . . . . 15

4 Extraterrestrial & Extrasolar Matters 154.1 Search for Extrasolar Planets . . . . . . . . . . . . 164.2 Possible Existence of Extrasolar Intelligence . . . . 164.3 Search for Extraterrestrial Intelligence . . . . . . . 174.4 Exam III . . . . . . . . . . . . . . . . . . . . . . . 184.5 Final Exam . . . . . . . . . . . . . . . . . . . . . . 18

5 Miscellaneous 185.1 Pseudoscience . . . . . . . . . . . . . . . . . . . . . 18

1

J. M. Veal, Life in the Universe 2

1 Introduction

1.1 Cosmic Perspective

• What is large? What is small? Consider walking from SanDiego to Los Angeles. An example of a scale model is aglobe. Perhaps the scale is 1 inch = 1000 miles.

• Solar System.

What is the solar system? For now, let’s say Sun, planets,moons, asteroids, & comets (no other stars!). (“My VeryExcellent Mother Just Served Us Noodles.”)

The scale is 1:1010. (In astronomy we use scientific notationfor large numbers.) No units are needed because they’re thesame on both sides. (A person would be 107 miles tall inthis model.)

The relative sizes are easy to show in a book or on a screen,as are the relative distances. [sketches 1 & 2] But showingboth the relative sizes and distances in the a single scalemodel can’t be done in a book or on a screen. Could wewalk to α Centauri, 4.4 light years away?

• Milky Way.

What is the Milky Way? It’s our galaxy, full of stars, dust,& gas. There are more than 1011 stars in our galaxy. Howmuch is a hundred billion? (Consider salt in Qualcomm...)

The scale is 1:1019. Or, 1 mm = 1 light year. (A light year isthe distance light travels in one year, roughly equal to 1013

km.) Or, atom-size = star-size. In this model, our MilkyWay would be the size of a football field – with the Sun atabout the 20 yd. line.

• Alternate Milky Way model.

If the d is a grain of NaCl, the next grain is 7 miles away.This model is useful when visualizing galaxy collisions.

• Universe.

The universe is everything that exists. There are more than1011 galaxies in the universe. Ñ 1022 stars. How much is1022? (Consider grains of sand... )

• In astronomy, it’s not acceptable to confuse the solar system,the galaxy, and the universe.1

• Time.

Now the scale model is 1 year = age of universe. (The ageof the universe is about 13.8 billion years.)

January 1: big bang

(the beginning of space, time, matter, energy, force, ...)

February: Milky Way forms

September 3: Earth forms

September 22: earliest life on Earth

December 26: rise of dinosaurs

December 30: extinction of dinosaurs

Dec. 31, 9 p.m.: earliest human ancestors

Dec. 31, 11:58 p.m.: first modern humans

Dec. 31, 11:59:25 p.m.: rise of agriculture

1htwins.net; The Known Universe by AMNH; cosmic eye; atlasoftheuni-verse.com; Cosmic Voyage.

Dec. 31, 11:59:49 p.m.: pyramids at Giza

Dec. 31, 11:59:59 p.m.: Kepler, Galileo

• Motions.

How does Earth move? Like a top on a record player on amerry-go-round.

motion speed periodrotation 600 mph 1 dayprecession 26,000 yrsrevolution 60,000 mph 1 yeargalactic orbit w/ “bounce” 600,000 mph 230� 106 yrsMilky Way toward Andromeda 180,000 mphexpansion of universe v � H0r

• Reading.

– TCPF2: Chapter 1, Sections 1 - 2.

• Homework.

1. (3 points) List the eight planets in order of increasingdistance from the Sun.

2. (5 points) Planet A is roughly how many times furtherfrom the Sun than the Earth is? (Planet A can be anyof the seven other planets.)

3. (8 points) Consider our ball-of-string model of our so-lar system. How far would we have to walk to reachthe d’s nearest neighbor, α Centauri? (To answer thisquestion, you must set up the appropriate ratio andsolve for the correct quantity.)

4. (8 points) Consider our two visualized models of ourgalaxy, the Milky Way. In the first, stars were the sizeof an atom, α Centauri was 4.4 millimeters from thed, and the galaxy was 100 yards across. In the second,stars were the size of a grain of salt, α Centauri was7 miles from the d, and our galaxy would be how faracross? (To answer this question, you must set up theappropriate ratio and solve for the correct quantity.)

5. (2 points) Briefly explain the difference between rota-tion and revolution.

1.2 Nature of Science

• Science doesn’t address the “Truth”.

If a statement can be proven, then it can be true.

– Karl Popper – “A theory can never be proven, onlydisproven.” To prove a theory, one must prove thatit can’t be disproven; it must be shown that no ex-periment can disprove the theory. One would have toperform all possible experiments and show that none ofthem disprove the theory; this is impossible.

– R. Descartes? What can we be absolutely sure of?Might we be dreaming? He says cogito ergo sum, andthat we can be sure of it. His argument appeals to ourintuition and common sense, but these cannot be reliedupon in these matters. (Consider “eivocae”...)

• Science seeks to describe what we observe, to find patterns,and to make subsequent predictions. (Consider learing grav-ity...)


• When evidence contradicts a theory, the theory must bechanged or dismissed in favor of a new theory. (Considera helium balloon...)

• Science seeks to explain these patterns with the simplestanswer (like Ockham’s Razor).

What does simple mean2 and what is a simple theory? Toanswer this, we must consider the number of initial condi-tions needed for the theory to happen (like the ingredientsin a recipe) and the number of characteristics or parts of thetheory (like the steps in a recipe).

– Consider a statement that describes a piece of chalk.How many statements are necessary to describe thechalk completely and exactly? One for the velocity andposition of every electron, proton, neutron, etc. We seethat even a piece of chalk is far from simple. (Simple:photon(1), black hole(3).)

– Consider an example in which science must choose,based on simplicity, between two theories. (Neither canbe proven; neither can be disproven.) A) The universewas created 5 minutes ago with all of our memories intact – far from simple. B) The big bang theory – simplecompared to A.

• Further lines of thought include the following.

“Absence of evidence isn’t evidence of absence.”

“What cannot be settled by experiment is not worth debat-ing.”3

Benjamin Franklin was once asked, “What is the use of pureresearch?” He replied, “What is the use of a newborn baby?”Consider the example of knot theory and its intersectionwith virology. This has implications for voting today.

• This is a science course. We will discuss what is observed:the clues. We will discuss the laws of nature: the rules ofthe game – more clues. We will put all the clues together tosolve the mystery: doing science.

Most scientists can control their experiments, but as-tronomers are observers without such control. Observationis not experimentation.

• Reading.

– TCPF2: Chapter 3, Section 2.

• Homework.

1. (8 points) Science seeks to describe what is observed,to find patterns in those observations, and to make pre-dictions based on those patterns. And when observa-tion produces evidence contrary to our theory, we throwaway the theory and create a new one. As we discussedin lecture, you all did all 4 of these acts as infants whenyou learned about gravity and then helium balloons.Discuss another example in which you have naturallybehaved as a scientist – include all 4 parts.

2In the philosophy of science, “simple” does not imply “easy to under-stand”.

3This is known as “Newton’s Flaming Laser Sword”.

2. (5 points) Given 2 theories which explain some phe-nomenon, and neither theory can be disproven, Ock-ham’s Razor helps the scientist choose between them.This is how we chose the big bang universe over the 5minute old universe during lecture. Discuss an exam-ple in which you have applied Ockham’s Razor. How is“simplicity” relevant to your example?

2 Light & Other Basics

2.1 Electromagnetic Radiation (a.k.a. Light)

• Why is light so important to astronomers? Almost every-thing we know about the universe comes to us in the en-crypted form of light.

• White light (for example, sunlight) is actually a sum of col-ors. The speed of light is c � 300, 000 km/s. This is likegoing around the ` 7 times in one second.

• Light is a wave. (Consider a sine wave, not a breaking wave.)[sketch 3] Wavelength is the distance from one crest to thenext. Amplitude is the height of the wave; this is related tothe wave’s power. Frequency is the number of crests passingby each second. [sketch 4] Generally, c � wf .

• What is the physical difference between red light and bluelight? Consider spectral bands: radio, microwave, infrared,visible, ultraviolet, x-ray, and γ-ray.

• Consider the electric and magnetic waves. [sketch 5]

• Light is also a particle;4 it’s called a photon.

• The Duality of Light: Light is a particle and a wave andneither!

This is not intuitive – we don’t grow up with this kind ofthing as part of our daily reality. So we find that the universeis a strange, exotic, wonderful place. And we learn thatthings can exist even if they don’t fit our common sense andintuition.5

• Reading.

– TCPF2: Chapter 5, Section 1, page 80.

• Homework.

1. (3 points) The following are the bands of light that wediscussed. Please list them in order of decreasing wave-length (i.e., start with longest wavelength). [gamma-rays, infrared, microwave, radio, ultraviolet, visible, x-rays]

2.2 Temperature

• What is it? What is the physical difference between hotwater and cold water? Temperature is the measurement ofthe speed of the particles (e.g., atoms, molecules).

4We may compare interference and diffraction with the photoelectric ef-fect and the behavior of CCD’s.

5All these years this amazing object, light, has been all around you; didyou ever stop to consider its true nature?


• Consider scales and units. � F, � C, K.

scale freeze boil� F 32 212� C 0 100K 273 373

How cold can something get? In accordance with our defi-nition, it’s when the motion of atoms is as slow as possible.This is “absolute zero”; this is 0 Kelvins.

• Reading.

