the private life of a proton

7/28/2019 The private life of a proton

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The Private Lie

o a ProtonHelen Johnston


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Very ew things are orever.Molecules re-arrange and re-

make themselves constantly, incountless chemical reactions; and

even atoms can be made and destroyed in theinteriors o stars, orging entirely new chemicalelements. Only protons are truly unchanging:every proton in every atom in the universe hasbeen there since the very beginning1. But overthe lie o the universe, those protons may havebeen through many dierent guises. I one othose protons could tell its story, what a story

that would be ...Well, here is that story.

In the beginning, there was a proton.

Actually, it wasnt quite at the beginning. At thevery beginning was the Big Bang, a momento innite temperature and density. The wholeo the universe we see today was compressedinto a region smaller than an atomic nucleus.

1 Actually, under some circumstances, protons can turninto neutrons and vice-versa. However, it takes energyto convert a proton to a neutron, so let to themselvesneutrons will decay into protons but not vice-versa.


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200|Genes to Galaxies

Figure 1:History o the Universe. Microcosm CERN


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The Private Lie o a Proton|201

We have no laws o physics to describe whatwas going on under these conditions, so ourdescription o the universe has to start a tinyraction o a second ater time zero. Ater thattime, the universe began to expand, and as itdid so, it cooled.

The story o our proton begins just 0.01 sec-onds ater the Big Bang. Beore that, the uni-verse had been too hot and dense or protonsto exist: instead, it was a seething mass o pho-tons2, electrons, positrons, neutrinos, quarksand antiquarks. It was not until the tempera-ture o the universe had dropped below severalmillion million degrees that quarks could bindtogether to orm protons and neutrons.

This is where we meet our proton or the

rst time.

For a while, the existence o the proton isvery transitory: every time it meets an anti-proton the two annihilate, converting theirmass-energy into a pair o energetic photons.These photons then spontaneously convertthe energy back into mass, producing a newproton/anti-proton pair, which speed awayrom each other.

There comes a time, however, when the

universe has cooled just enough that thephotons no longer have enough energy toproduce new particles. When that happens,most o the particles and antiparticles annihi-late each other one last time. Our proton wasone o the ew the very ew that did notnd an antiparticle. For each lucky particle,30 million other particles did not make it.One second ater the Big Bang, our protonnds itsel in a universe made up o energyand matter, with essentially no antimatter.

For reasons we still dont understand, there wasa tiny imbalance o matter over antimatter orevery 30 million antiparticles there were 30million and one particles. Ater the annihila-tion had nished, only this small amount olet-over matter remained: the rest had disap-peared into radiation. So about 1 second ater

2 Electrons and positrons are examples oantiparticles,which have the same mass but opposite electric charge.

Particle-antiparticle pairs can annihilate one another andconvert their entire energy into two photons. Matter ismade o protons, neutrons and electrons, while antimatteris composed o antiprotons, antineutrons and positrons.

the Big Bang, there was about one proton orneutron or every billion photons or electronsor neutrinos.

The universe is a seething maelstrom.Protons and neutrons smash into each other,moving much too ast to stick together. The

entire universe still consists entirely o sub-atomic particles.

As the temperature drops, the particles startmoving more slowly. Now when protons andneutrons meet, they can stick together; thestrong nuclear orce grabs them and bindsthem together into the rst nuclei. All aroundour proton, particles are sticking togetherin clumps: rst two, then three and our.The our-nucleon clump two protons andtwo neutrons is the most stable: a heliumnucleus. But not ve: when a our-particlehelium nucleus is struck by another particle,the whole lot is split apart again.

By the time the universe is a bit more thanthree minutes old, nearly all the neutronshave combined into nuclei, while most othe protons (including ours) are still ree.About 90% o the universe is hydrogen, withnearly all the rest made up o helium. Thereis some deuterium3, and tiny amounts o

lithium and beryllium, but nothing else. Therst elements have been born, albeit withoutelectrons: it is still too hot or the electronsto combine with the nuclei to orm atoms.Shortly aterwards, when the temperaturebecomes too low or nucleosynthesis totake place, the production o nuclei stops.No more elements will be ormed or along time.

This story o how the rst elements were

ormed is extremely well understood. As theuniverse cooled, new particles could be madeout o old ones. We can measure the rates atwhich these reactions occur in particle ac-celerators, and by applying the concepts othermodynamic equilibrium, we can predictwhich particles will be ormed as the universecools. It turns out that the nal compositiono the universe depends only on the baryon

3 Deuterium is the name given to heavy hydrogen,hydrogen-2, whose nucleus consists o one proton and oneneutron bound together. The ordinary hydrogen nucleus,hydrogen-1, has just a single proton.