– TCPF2: Appendix C, Section 6.

2.3 Thermal Radiation

• Also known as black body radiation or incandescence.

• Hot objects glow. There are many familiar examples: aglowing hot solid like an iron poker or an electric stove coil,a glowing hot liquid like lava, or a glowing hot gas like ourSun or any other star. (There is no fire on or in the Sun orstars.)

• Why is it called a “black body”? An object that is perfectlyblack is a perfect absorber & emitter of e.-m. radiation.(Consider what you might wear on a hot day in direct sun.Or in the shade.)

• Black body curves help us understand the Sun and stars.6

In astronomy, we’re usually concerned with how stars lookthrough our telescopes on the surface of the Earth. Thus,when we consider black body curves for stars, we’re usuallyinterested in starlight after it’s passed through the Earth’satmosphere. Consider a graph of wavelength vs. intensity– with peaks for 8000 K (blue), 6000 K (yellow), 4000 K(red).7 These curves show peaks for radiation influenced bythe air in our atmosphere. [sketch 6]

Three rules go along with the curves: hotter is brighter,hotter peaks at shorter wavelength, and hotter is brighter atall w’s.8

Also consider the conventional use of the words “blue” &“red” in astrophysics. By “blue” we mean short wavelength,and by “red” we mean long wavelength. So the second rulecan also be stated as “hotter is bluer”.

• At the Earth’s surface, the Sun appears yellow. (Considerthe evolution of cones and rods?) Arcturus appears orange;Vega appears blue. 9

• Reading.


• Homework.

6The curves are described by the function

Bpwq � 2hc2{w5

ehc{wkT � 1.

712000 K peaks in the UV; 3000 K peaks in the IR.8WU 100 4-1, 4-2 (21’)9Anyone interested in how the eye perceives colors, or why most stars

appear white, should investigate the chromaticity diagram.

1. (5 points) When considering blackbody radiation, wecan safely say that a blue object emits more red lightthan a red object of the same size. Explain this conceptby drawing two blackbody curves on the same graphand labeling all relevant items, including axes.

2.4 Scattering

• What is it? Photons bounce off of tiny particles such asatoms, molecules, microscopic dust. [sketch 7]

Why does blue scatter more than red? It’s related to 1{w4.Consider ratios. [sketch 8]

• Why is the sky blue?

Visualize a blue photon arriving from d and bouncingaround until it reaches your eyes. You say, “I see blue inthat direction.” With all the blue photons, you see bluefrom all directions and say, “The sky is blue.” [sketch 9]

Consider photons at sunset.10 [sketch 10]

• Why isn’t the sky purple? Consider the BB spectrum of thed and the eye’s sensitivity. [sketches 11 & 12]

What color do things look in the moonlight?

• Homework.

1. (5 points) What would be the color of the sky at noon ifscattering were related to w4? What would be the colorof the setting sun in this case? Justify your answers.

2.5 The Classical Atom

• “Classical” implies an older, less accurate, but simpler idea.Consider a simple atom: nucleus and electron. [sketch 13]

• The atomic number tells us the element and equals the # ofprotons in the nucleus.

The atomic weight tells us the isotope and equals the # ofnucleons in the nucleus. (A nucleon is a proton or a neutron.)

The atomic charge tells us the ion and equals the # of pro-tons in the nucleus minus the # of electrons in the atom.

• Consider examples. How many protons, neutrons, and elec-trons are in each? 1H, 2H, 4He, 3He�, 3He��, 8Li, 60Fe��

p�: a.n.

n: a.w. - a.n.

e�: a.n. - a.c.

• Homework.

1. (5 points) Consider an atom represented by yXz. Thatis, some element X; its atomic number is x, its atomicweight is y and its atomic charge is z. Answer the fol-lowing three questions in terms of x, y, & z, and justifyyour answers. How many protons does yXz have? Howmany neutrons does yXz have? How many electronsdoes yXz have?

10The green flash is a well-known phenomenon, but not as simple as manythink. It’s more than just refraction and chromatic dispersion. To be seenwith the unaided eye, a mirage is required for vertical magnification, andthe observer must be in a cooler layer above a warmer layer.


2.6 Quanta, Absorption, & Emission

• Consider the ground state and an excited state. Ground statemeans the e� has the minimum possible energy, and excitedstate means e� has more than the minimum possible energy.There are many levels (a.k.a. states, orbitals) available to ane� in an atom. A higher level for an e� means it has moreenergy. [sketch 14]

Only certain levels are allowed for the e�; anything in be-tween is forbidden. [sketch 15] This is the Bohr model ofthe atom.

The atom can only have specific quantities of E. We say “en-ergy is quantized”. What does this imply? This is differentfrom the way things work in our everyday world. Imag-ine a car accelerating from 50 mph to 60 mph. Can thecar go from 50 to 60 without at some point having everyspeed in between? But jumping this way is exactly whatelectrons do. If cars behaved like electrons, they could in-stantaneously leap from 50 mph to 60 mph. When electronsjump from one level to the next, they never have the energyin between; they never even occupy the space in betweenthe levels. They make a “quantum leap”. [previous sketch](Imagine if you have no money and I then give you a $50bill. Did you at any time have $20?)

• What makes an electron jump up from one level to the next?The atom absorbs energy. What kind of energy? Light (pho-tons). When an atom absorbs a photon, the e� jumps up toa higher level; this is absorption. [sketch 16]

E � hf : E is energy of the photon, f is frequency, and his a constant (a number, sort of like G in gravity, but evensmaller). So we see a blue photon has more E than a redphoton

Since only specific levels are allowed, only specific colors canbe absorbed by an atom. That is, the photon has to havejust the right E (which determines w and f) in order to beabsorbed and make the e� jump from one level to exactlyanother; if the photon has too much or too little, then itwill pass right through the atom. There must be a perfectmatch.

Absorption is basically photon in, e� up.

• Electrons, when left alone, seek the ground state. This isbecause the ground state has the lowest energy. This ismuch like how a ball, left alone on a hill, will roll to thelowest point. This is because the lowest point has the lowestenergy.

Emission of a photon occurs when an electron jumps to alower level. This is the opposite of absorption. [sketch 17]

Since only specific levels are allowed, only specific colors canbe emitted by an atom.

Emission is basically e� down, photon out.

• Consider the example of phosphorescence, which is delayedemission (glow-in-the-dark), involving a metastable state.[sketch 18]

• Reading.


• Homework.

1. (2 points) Please briefly explain the difference betweena photon and a proton.

2.7 Spectral Lines

• Consider a blackbody, a H cloud, 3 prisms, 3 spectra, and 3graphs (continuous, absorption, emission). [sketch 19]

A continuous spectrum is a blend – no gaps between thecolors. [sketch 20]

Light tries to pass through the H cloud; does all of it makeit through? No. Why not? Because some of the colors hadjust the right E (which determines w and f) to be absorbedby the H atoms in the cloud and make them excited. So,particular colors will be missing, but the rest will pass rightthrough – hence, a spectrum with a few dark lines. This isan absorption line spectrum.

Imagine a day when you can see the Sun through the thinclouds above you. Not all of the light makes it through.

Now look at the cloud itself. What will the spectrum looklike? A few bright lines. In fact, exactly the colors missingin the absorption spectrum will be found. This is becausethe excited H atoms proceed to de-excite and emit photonsthat have the same energies that were absorbed. This is anemission line spectrum.

• Every element, isotope, ion, molecule, etc. has its own setof energy levels. Therefore, each has its own set of colors itcan absorb and emit. Therefore, each has a unique set ofspectral lines.

• This means we can tell what something is made of by lookingat its spectrum. This is spectral analysis. This lines, ingeneral, are called spectral lines.

For example, if a star’s spectrum shows a set of lines uniqueto H, then we know that the star’s atmosphere contains H.Spectral analysis is probably the most powerful tool of ob-servational astrophysics.11

• Consider colors: thermal radiation, scattering, fluorescence,refraction, thin-film interference, diffraction, and so on. Inaddition to physics, we must also consider physiology andpsychology.

• Reading.


• Homework.

1. (5 points) Please explain how the sum of an emissionspectrum plus an absorption spectrum can equal a con-tinuous spectrum. (Hint: quantized energy)

2. (6 points) Consider the diagram with the prisms in thesection Spectral Lines. Can one be sure that the emis-sion spectrum is really an emission spectrum? Or canthis spectrum arise from scattering? Consider if it’spossible that the atoms in the cloud scatter only thethree colors (red, cyan, violet) in the spectrum by com-paring their graphs of brightness vs. wavelength. Jus-tify your answer.

11WU 100 2-2 (9’)


2.8 Ionization, Recombination, & Cascade

• Ionization is the process of creating an ion. Most commonly,a neutral atom loses an e�.

How can an atom lose an e�? If the photon it absorbs hasenough E to send the e� past level 8. Note that level 8has finite E and a finite distance from the nucleus. How?12

Consider stepping half way to the wall over and over. Howmany steps to reach the wall? So we see that it is possibleto have an infinite number of steps in a finite distance. Thisidea applies to the e� as it jumps up from one level to thenext. [sketch 22]

Ionization is basically photon in, e� out. [sketch 23] Theresult is 1 ion & 1 free e�.

• Recombination is the opposite of ionization. It is basicallye� in, photon out. [sketch 24]

• Sometimes the recombining e� lands in an excited state,rather than the ground state. It will then proceed to jumpdown level by level, releasing a photon at each jump. Thisseries of jumps is known as a cascade. [sketch 25]

2.9 Collisional vs. Radiative Processes

• So far, we have discussed radiative excitation and radiativeionization. Thus called because the energy required to makethe e� jump was supplied by radiation (that is, electromag-netic radiation, which is light – or photons).

• Energy can also be supplied by a collision between 2 atoms.

For example, consider a collision between 2 cars. Energy isneeded to bend the metal, to do that work. Where does theE come from? From the velocity of the cars.

So, energy can come from the velocity of the particles.The result is collisional excitation or collisional ionization.[sketches 26 & 27]

• There is also collisional de-excitation. [sketch 28]

• Consider the everyday phenomenon of radiative excitationfollowed immediately by collisional de-excitation.

• Homework.

1. (5 points) Almost every day, all around you, you wit-ness radiative excitation followed immediately by col-lisional de-excitation. What is this commonly called?Justify your answer.

12Recall Zeno’s Paradox, a.k.a. Achilles & the Hare. Achilles runs 10times faster than the rabbit, but the rabbit has a 10 m head start. [sketch21] How many decreasing steps are needed for Achilles to catch the hare?“Infinite number.” So it seems Achilles will never catch the hare. However,in another second, when Achilles has run the 2nd 10 m stretch, the harehas only run another 1 m. Obviously, Achilles overtakes the hare. This isthe paradox. Two logical lines of thought lead to different answers. TheGreeks knew Achilles caught the hare, but they couldn’t reconcile this withthe series of numbers. It wasn’t until Isaac Newton, who invented calculus,that the paradox was resolved. Calculus tells us that the infinitesimal isvalid and real and significant.

2.10 Example: Shooting Stars

• Consider the actual size of a typical shooting star. [sketch29] So we see that they’re not stars at all.

• The `, in its orbit (�60,000 m.p.h), runs into little rocksfloating around the solar system.

The rock enters our atmosphere at that speed. Friction be-tween the rock and the air heats the air and the rock. (Con-sider rubbing your hands together at 60,000 m.p.h!)

Remember that high temperature means fast particles. Socollisional ionization and collisional excitation occur, im-mediately followed by recombination, cascade, and de-excitation – all of which produce photons. The result is thatthe path of the rock glows momentarily; we see a shootingstar (a.k.a. meteor).13

• There are meteor showers. For example, the Perseids arearound August 11 every year.

Consider that a comet (sort of a mountain-sized dirty snow-ball) will sublime as it approaches the Sun. [sketch 30] (No-tice the direction of the tail.) So we see that comets litterthe solar system with debris. And we see why showers occurannually.

2.11 Magnetic Fields

• The abbreviation used is ~B.

How shall we represent something invisible? With ~B lines.[sketch 31]

Consider a sketch of a river showing a “velocity field” ofwater flows. [sketch 32] What do the arrows mean? It’s thesame when drawing magnetic fields; they’re not meant toexist only where the lines have been drawn.

• Consider ~B for a bar magnet. [sketch 33] The lines representthe shape of the force field and the “polarity” of the force.14

[demonstration]

• What happens if we break a magnet in half? What aboutmagnetic splinters, atoms, and subatomic particles? [sketch34]

• Charged particles interact with ~B’s. They can run alongthem, but they can’t freely cross them. [sketch 35]

2.12 Doppler Shift

• [Doppler shift demonstrator (UNL), Doppler applet (CBU)]

• The Doppler effect is a measurable shift in f as a resultof relative motion between the source (of waves) and theobserver.

Imagine a car or train pass by you; there is a drop in pitch(not loudness!). Consider a wave: w, f � pitch; A � loud-ness. [sketch 36]

Consider water, sound, light.

Recall c � wf , so v � wf is also true. (Be careful here, vis wave speed, not speed of relative motion between sourceand observer).

13Consider the similarities to lightning.14Inside the magnet, the lines run from S to N; outside they run around

from N to S. Note that the Earth’s south magnetic pole is the geomagneticnorth pole.


• A picture in your mind is useful... picture water waves, butbe wary. There can be differences between some scenarios:stationary source with moving observer, moving source withstationary observer, or both moving.

stat. so. mov. so. bothmov. obs. stat. obs. movec const. c const. c const. know because

light w shift w shift w shift c is constantf shift f shift f shift and c=wf

water v shift v const. v shift should beand w const. w shift w shift able tosound f shift f shift f shift visualize

Notice there is always a shift in f .

Try to think and visualize until all of the elements of thetable make perfect sense.

• The Dopper shift does not depend on relative distance; itdepends on relative motion.

• Which lines show up in a spectrum tells us what the universeand its contents are made of; how lines are shifted tells ushow the universe and its contents are moving! Considerspectra from the lab and three stars. [sketch 37]

• Reading.


• Homework.

1. (6 points) The Doppler effect takes on two forms:blueshift and redshift. Where do these names comefrom? How might it be possible to explain their sig-nificance in terms of an approaching or receding sourceof water waves? Is the water situation identical to thelight situation? Why or why not?

2. (6 points) If a light source moves away from you, youdetect a redshift; if a light source moves toward you,you detect a blueshift. If the source is stationary, butyou, the detector, are moving, do you detect a Dopplershift? Why or why not? Can you relate this to thewater case? If so, how?

3. (5 points) If a light source is directly north of you, andit is moving due west, what sort of Doppler shift (blue,red, or none) could you detect? Why? (Note: thewording of this question is exact.)

2.13 Newton’s Universal Law of Gravitation

• F � GMm{d2

Here, F is the gravitational force between two masses, G isthe gravitational constant (a.k.a. Newton’s constant), M &m are the masses, and d is the distance between the centersof the two masses. This is an “inverse-square law”; considerexamples,15 including weight.

• Reading.


15WSS 180 6-1 (15’), WU 100 3-1, 3-2 (25’)

• Homework.

1. (6 points) The mass of Mars is roughy 11% of theEarth’s mass. If you could visit Mars, would yourweight be 11% of your weight on Earth? If not, wouldyour weight be more or less than 11% of your weighton Earth? Justify your answer using F � GMm{d2.

2.14 Exam I

Exam 1 covers material up to here.

• The following gives the instructions found on the first pageof the exam.

– This is a 50 minute, closed-book, closed-note exam.You may not have enough time to finish if you don’tpace yourself. There are XX fill-in-the-blank questions,worth one point each. There are multiple “written”questions, worth several points each, the answers ofwhich are to be legibly written on your scantron form,886-E.

– At the top of this exam and on your scantron form,please write your name. Both the exam and the scant-ron form must be turned in. If you forget, you willreceive a zero grade.

– For each numbered blank, choose the most appropriateanswer from the alphabetized list at the bottom of thepage. If none are appropriate, choose “de. none of theabove”. For each page, use only the answers at thebottom of that page. Some of the answers may be usedmore than once; some of the answers might not be usedat all.

– An answer such as “ab” means to fill in both “a” and“b” for the same blank.

– The scantron boxes don’t need to be completely filledin; a single, heavy, dark line should suffice.

– If you need to erase your marks, do so very thoroughly.If the scantron grading machine detects marks that arenot thoroughly erased, you will be marked off for anincorrect answer, and no scoring adjustments will bemade later.

– If you are asked to fill in blanks in alphabetical order,alphabetize the answers that fill in the blank, not theletters that fill in the scantron boxes.

3 From Subatomic Particles to Intelli-gence

3.1 Early Universe

• Planck Time: before 10�43 sec.16,17 It is thought that onlyone force existed in the universe: the TOE force (We are not

16Consider that the mass in the universe is a form of energy (as in E �mc2), but the gravity between these masses is a form of negative energy (asin gravitational potential energy). Perhaps the total energy in the universeis actually zero.

17WSS 170 6-4 (12’)


even close to finding the “theory of everything”, since wedon’t yet know the GUT. The TOE might not even exist.).

• GUT Era begins: t � 10�43 sec, T � 1032 K. It is thoughtthat two forces existed in the universe: gravity and the GUTforce. (We are not even close to finding the “grand unifiedtheory”, since we don’t yet understand quantum chromody-namics well enough to say why a proton has spin one-half.The GUT might not even exist.)

• Electroweak Era begins: t � 10�38 sec, T � 1029 K. Therewere three forces in existence during this era: gravity, thestrong nuclear force, and the electroweak force. The universecontains quarks from now on.

The strong nuclear force holds protons and neutrons togetherin atomic nuclei.

(The GUT era lasted how many times longer than the Planckera?)

• Particle Era begins: t � 10�10 sec, T � 1015 K. Therewere four forces in existence during this era (and so it istoday): gravity, the strong nuclear force, the weak nuclearforce, and the electromagnetic force. The universe containsleptons from now on.

The weak nuclear force is responsible for radioactive decay.18

The electromagnetic force holds electrons in orbit of nuclei tomake atoms, and it includes all electric and magnetic forces.

• Late Particle Era: t � 10�3 sec, T � 1012 K. Collisionscool and slow enough that protons and neutrons can existwithout breaking into quarks.

• Very Late Particle Era: early in 1st sec.

There is much pair production & pair annihilation. [sketches38 & 39]

• End of Particle Era: after 1st sec. The γ’s collide less andpair production halts. Pair annihilation proceeds (why?)and antimatter disappears. The universe contains matterfrom now on.

Symmetry breaking:

particles

antiparticles�

1, 000, 000, 001

1, 000, 000, 000

Protons, electrons, and neutrons are left over.

• Era of Nucleosynthesis: 1st 3 minutes, T � 109 K. The γ’smostly kept nuclei from forming, but some He, Li, Be wereformed. All other than first 4 were formed later

• Era of Nuclei: 1st 500,000 yrs. No neutral atoms formeddue to continual ionization.