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This period when electrons were trapped intoatoms the recombination era4 has one impor-tant consequence. Once the photons were nolonger continually bouncing o electrons, theycould begin fying reely. Most have been fyingreely ever since. We can observe these photons

today as the cosmic microwave background.Since they began travelling, the universe hasexpanded by a actor o 1000, so the tempera-ture has dropped rom 3000 degrees to just 3degrees above absolute zero.

The cosmic microwave background radiation isalmost uniorm in all directions. The WilkinsonMicrowave Anisotropy Probe (WMAP) was asatellite launched in 2001 to measure the tinyvariations in the temperature o the radiation:

the temperature over the sky ranges rom2.7251 to 2.7249 degrees Kelvin.

The universe is completely dark. There areno stars; nothing is hot enough to produceany visible radiation. There is no source olight anywhere.

Once the temperature o the universe haddropped below 3000 K, the wavelength o theaverage photon making up the background

4 Actually, it should be called the combination era, sincethe electrons and nuclei had never been united beore.

density how many protons and neutrons therewere in a given volume o the early universe.By measuring the abundance o elements likedeuterium in the oldest stars, we can determinethis baryon density.

It was still too hot or the electrons to com-bine with the nuclei to orm atoms, so ourproton ound itsel ree in a sea o protonsand helium nuclei, surrounded by photonsand electrons.

The entire universe consisted o nothing butthis ionised gas: aplasma. Electrons and nucleimoved reely about. Because electrons are verygood at scattering photons, light cannot travelar beore hitting an electron and fying o inanother direction. This has the eect o makingthe universe opaque: light is scattered aroundjust like being inside a og.

Nothing else much o note happens or along time, while the universe continues toexpand. About 380,000 years later, the tem-perature has cooled to about 3,000 degrees.Finally, it is cool enough or electrons tocombine with nuclei to orm stable atomswithout being ripped apart again. Once ithas captured an electron, our proton is no

longer a ree proton. It is now the nucleus oa hydrogen atom.

Figure 2: The cosmic microwave background, observed by the Wilkinson MicrowaveAnisotropy Probe (WMAP). The average temperature is 2.725 K; the colours represent tinyfuctuations (0.0001 degrees) rom this mean, with red regions warmer and blue regionscolder.Image: NASA/WMAP Science Team


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radiation had shited into the inrared. Nothingin the universe was emitting visible light.

This cosmic dark age lasted or perhaps a hun-dred million years.

Our hydrogen atom moves aimlessly aspart o the gas that makes up the universe;almost, but not quite, completely uniorm. Byrandom chance, in some regions the atomsare slightly closer together than in other

regions. This means they have slightly moremass, so their gravity pulls in more material,so they get denser still. By such means, littleby little, the universe gets lumpier.

In the dark, things were happening. The mat-ter was distributed almost but not quite evenlythrough the universe: we still see the imprinto these tiny fuctuations in the backgroundradiation. As time went on, the clumps o mat-ter grew; gravity was assembling the compo-

nents o the universe. The physics o how thishappened is extremely complicated: we needsupercomputer simulations to understand howstructures grew. These simulations show thatthe matter develops into a web o laments,with voids separating the denser regions. Wesee these laments today in maps o galax-ies. The 2dF Galaxy Redshit Survey, whichwas done at the Anglo-Australian Telescope,measured the distances to nearly a quarter o amillion galaxies, enabling astronomers to makea three-dimensional map o the universe. Thedistribution shows clusters and laments, sepa-rated by vast voids almost devoid o galaxies.

For a long time, our hydrogen atom drits.For millions o years the drit is barely per-ceptible, but gradually it becomes apparentthat the gas is getting denser, and that itis driting in a particular direction. The gascloud containing our hydrogen atom is nowstretched out like a long thread. Then at last,

something has changed. Millions o lightyears away in the direction the cloud is drit-ing, there is light: the rst light that has been

Figure 3: Snapshots o a very largevolume o space in the MillenniumSimulation, which ollowed the ormationo galaxies and clusters. The yellow pointsrepresent individual galaxies.Images: G.Lemson and the Virgo Consortium


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Figure 4: A slice o the universe rom the 2dF Galaxy Redshit Survey: each dot isa galaxy. Matthew Colless

at higher redshit have all their blue light ab-sorbed by neutral hydrogen gas, while quasarsat lower redshit do not. Re-ionisation almost

certainly did not happen all at once; instead,bubbles o ionised gas ormed around stars (orquasars). As the number o bright objects grew,the bubbles merged together and cleared upthe og o neutral hydrogen, allowing the bluelight to travel reely.