• Era of Atoms: after 500,000 yrs, T � 3000 K. Photons arespread out due to expansion, and atoms form as recombi-nation occurs everywhere; the universe becomes transparent– photons race across universe and we see them as CMBR.[sketch 40]

• Era of Galaxies: Nowadays photons outnumber atoms by109 to 1.

18Nuclear forces act only over 10�12 mm.

• Reading.


• Homework.

1. (5 points) The electroweak era lasted how many timeslonger than the GUT era? Justify your answer.

2. (7 points) Consider the era of atoms, and the “beforeand after boxes” as discussed in class. If the “after box”had eight H atoms, two He atoms, one Li atom andone Be atom, how many particles were in the “beforebox”? How many of each kind of particle? Justify youranswer.

3.2 Overview of Galaxies

• Galaxies come in four basic types: elliptical, barred spiral,unbarred spiral, and irregular. [sketches 41, 42, 43, & 44]All types come in a wide range of sizes. Our Milky Way isa barred spiral galaxy.

• Stars in elliptical galaxies have complex, irregular orbits.[sketch 45] The density of stars in elliptical galaxies is sohigh that type I supernovae keep the gas hot, and there islittle or no star formation.

• Stars in spiral galaxies move on nearly circular orbits in the“disk”, but have irregular orbits in the “bulge”. [sketch 46]The gas is cool, and stars form along the spiral arms; theformation is triggered by spiral density waves.

• Reading.

– TCPF2: Chapter 11, Section 2, pages 189 - 190.

3.3 Star Formation

• Macroscopically speaking, a galaxy is composed of threethings: stars, dust, and gas. The dust and gas make up theinterstellar medium, or ISM. The ISM contains interstellarclouds, some of which are star formation regions.

Check out APoD; image search “spiral galaxy”, “milky way”,& “eagle nebula”.

• First we consider four basic concepts related to star forma-tion.19 For our Sun, the process lasted about 35 millionyears.20

– A star formation region is likely to be somewhat ion-ized by cosmic rays, which are usually very high-speedprotons coming from supernovae.21 [sketch 47]

– The Milky Way has a ~B that runs, for the most part,along the spiral arms of our galaxy.22,23 [sketch 48]

As a result of the ionization along with the presence ofthe galactic ~B, the star formation region is a plasma,which is a charged, magnetic gas. (Remember the way

in which charged particles interact with ~B’s?)

19WSS 180 1-2 (11’)20There is a quite a range in the amount of time it takes, depending on

the mass of the star. If the star will be about 10 Md, then it can take aslittle as 105 years. If the star will be about .5 Md, then it will take longerthan it did for the Sun.

21A supernova is an extremely energetic, explosive event at the end of thelife of a very massive star.

22Our Milky Way is actually a barred spiral galaxy.23The origin of this galactic ~B remains a mystery.


– Eventually, the star formation region collapses due togravity. [sketch 49] In addition to the star, other solarsystem objects (planets, moons, asteroids, and comets)form as part of this process.

– During the contraction, the region obeys the law of con-servation of angular momentum, which can be thoughtof as the “spinning ice-skater effect”. [sketch 50] If themass is constant, then the rotation rate varies inverselywith the size or radius.24

• We consider the actual process of star formation in threestages, the first of which is a combination of the previouslyconsidered four basic concepts.

– The star formation region undergoes a modified gravi-tational collapse.

The collaspe must begin along the field lines. Why?[sketch 51] In the beginning, the particles are relatively

far apart, allowing the ~B to dominate the collapse. Onemight ask why the neutral particles also follow the fieldlines; the answer is collisional coupling between thecharged and the neutral particles. (Imagine trying towalk diagonally through a mass of people all headed inthe same direction; surely you would be forced, as aresult of collisions, into their stream. [sketch 52])

When the particles are, on average, closer together, thecollapse is then dominated by the gravitational field(rather than the ~B). Why? This produces a “pinched”~B. [sketch 53]

Since the collapse isn’t 100% efficient, there is still someleftover plasma outside the collapsed region. [sketch 54]

The ~B permeating the leftover plasma tends to dragit. Why? This effect is much like a snowplow or asail, and it’s called magnetic braking. As a result, therotation of the collapsed region is slowed. If not formagnetic braking, conservation of angular momentumwould have increased the rotation rate so much thatthe region would be more likely to fly apart than tocollapse further to form a star.

– In the core of the collapsed region, the density and tem-perature have become much greater. (Remember themeaning of temperature?)

– When protons are fast enough to overcome their mutualelectric repulsion, they can stick together (fuse).25 (It’sa bit like rolling two balls up a hill with a well on top?)[sketch 55] Recall that a proton is a hydrogen nucleus,so when two protons stick together, it’s an example ofnuclear fusion (not the same as nuclear fission). Nuclearfusion is what makes a star a star.

• Reading.


• Homework.

24An alternate way of thinking of this is in terms of Kepler’s second lawof planetary motion: the law of equal areas. Extended arms sweep slowly,and retracted arms sweep quickly.

25They do this by getting close enough for the strong nuclear force to takeover. Did you ever wonder why, since like charges repel, the protons in anucleus don’t repel each other?

1. (5 points) If the star formation process didn’t includemagnetic fields, would the Sun’s rotation rate be anydifferent from what it is now? Why or why not?

2. (5 points) If the gas in a star formation region werenever ionized, would the gravitational collapse of thisregion be any different? Why or why not?

3.4 Nuclear Fusion

• The principle underlying all nuclear reactions, fusion or fis-sion, is the interchangability of mass and energy: E �mc2. The total amount of “mass-energy” in the universeis thought to be constant; it can be neither created nor de-stroyed.

• There are a few rules that go along with nuclear fusion reac-tions. Here we are concerned with two of them: conservationof charge and conservation of nucleons (a proton or a neu-tron is a nucleon). In physics, when something is conserved,the total amount doesn’t change.

• The most common nuclear fusion reaction in our Sun andin most stars is called the proton-proton chain reaction. Wecan imagine it in three stages, each of which obeys the rules.

The first stage begins with two protons colliding, which be-come a deuterium nucleus (one version of heavy hydrogen),a neutrino (a nearly massless neutral particle, which hap-pens to conserve spin), and a positron (this most commonexample of antimatter is just like an electron, but with oppo-site charge; it can also be thought of as an electron movingfrom the future to the present). The positron then meetsan electron, annihilates with it, and forms two gamma rays.[sketch 56] Consider conservation.

The second stage begins with a deuterium nucleus and aproton colliding and ends with a 3He nucleus and a gammaray. [sketch 57] Consider conservation.

The third stage begins with two 3He nuclei colliding and endswith a 4He nucleus and two protons. [sketch 58] Considerconservation.

• We can imagine all three stages together in one reactionwhich obeys the rules. [sketch 59] Consider conservation.

• Consider some details.

61H� 2e� Ñ 4He� 21H� 6γ � 2ν

We can ignore the neutrinos and the electrons here. We canalso subtract two protons from each side. So the reactionboils down to something less complicated.

41H Ñ 4He� 6γ

The essence of this is simple.

fuel Ñ exhaust� energy

• It turns out that the mass of four protons (6.69� 10�27 kg)is more than the mass of the 4He nucleus (6.64� 10�27 kg).Where did the missing mass (.05 � 10�27 kg) go? (It’s notrelated to the electrons that we ignored, they’re combinedmass is only about .002 � 10�27 kg.) It actually turnedinto energy! The amount of energy produced is about 4 �10�12 Joules, which is about one trillionth of a dietary calorie(Cal).


• The Sun’s luminosity (total power output) is about 4� 1026

Watts (a Watt is one Joule per second). So how many re-actions, at 4� 10�12 Joules per reaction, must happen eachsecond to produce this amazing power?26 The answer is1038 reactions per second! And this rate lasts for ten billionyears! So how much mass is converted to energy each secondin our Sun? The answer is about 5,000,000 metric tons persecond!27

• Reading.

– TCPF2: Chapter , Section 1, pages 129 - 130.

• Homework.

1. (5 points) Describe the conservation of charge in the(a) first step of the proton-proton chain, (b) secondstep of the proton-proton chain, and (c) third step ofthe proton-proton chain.

2. (5 points) Describe the conservation of nucleons in the(a) first step of the proton-proton chain, (b) secondstep of the proton-proton chain, and (c) third step ofthe proton-proton chain.

3. (4 points) Consider the reaction 6Li + 2H Ñ 4He +. How many charges must exit this reaction? How

many nucleons must exit the reaction? Which elementmust fill in the blank? Which isotope must fill in theblank? Justify your answers.

3.5 Stellar Evolution & the H-R Diagram

• Stellar evolution28 is the branch of astronomy that studieshow stars change over time. (The Hertzsprung-Russell dia-gram, the cluster-aging technique, the life of our Sun, andthe life of a supergiant are all under the umbrella of stellarevolution.)

• Consider some basic ideas relevant to the Hertzsprung-Russell diagram.

Imagine measuring luminosity and surface temperature(how?) of 100 stars chosen at random. You would havetwo columns of numbers, which you could plot against eachother on a graph. [sketch 60] Notice the trend obeys blackbody rule #1.

Most (but not all) stars fit the trend. Why? (Imagine asample of 100 people chosen at random. Place them into fivebins: infant, child, adolescent, adult, & elderly. Which binhas the most people in it? Why?) For a star, the main se-quence is the longest stage of its life (there are many stages).A star is on the main sequence if it’s fusing H Ñ He in itscore, and this leads to the special relationship between Land Ts.

A star with H fusion in the core will be in hydrostatic equi-librium; usually a star is in balance with itself. The inwardforce of gravity is balanced by the outward force related tothe energy of nuclear fusion. [sketch 61] As a result, ourSun’s size stays relatively constant.

26We need only divide the power by the energy per reaction to find theanswer.

27WU 180 2-3 (11’)28WSS 180 1-5 (11’)

• Hydrostatic equilibrium lasts for about 1010 years for ourSun with core H fusion. A very massive star,29 about120 Md, will stay on the main sequence for about 3 � 106

years, while the least massive star, about .08 Md, will stayon the main sequence for about 1012 years.30 On the HR dia-gram, the more massive stars are at the top; the less massiveat the bottom. [sketch 62] Is this reasonable?

When the fuel runs out, and there’s no more H in the core,the star leaves the main sequence. But no one has ever livedlong enough to see a star do this, so how can we know it’strue? (Have you ever watched someone go from birth to oldage? No. So how do you know you’re going to get old?) It’ssimple; we “connect the dots”, so to speak.

• Consider the cluster-aging technique. In a star cluster, thestars formed at nearly the same time. Imagine observing fourclusters, and measuring L & Ts for 50 stars in each cluster,chosen at random. [sketch 63] We see the “main-sequenceturnoff point” tells us the age of the cluster.

• Reading.

– TCPF2: Chapter 8, Section 3 and Chapter 9, Section1.

• Homework.

1. (3 points) Briefly describe hydrostatic equilibrium.

2. (6 points) Pretend black body rule #1 tells us thatcooler is brighter (instead of the actual hotter isbrighter). Answer this question by creating one HRdiagram containing both the real main sequence andthe hypothetical main sequence that would occur as aresult of this hypothetical rule #1.

3. (6 points) Generally, if one main-sequence star is hot-ter than another, it is also bigger. What if all main-sequence stars were the same size as our Sun? Answerthis question by creating one HR diagram containingboth the real main sequence and the hypothetical mainsequence that would occur if all main-sequence starswere the same size.

3.6 Life of Our Sun

• The Sun’s life can be described, and plotted on the HR dia-gram, in eight stages. [sketch 64]

• The first stage is the zero-age main sequence. This is theonset of H fusion in the core, when the core temperaturereaches about 15� 106 K.

• The second stage, the main sequence, lasts about 1010 years.Here we have steady H fusion in the core: the proton-protonchain.

• The third stage, the red giant branch, lasts about 109 years.By the end of this stage, the radius is about 102Rd, theluminosity is about 103Ld, and the surface temperature is

29The “Eddington limit” means if there is too much mass, the fusion willbe too great for the gravity; hydrostatic equilibrium will not be attained andthe star won’t be able to form. This limit is around 300 Md.

30The lifetime on the main sequence can be estimated with t �1010yrs{M2.5.


about 3000 K. So Mercury will be vaporized! Imagine vis-iting the Earth at this time; what would the Sun look like?The Earth will be uninhabitable.

Does it seem odd that the star is both cooler and brighter?This seems to conflict with black body rule #1. The answeris in the large radius.31 While Ts has decreased only a little,R has increased very dramatically.

The chain of logic follows something like this.The core runs out of H.

So the core fusion ceases.So hydrostatic equilibrium is lost.

So the core contracts.So the pressure rises in the H shell around the He core.

So the H in the shell heats and fuses.So the inner envelope heats.

So the envelope expands, becoming a giant.So the surface cools.

So the surface gets red. [sketch 65]And the surface gravity becomes weak.

And so the wind increases.

• The fourth stage is the helium flash, which lasts a few hours.It occurs when the core temperature reaches about 108 K.32

The helium in the core begins to fuse and rapidly becomesa “thermonuclear runaway”.

The fusion reaction is called the triple-alpha process (3-α).We can imagine it in two stages. The first stage beginswith a collision between two 4He nuclei33 and results in a8Be nucleus. [sketch 66] Consider conservation. Since the8Be nucleus is so unstable (it will decay in 10�16 seconds!),it should break apart into the 4He nuclei. But when thetemperature is so hot – and the particles are so fast, a third4He collides with the 8Be. This is the second stage, and theresult is a 12C nucleus and a γ-ray. [sketch 67] Considerconservation. If another 4He collides with the 12C, it willform 16O.

Due to the tremendous energy released, the core is able toexpand and the “degeneracy” is removed.

• The fifth stage, the horizontal branch. lasts about 108 years.Hydrostatic equilibrium returns and there is steady heliumfusion (3-α) in the core. [sketch 68] Examples of such starstoday are Albebaran in Taurus and Arcturus in Bootes.[sketches 69 & 70] Such stars have the same luminosity,but slightly different surface temperatures; this is the rea-son for the “branch”.

• The sixth stage, the asymptotic giant branch, lasts roughly106 years. The AGB star is larger and more luminous thana red giant and slightly cooler. Venus will be vaporized; theEarth may just barely escape total destruction.

The chain of logic for the AGB is very similar to the RGB,except that it begins with the core running out of He, andthere are two fusion shells: H and He. Also, the shell burningis unstable, causing thermal pulses, which can lead to theenvelope pulsating! [sketch 71]

31L � 4πR2σT 4s .

32The pressure and density in the core don’t increase as the temperaturerises because the electrons are “degenerate”. Of course, the nuclei aren’t!

33A 4He nucleus is a.k.a. an α-particle.

• The seventh stage is the planetary nebula. This occurs whenmost of the mass of the star is ejected into space as an ex-panding gas shell. The relatively small, hot core that is leftbehind will emit much UV radiation, thereby causing radia-tive excitation and ionization in the gas shell. Of course,this will result in de-excitation, recombination, and cascade:all of which produce photons. Hence, the shell glows and isa PN. [sketch 72] (This has nothing to do with planets; thename comes from the look of the expanding gas shell whenviewed with a small telescope.)

Did you ever wonder where the C in your DNA was built?Or the O in the air you breath and the water you drink?Now you know!

• The eighth stage is the lone white dwarf, which is the hot corethat’s left behind.34 It’s about the size of the Earth, with asurface temperature of about 25,000 K; the black body curvepeaks in the UV. [sketch 73] Consider the density of about103 kg/cm3!

Consider the strange idea of electron degeneracy, and howthis purely quantum mechanical effect, which is not a forceat all, is able to withstand the tremendous force of grav-ity. How bizarre! And yet, this has been confirmed throughthe consistency between observations and the Chandrasekharlimit, which states that MWD ¤ 1.44Md if the WD is madeof C & O.

• Reading.

– TCPF2: Chapter 9, Section 2, pages 152 - 155 andChapter 10, Section 1, page 167.

• Homework.

1. (7 points) Consider part of the life of a proton. Beginwith it floating in the core of our Sun when the Sun firstformed and follow it to the white dwarf stage. Assumeit is not involved in fusion. Make a graph of the proton’sspeed vs. time.

2. (9 points) Consider part of the life of a proton. Beginwith it floating in the photosphere of our Sun when theSun first formed and follow it to the planetary nebulastage. Assume it does not leave as wind, but is ejectedwith the planetary nebula. Make a graph of the pro-ton’s speed vs. time.

3. (10 points) The third stage in the section Life of OurSun follows a specific chain of logic, as given in thenotes. The sixth stage follows a similarly specific chainof logic; construct this chain of logic.

4. (8 points) Consider the seventh stage in the section Lifeof Our Sun. Also consider the Ring Nebula, M57, whichis an easily-seen planetary nebula in the constellationof Lyra. If the white dwarf had a surface temperatureof 3,000 K instead of 25,000 K (so that its black-bodycurve peaked in the IR instead of the UV), would westill be able to detect M57? Justify your answer.

3.7 Exam II

Exam 2 covers material between exam 1 and here.

34WU 180 1-4 (12’)


3.8 Supergiants & Supernovae

• Some high-mass stars can become supergiants (15 Md to 120Md).

• Eventually, the H in the core runs out. The star exits theMS, becomes a hot, blue supergiant, and subsequently be-comes a cool, red, He-burning supergiant. The size of thecore is only about .01Rd, but the size of the star is tremen-dous: about 103Rd! If we replaced our Sun with a redsupergiant, it’s surface would be at Jupiter! So we can fit amillion Earths inside our Sun, but we can fit a billion Sunsinside a red supergiant! Think of this next time you gaze atBetelgeuse in Orion or Antares in Scorpius. [sketches 74 &75]

• The core of such a star exceeds the Chandrasekhar limit andcannot become a WD. There are several burning stages in asupergiant core. Each time a fuel source runs out, the ash ofthe reaction becomes the fuel for the next reaction at highertemperature. And an additional fusion shell will surroundthe core, thereby producing a layered structure like that ofan onion.

Before the core H runs out, it burns for about 107 yearsat a core temperature of near 108 K. The subsequent coreHe-burning (3-α) stage lasts about 106 years at a core tem-perature about 108 K.

The core C-burning stage, at about 109 K, lasts only a couplehundred years. At this temperature, 12C fusion leads to 16O,20Ne & 24Mg (α processes, a.k.a. He-capture reactions).

At this point, there is no C left in the core. The core Ne-burning stage, at nearly 2� 109 K, lasts only about a year.At this temperature, 20Ne will fuse into 16O and 24Mg (pho-todisintegration and He-capture, respectively).

When all the neon in the core is gone, the O-burning stageensues for about half a year. At a temperature of nearly3� 109 K, a set of reactions beginning with two 16O nucleiwill result in the formation of 28Si and 31S.

The next reaction begins in the core at a temperature ofnearly 4� 109 K and lasts only a couple days. Here, a set ofreactions involving 28Si nuclei will result in the formation of56Fe.

• At this point [sketch 76] there is an Fe core, but no morefusion reactions will occur. A nucleus with 56 nucleons hasthe lowest mass per nucleon, and therefore any further fusionreactions would have to be endothermic.35

So the core continues to contract and heat to such a degreethat the Fe photodisintegrates and then electrons combinewith protons to produce neutrons and neutrinos. [sketch77] Subsequently, the core rapidly collapses (at nearly onequarter of the speed of light!) to such a small size thateverything bounces out from the very center.36 Usually, aneutron star is left behind.

This rebound and the tremendous outward flux of neutrinoscause most of the star to explode outward. The energy ofthis event, produced by the gravitational collapse of the core,is more than 100 times what our Sun will generate over itsentire MS lifetime! This event is a type II supernova.

3556 nucleons is also a dead end for fission reactions.36The halt of the collapse is probably a combination of neutron degeneracy

and a repulsive neutron-neutron interaction mediated by the strong nuclearforce.

• This outward shock wave induces all sorts of nuclear reac-tions, resulting in the construction of all elements heavierthan Fe.37

• In the year 1054, the Chinese and the Anasazi both recordeda supernova; today we see the remnant as the Crab Nebulain the constellation of Taurus. (Why did the Europeansmiss it?) What if Betelgeuse or Antares went off tonight? Itwould be ten times brighter than the full moon and mightlast a few months!

• Reading.


• Homework.

1. (6 points) Consider part of the life of a proton. Beginwith it in the photosphere of a newly formed 50 Md starand follow it into a supernova shell. Assume it doesnot leave as wind, but is ejected with the supernova.Consider the mass of the nucleus in which this protonresides and make a graph of this mass vs. time.

2. (11 points) Consider part of the life of a proton. Beginwith it in the core of a newly formed 50 Md star andfollow it into a neutron star. Consider the mass of thenucleus in which this proton resides and make a graphof this mass vs. time.

3.9 Cosmic Recycling

• Where did the carbon and oxygen atoms in our bodies comefrom? Our iron atoms and our heavier elements?

The star-gas-star cycle is self-explanatory. Stars endtheir lives as planetary nebulae or supernovae, sendingtheir “newly constructed” elements back to the interstellarmedium. Some of the ISM becomes interstellar dust. Subse-quently, this interstellar dust and gas collapses to form newstars. The cycle continues. We are recylcled star material.38

• What if some of our protons could tell their stories? Eachwould have its own unique 13.8 billion-year story to tell.If we could speak with one of our protons, we could ask itquestions. Where were you ten billion years ago? Six billion?Of how many stars have you been a part? Where were youone billion years ago? Ten thousand? One hundred? Oneyear ago? Where do you see yourself in ten billion years?

Each of us is an astounding collection of nearly 1028 protons– many of the protons having journeys as old as the uni-verse – that have come together after billions of years. Howamazing!

• When I look into the sky I know the stars built you and me,and one day you and I will build them in return.

• Reading.

– TCPF2: Chapter 1, Section 2, pages 10 - 12 and Chap-ter 11, Section 1, pages 185 - 186.

• Homework.

37WU 180 2-4 (15’)38WU 180 2-1 (11’)


1. (4 points) Imagine if stars had no wind, and they never“exploded” as planetary nebulae, novae, supernovae,or in any other way. Would there be more, less, or thesame amount of life on Earth? Justify your answer.

3.10 Interstellar Chemistry

• Some massive stars, late in their evolution, send off muchwind. The wind is made of silicates (Si, Mg, Fe, & O),among other constituents.

As the silicates move away from the star and cool, theycondense into tiny grains.

• Consider the relative cosmic abundances of the five mostcommon elements, relative to H.39 Note that He is inert ; itdoesn’t react with other elements. (That is, it can’t play animportant role in life or the formation of life because if itcan’t be part of a molecule, then it certainly can’t be partof anything more complicated than a molecule.)

element relative abundanceH 10,000He 790O 8C 5N 1all others 2%

• Silicate grains react with cosmic materials. They do sothrough surface chemistry instead of gas-phase chemistry.

Imagine two molecules bouncing around a completely empty(vacuum), room-sized box. [sketch 78] What are the oddsthey will collide with each other and react with each other?(This is like gas-phase chemistry.) On the other hand, imag-ine this box also contains a table with a very special surface.When either molecule strikes such a surface, it sticks to thesurface and rolls around on it. [sketch 79] What are the oddsthe two molecules will end up rolling around on the surfaceand then collide with each other and then react? (This islike surface chemistry.)

It seems amazing, but the surfaces of interstellar silicategrains behave just this way. So when atoms run into a grain(those most likely to do so are, of course, the elements H, O,C, & N), they stick to the grain, roll around its surface, andthen react with each other.

• What sort of molecules would we expect to form on the grainsurface? Well, let’s think about it. If the most abundantelement, H, is combined with the second-most abundant re-active element, O,40 then H2O (water) is formed. If H iscombined with C,41 then CH4 (methane) is formed. Andif H is combined with N,42 then NH3 (ammonia) is formed.These three molecules are the most common molecules (afterH2, molecular hydrogen) in the universe. There are manyinteresting, yet simple, molecules that can be formed. Oth-ers include CO (carbon monoxide), CN (cyanide), and so on.So the silicate grains form simple organic mantles. [sketch80]

39A few other elements are about as common as N, but they’re not sorelevent for forming life.

40Oxygen forms two bonds.41Carbon forms four bonds.42Nitrogen forms three bonds.

But we are surely forgetting something: the most abun-dant particle in the universe. What is it? Light! Re-alistically, we must include photons in this mix; we mustconsider photochemistry. When similar grains (silicate corewith simple organic mantle) are made in a laboratory, theyare subsequently exposed to UV radiation – similar to whatthey would receive in space from nearby stars. The resultsare interesting, including (NH2)2CO (urea), NH2CH2COOH(glycine, the simplest amino acid), and so on.43 So a typi-cal grain cruising around interstellar space has a somewhatcomplex, organic surface. [sketch 81]

• Our exploration of interstellar chemistry so far has sev-eral intruguing ideas. But one of the main ingredientsof science is that it can be tested, either by experimentor by observation. So we must ask. Is there observa-tional evidence to support these ideas? The answer is yes.The following constitutes a small fraction44 of the organicmolecules that have been observed in space: CO2 (carbondioxide), HCN (hydrogen cyanide, used in the gas chamber),H2CO (formaldehyde, used to preserve bodies for dissection)HCOOH (formic acid, of ant bites and bee stings), CH3OH(methyl alcohol, a.k.a. methanol), CH3CH2OH (ethyl alco-hol, a.k.a. ethanol), (NH2)2CO (urea), and even CH3COOH(acetic acid, a.k.a. vinegar).

One might wonder, since molecules are so small, how can weobserve them in space many thousands of light years away?The answer is, of course, with spectroscopy.45

• Homework.

1. (4 points) (a) What if He weren’t inert? Pretend Hecould form molecules in the same manner as O, thusforming the hypothetical H2He. Would you expect theabundance of such a molecule in the universe to begreater than, equal to, or less than that of water? (b)Hydrogen sulfide, H2S, is a poisonous, flammable gasthat smells like rotten eggs. Would you expect theabundance of such a molecule in the universe to begreater than, equal to, or less than that of water? Jus-tify your answers.

3.11 Comets

• Comets are like little pieces of the ISM. Complex moleculesend up in comets, making comets like fossils of the cloudfrom which our solar system formed. As before, we remem-ber that one of the main ingredients in science is that itcan be tested. And so we ask. Is there observational evi-dence to support this idea? The answer is yes. The follow-ing constitutes a small fraction46 of the organic molecules

43Also of note are HOCH2CH(OH)CO2H (glyceric acid) andHOCH2CH(OH)CONH2 (glyceramide), which exhibit prebiotic mirrorsymmetry.

44There are well over 100 molecular species that have been observed inthe ISM.

45In a previous section, Spectral Lines, we discussed spectra producedby electrons jumping downward: electronic spectra, which tend to be in thevisible and UV spectral bands. Most interstellar molecules are observed withrotational spectra, which tend to be in the radio and microwave spectralbands, or observed with vibrational spectra, which tend to be in the IRspectral band.

46There are well over 100 molecular species that have been observed incomets.


that have been observed in comets: CO2 (carbon diox-ide), HCN (hydrogen cyanide, used in the gas chamber),HCO� (formyl ion, discovered in comets by Veal et al, 1997),H2CO (formaldehyde, used to preserve bodies for dissec-tion) CH3OH (methyl alcohol, a.k.a. methanol), and evenCH3CH2COOH (glycine, the simplest amino acid).47

Comets have an aggregate structure, meaning they are lightand “fluffy” like popcorn.

• Comets have bombarded the Earth at least hundreds oftimes. The water for at least half of the Earth’s oceanswas brought by comets. (Note that comets are made mostlyof water.) Given the amount of biomass48 in one comet, it’seasily possible that all of the Earth’s biomass was deliveredby comets.49

We expect that some of the organic molecules could be de-stroyed during the impact (which is very energetic), but notall of them.

• Some of the organic material actually survived the impact.We expect this to some degree through the combination ofaggregate structure of comets with the early Earth’s atmo-sphere having been much thicker50 than it is today.

In 2012, J. G. Blank performed experiments demonstratingthat “the building blocks of life could, indeed, have remainedintact despite the tremendous shock wave and other violentconditions in a comet impact.”

• So comets could give a head start to the formation of life.Amino acids may or may not have taken a billion years ormore to form in the ISM, but as soon as our solar systemformed, these amino acids – the building blocks of life – weredelivered, intact, to the surface of the Earth. The Earthdidn’t have to wait a billion years or more for amino acidsto actually form on its surface.

• Taken together, this is a set of processes probably takingplace all over the galaxy and all over the universe. Interstel-lar chemistry is ubiquitous. Comet formation and bombard-ment is likely common to all star systems with planets. It’sentirely plausible that the building blocks of life have been,and are being, delivered to planets throughout the galaxyand even the universe!51

• Homework.

1. (10 points) Trace part of the life of a proton. Beginwith it floating in an interstellar cloud. Follow it untilit winds up in your heart.

3.12 Origin of Life

• Oxygen destroys organic molecules, so a cell membrane isneeded for protection. But the early Earth had a predomi-nantly CO2 atmosphere, so organic chemistry worked okay

47Glycine was found in Comet Wild 2 by the Stardust Mission/NASA in2009.

48This is a term referring to the total mass of the molecules that can beused for the construction of living organisms.

49A single comet the size of Comet Hale-Bopp could deliver eighty timesthe biomass of all the living organisms on our planet.

50Up to twenty times thicker, and mostly CO2.51WU 180 2-5 (13’)

(with sunlight).52 However, such chemistry might not pro-duce enough biomass on its own. Fortunately, comets andasteroids are included as sources of organic molecules onEarth. There is also deep-sea vent chemistry.

When all three sources (atmosphere, comets/asteriods, &deep-sea vents) are taken together, there is plenty of biomassproduced.

• The transition from chemistry to biology is of great interestto astrobiologists.

– Whence came the first self-replicating molecule? (DNAis too complex to start with.) The most obvious can-didate is RNA. (But RNA and enzymes can’t replicatewithout each other.)

– Consider a molecular analog to natural selection.53

Consider a group of simple molecular species, namedafter letters of the alphabet: A, B, ..., S. Some of themolecules are built from others. For example, A+B=C.How might competition, and therefore natural selec-tion, arise between molecules? Consider a competitionbetween molecules named R and S.

∗ A+B=C and D+E=F. Then C+F=S. S is self-replicating.

∗ A+G=H, I+J+K=L, and M+N+O=P. ThenH+L+P=R. R is self-replicating.

∗ Since S is constructed more easily, it could occurmore often. But if it occurs more often, then itwill end up “stealing” all of the A molecules. Sub-sequently, R will not be able to form.

– Free-floating RNA bases (a.k.a. nucleotides) use miner-als on clay as surfaces for surface chemistry. (Remem-ber a previous section, Interstellar Chemistry, in whichsurface chemistry was found to be relatively efficient.)Hence, RNA bases combine to form RNA strands (upto nearly 100 bases long) in the laboratory.

– Base-pairing rules then build complementary strands,which then serve as templates for copying (again viabase-pairing rules) the original RNA strands. [sketch82]

• Confining organic molecules with a pre-cell does two things.First, it keeps the molecules closer together, thereby mak-ing chemical reactions more probable. Second, it facili-tates a molecular analog to natural selection between RNAmolecules (tougher competition).

– When a warm-water solution of amino acids is cooled,the amino acids can form bonds among themselves tomake an enclosed, spherical structure. These are notalive, but...

∗ They can grow in size by absorbing more shortamino-acid chains until they reach an unstable sizeat which they split to form “daughter spheres”.

52In the famous Miller-Urey experiment, there were some faulty assump-tions about the Earth’s early atmosphere: too much CH4 & NH3, and toolittle CO2.

53This is one of the two main components of evolution; the other is mu-tation. Consider simple examples like the moths in England during theindustrial revolution or why humans have tailbones but no tails. (We mightconsider positive, negative, and neutral evolutionary pressures.)


∗ They can selectively allow some types of moleculesto cross into or out of the enclosure.

∗ Some even store energy as electrical voltage acrosstheir surfaces, discharging to facilitate chemical re-actions inside.

– The second type of membrane forms spontaneouslywhen lipids are mixed with water.

• Natural selection among RNA molecules in pre-cells leads tocomplexity,54 which leads to life.

Natural selection leads to DNA, which leads to evolution.55

3.13 Nature of Life

• A cell is a tiny chemical factory that makes reactions morerapid, thereby helping simple molecules become complex.

Cells are carbon-based.

– Carbon forms from one to four bonds. This versatilityleads to complexity.

– In comparison, silicon is less abundant, it has weakerbonds, it doesn’t usually form double bonds, and com-plex Si-based molecules cannot exist long in water.

Cells have four main molecular components: carbohydrates,lipids, proteins, and nucleic acids.

– Carbohydrates provide energy to cells and they play animportant structural role.

– Lipids form membranes.

– Proteins are built from long chains of amino acids.Some are enzymes, which are catalysts that greatly ac-celerate chemical reactions.

– Nucleic acids allow cells to function according to pre-cise, heritable instructions.

Cells come in two types: prokaryotic and eukaryotic.

– Prokaryotic cells have no nucleus, they are simple &small, they constitute more biomass on Earth, theydon’t depend on eukaryotes, and they give rise to twodomains: bacteria and archaea.

– Eukaryotic cells have a nucleus (enclosing DNA), theyare complex & large, the constitute less biomass onEarth, they can’t exist without prokaryotes, and theygive rise to one domain: eukarya.

• Metabolism is biochemical manufacturing, which requiresraw materials (C, etc.) and energy (to fuel metabolic pro-cesses of manufacture).

Storage and release of energy in any living cell56 happenswith the following reactions.57

ADP + phosphate group + energy Ñ ATP

ATP Ñ ADP + phosphate group + energy

54How complex can a molecular-biological system be? Check out DrewBarry’s TED talk: Animations of unseeable biology.

55WSS 180 5-3, 5-4, 5-5 (42’)56The fact that it’s true for any living cell is a good argument for a common

ancestor of all life.57ADP and ATP are, respectively, adenosine diphosphate and adenosine

triphosphate.

– Organisms can acquire raw materials either from foodor from the atmosphere: heterotrophs or autotrophs,respectively.

Organisms can acquire energy either from food (or in-organic chemicals) or from the Sun: prefixes chemo- orphoto-, respectively.

carbon energyclassification source source examplesphotoautotroph CO2 Sun plants/

photosynthetic

chemoautotroph CO2 inorganic extremophileschemicals (prokaryotes)

photoheterotroph food Sun someprokaryotes

chemoheterotroph food food animals(organic

compounds)

• Water plays a prominent role. Chemicals dissovle in it, sothey’re ready to react. It transports chemicals in and wasteout. It is part of many reactions, like ATP.58

• Consider tardigrades.

• Reading.


3.14 Mass Extinction

• There is much evidence of recent impacts in our solar sys-tem.59

Consider Meteor Crater (APoD image search: “meteorcrater”), Tunguska (Google image search: “tunguska”), andComet Shoemaker-Levy 9 striking E (APoD image search:“shoemaker-levy 9”).

Consider the extinction of the dinosaurs, the K-T boundary,irridium, and the Yucatan.60

• Reading.

– TCPF2: Chapter 6, Section 2, pages 108 - 109 andSection 3.

4 Extraterrestrial & Extrasolar Mat-ters

• The word “extraterrestrial” means nothing more than “out-side the Earth”. For example, the Moon is extraterrestrial.

• What are the characteristics of life? Can we define life?We would like a definition of life that is not circular. Forexample, we can’t define life in any way related to death ifdeath is defined as the end of life. We would like a definitionthat is unique. How do we go about finding such a definition?One way is to consider necessary & sufficient conditions.

58WSS 180 5-1, 5-2 (20’), WU ALL 4-5 (13’)59WSS 120 4-3 (14’)60Note that this coincided with 105 years of volcanos in India.


– If no A, then no B. Here A is necessary in order for Bto happen. Can we find an A that allows us to makethe following statement? “If it doesn’t have A, it’s notalive.” The answer is yes, and there are many examples,one of which is metabolism. If it doesn’t metabolize,it’s not alive. So metabolism is a necessary conditionfor life. (What about a virus?)

In contrast, consider emotions. Is the following state-ment true? “If it’s not emotional, it’s not alive.” Theanswer is no. Clearly, mushrooms are not emotional,yet they are alive. So emotions are not a necessarycondition for life.

– If A, then B. Here A is sufficient in order for B tohappen. Can we find an A that allows us to makethe following statement? “If it has A, it’s alive.” Theanswer is yes, and there are many examples, one ofwhich is emotions. If it’s emotional, it’s alive. (There isnothing that’s emotional that’s not alive.) So emotionsare a sufficient condition for life.

In contrast, consider metabolism. Is the followingstatement true? “If it metabolizes, it’s alive.” Theanswer is no. Clearly, factories metabolize, yet they’renot alive. (A factory uses raw materials and energy tomanufacture.) So metabolism is not a sufficient condi-tion for life.

Ideally, we want a condition that is both necessary and suf-ficient. The challenge is to think of one. If we could, thenwe would have life defined.

• We can play the same game of necessary and sufficient con-ditions for intelligence. Can intelligence be defined?

• So we find that we cannot define life. And we cannot defineintelligence. This established, we can still agree to proceedwith the discussion. We can still consider, without stum-bling over the lack of exact definitions, concepts related tothe possible existence of extraterrestrial life and extrasolarintelligence.

• Reading.


• Homework.

1. (8 points) Consider two attempts to define human. Ineach case, state whether the phrase is (i) necessary,(ii) sufficient, (iii) both, or (iv) neither. (a) Has twohands and two feet. (b) Born of a human. Justify youranswers

2. (8 points) Consider two attempts to define human. Ineach case, state whether the phrase is (i) necessary,(ii) sufficient, (iii) both, or (iv) neither. (a) Thinkslogically. (b) Writes poetry. Justify your answers

4.1 Search for Extrasolar Planets

• Long before extrasolar planets (a.k.a. exoplanets, thosearound stars other than our Sun) were detected, they werethought to exist as a result of the star formation process.

While not all stars were thought to have planets, it was ex-pected that roughly one third would.61 The first exoplanetwas found in 1992. As of 2012, more than 750 exoplanetshave been found.

• The detection involves a very slight “wobble” in the star thatthe exoplanet is orbiting. That is, the star’s center of massis “orbiting” around the center of mass of the “star-planetsystem”. [sketch 83] The technique involves Doppler-shiftedspectral lines. [sketch 84]

• Reading.


4.2 Possible Existence of Extrasolar Intelli-gence

• The Drake equation is used to help us organize our thoughtsabout the possible existence of extrasolar intelligence. Andwhile it provides a quantitative answer, this answer is notmeant to be reliable in any way. It can’t be reliable becausemost of the values in the Drake equation are completelyunknown.

• The equation is currently written as

N � R�fpnef`fifcL.

– N is the number of transmitting civilizations in ourGalaxy.

– R� is the galactic birthrate of stars per year suitablefor hosting life.

– fp is the fraction of such stars having planets.

– ne is the number of planets and/or moons per star sys-tem that have an environment favorable for life.

– f` is the fraction of such planets and/or moons on whichlife has developed.

– fi is the fraction of inhabited worlds on which intelli-gence has developed.

– fc is the fraction of “intelligent planets” that producea civilization capable of interstellar communication.

– L is the lifetime that such civilizations are broadcastingsignals.

If we consider the units of the equation, we find the following.

# of t.c. �

��

yr

��

�

�pla.

�

�i.w

pla.

�int.

i.w.

�t.c.

int.

pyrq

Hence, we see all the units on the right hand side cancelexcept the number of transmitting civiliations.

• Estimate.

• Reading.


• Homework.

1. (15 points) What are your personal estimates for theterms in the Drake equation? Why? What total num-ber results (i.e., multiply your terms together) fromyour estimates? (Beware, most terms are between 0and 1.)

61This is because it was estimated that roughly half of the “points of light”in the night sky are binary stars or multiple stars.


4.3 Search for Extraterrestrial Intelligence

• Whenever the search for extraterrestrial intelligence(a.k.a. SETI) is considered, anthropocentrism must be con-sidered with it. That is, one must not assume any processesin the universe are at all affected by the human race; theuniverse doesn’t “revolve” around us.

• There are at least three options for possible communicationwith an extraterrestrial civilation. Efficiency and practical-ity vary widely among the options.

– A round trip for a small space craft to the nearest starat 70% of the speed of light (an eleven-year time frame)would cost $50 trillion and use the same amount ofpower that the USA would use in 100,000 years. Asimilar trip to the center of our Galaxy would cost anduse 10,000 times as much.

– To send a single electron to the nearest star and backin an eighty-year time frame at 10% of the speed oflight would require very little energy: about 10�16 J.However, compare this with a photon.

– To send a single radio photon, which automaticallytravels at speed c, to the nearest star and back wouldonly take about eight years, and it would require onlyone billionth as much energy as the electron previouslyconsidered. A standard 100-W bulb uses about 1027

times as much energy in an hour.

The photon is the obvious choice to begin with. (Is thischoice anthropocentric?)

• Once the decision for a photon is made, the spectral bandmust be chosen. It’s best to choose a spectral band thatisn’t noisy62 so that the signal will stand out against thebackground.

The visible, UV, x-ray, & γ-ray spectral bands are attenu-ated by interstellar dust clouds. So these frequencies cannottravel well across a galaxy. This leaves the radio, microwave,& IR spectral bands. Check out APoD; image search “milkyway”.

• Within these three spectral bands, there are at least foursources of noise: the universe, the Galaxy, the quantum limit,and the Earth’s atmosphere. [sketch 85]

– The universe is noisy at frenquencies below about 100GHz as a result of the cosmic microwave backgroundradiation from the big bang.

– Our Galaxy is noisy at frequencies below about 2 GHzas a result of synchrotron radiation, which is producedwhen electrons spiral around the galactic ~B. This elim-inates the the low-frequency side of the radio spectralband.

– The quantum limit gives rise to radio receiver noiseat frequencies above about 12 GHz as a result of theuncertainty principle, which describes one of the funda-mental properties of the universe. This eliminates theIR spectral band.

62For example, getting a distant person’s attention when surrounded byloud voices and music can be a challenge. Yelling doesn’t help because itjust blends in with the noise. There is at least one thing that stands outagainst the noise: broken glass.

– The Earth’s atmosphere is noisy at frequencies betweenabout 5 and 300 GHz, as a result of water vapor andoxygen in the air. Note that of the four sources, this isthe only one that is anthropocentric.

So we are left with a frequency range roughly from 1 to 5GHz. This is in the microwave and the high-frequency radiospectral bands.

• This spectral region still contains many (at least billions)of possible frequencies. Is there a way to choose just one?As it happens, there is sort of a “magic frequency” in thisrange: 1.42 GHz. It’s “magic” because it is one of the mostwell-known frequencies to anyone with knowledge of basicatomic physics and spectroscopy.

This radio photon is produced as a result of the “H spinflip”,63 when the electron’s spin transitions from parallel toantiparallel relative to the proton’s spin. [sketch 86]

Not all astronomers agree on the 1.42 GHz. Should we ex-pect an extraterrestrial civilization to agree on it?

• Communication involves transmitting and receiving infor-mation. It’s much easier to receive than transmit since itrequires so much less energy. On Earth, the SETI Project ishoping to receive radio signals. The very simplest messagethat can be sent or received is “I am here.”.64 All that is re-quired is the reception of a signal that stands out against thebackground noise in such a way that there is no ambiguity– a signal that must have been artificially generated.

• Consider a series of ones and zeros, in which the onesrepresent signal pulses and the zeros represent pausesbetween the pulses. What if we received a verylong series of 1010101010101010101010101010101010....?Should that be interpreted as artificial? The an-swer is no, as this is the very pattern we receivefrom pulsars.65 What about the following sequence?1011011101111101111111011111111111011111111111110

• How likely is it that the SETI Project will receive the sig-nal they are hoping for? Not likely, given that the firstsuch project was in 1960. It’s like searching for the needlein a haystack, except this is a 5-D haystack! From whichdirection will the signal arrive? When? At what (likelyDoppler-shifted) frequency? What polarization? How willit be coded? Will the signal be strong or faint?

What if you want to win a billion-dollar radio contest? Pre-tend the contest is simply to call the number with your phonewhen the DJ says, “Call this number!” But you don’t knowwhich band: AM, FM, or satellite. You don’t know whichstation. You don’t know which city, in which country. Youdon’t know when the annoucement will be made. And youdon’t know which language.

How will you decide upon a strategy? Will you spend allof your money to hire people all over the world to listen foryou? If you lose, you’ll be out a lot of money. Will you buy

63This hyperfine transition is actually 1 2S1{2 F � 0 Ñ 1.64This message is so simple that it can be communicated to a tick on the

ground. Even though the tick doesn’t speak words, it will clearly receiveyour message the instant before you squash it underfoot.

65A pulsar is a type of neutron star that sends synchrotron radiation inthe direction of Earth. The signal comes in the form very regularly-spacedpulses of radiation. As a joke, before the cause was understood, they werenicknamed LGM’s, which stands for “little green men”.


lots of different radios so you can listen to all the stations onall the bands (AM, FM, satellite) at once? Will you fly fromcountry to country, hoping to get lucky? Will you decidenot to sleep anymore, so that you can listen every second ofevery day? Will you buy a huge antenna, in case the signalis faint?

• With the SETI Project, the “trade-offs” must be accepted.A large-diameter telescope will detect faint signals, but itwill limit the number of directions observed at any giventime.66 A long exposure time will detect faint signals, but itwill limit the number of directions you can observe over anextended period of time.67 [sketch 87]

If a few strong signals are expected, the choice is smaller di-ameter and shorter exposure times. If many weak signals areexpected, the choice is larger diameter and longer exposuretimes.68 But there’s no way to know what to expect!

• Reading.


4.4 Exam III

Exam 3 covers material between exam 2 and here.

4.5 Final Exam

The final exam is cumulative up to this point.

5 Miscellaneous

5.1 Pseudoscience

• Just for fun, we may consider pseudoscience associated withStonehenge, the Pyramids of Giza, The Nazca lines, the“face” on Mars (or a tortilla), the Loch Ness monster, Big-foot, the Bermuda triangle, crop circles, alien abductions, orany other suggestions.

66This is just like using binoculars. Without the binoculars, your eyes seemany directions at once: almost everything in front of you. With binoculars,you can clearly see objects much farther away, but you only see a tiny fractionof the things in front of you.

67This is like taking photos at night. You have to use your camera’s modethat leaves the shutter open for a long time. But this means you can’t takeas many photos in an hour.

68Actually, the sensitivity is mathematically related to diameter and time:S 9 pD2

?tq�1.

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