Evidence is beginning to suggest that it wasstars that rst re-ionised the universe. The mostdistant quasars yet observed already show thepresence o elements heavier than hydrogen

and helium, which means (as we shall see) thatthere must have already been massive stars be-ore these quasars were born.

Composed only o hydrogen and helium (and atiny amount o lithium), these rst stars wouldhave been very dierent rom the stars ormingtoday. Theory predicts they would not onlyhave been much hotter than stars orming now,but also that they could have been much moremassive. Stars orming today cannot be more

massive than about 150 times the mass o ourSun. Beyond that mass, the star produces somuch radiation that the outward pressure o

seen since the universe cooled below 3,000degrees. Far ahead, one o the rst stars inthe universe is shining.

Eventually, the rst objects which producedlight were born. This light not only illuminatedthe darkness, but also stripped the electrons othe atoms in the interstellar gas again. We arestill not sure which objects were responsibleor this cosmic re-ionisation. They had to beobjects producing large numbers o energeticphotons: hydrogen is ionised by radiation withwavelengths shorter than 91.2 nm, which areultraviolet photons. The most likely candidates

are either quasars or the rst stars. Quasarsare supermassive black holes, which are ac-creting gas rom a swirling disk and sendingnarrow jets o high-speed particles and radia-tion towards us. We know the universe wasalready re-ionised by the time the universe was1 Gy old5, at a redshit o 6, because quasars

5 Redshift describes how much the wavelength o the lightwhich reaches us has been stretched by the expansion othe universe since it let the source. More distant objects

have higher redshits, and so when we observe an object athigh redshit, the light has been travelling since a long timein the past. The relation between redshit and age o theuniverse is given in Table 1, see end o chapter.


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Figure 5: Cosmic reionisation in the history o the Universe. S. G. Djorgovski et al., Caltech.Produced with the help of the Caltech Digital Media Center.


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the radiation exceeds the inward pull o grav-ity, and the star tears itsel to pieces. The veryrst stars, however, could potentially grow tobe much more massive: several hundred, per-haps even up to a thousand solar masses. Suchstars would have extremely short lietimes amillion years or less ater which they would

explode, seeding the interstellar medium withheavy elements. Their collapsing cores mayeven have provided the seeds which grow intothe massive black holes we see at the centres oquasars and galaxies.

The region towards which our hydrogenatom is alling is now perceptibly a proto-galaxy. At its centre is a black hole, ormedrom the embers o one o the dying rststars. Since its ormation it has grown con-siderably, by merging with other black holes,and by sucking down enormous quantities ogas. Surrounding it is a cloud o stars ormed

rom the gas that continues to accumulate.The gas cloud containing our proton gradu-ally swirls towards the centre o the growinggalaxy. As smaller clumps come too closeand are pulled in, some o the gas is fungcompletely away, doomed to swirl oreverin the almost empty regions o intergalacticspace. Other gas nds itsel being hurled

towards the centre o the galaxy. There it ispulled into a swirling, super-heated accretiondisk around the black hole, where it will even-tually disappear into the event horizon andbe lost orever, or else squirted at nearly thespeed o light right out o the galaxy in twinjets. Our proton avoids both o these ates;instead, it nds itsel near the centre o adense cloud, which gets denser as more gascollides with it and compresses it.

Whether they were ormed in the rst stars,or collapsed directly rom the gas, or grewrom seeds o primordial black holes created

Figure 6: The radio galaxy Centaurus A, showing twin jets o matter being ejected rom

the central black hole. These jets can be seen at X-ray and radio wavelengths.NASA/CXC/SAO


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in the rst instants o the Big Bang, we knowthat massive black holes already existed andwere growing less than a billion years ater theBig Bang. The most distant quasar currentlyknown has a redshit o 6.43, so it was ormedwhen the universe was only 0.87 Gy old. Radiogalaxies are also powered by supermassive

black holes, but the jet is seen side-on, so wesee radio emission rom the jets. Radio galaxiesare known at redshits o up to 5.19, when theuniverse was just over 1 Gy old. So the blackholes must have been ormed very early onin the universe. Did they exist rst and thengalaxies grew around them? Or did the galaxyand the black hole both orm together? We stilldont know. We do know, however, that almostevery galaxy has a massive black hole at itsheart, and that the bigger the galaxy, the bigger

the black hole. This suggests that the growth

o the galaxy and the black hole are somehowintimately linked.

However they began, the evidence suggests thatboth the galaxy and the black hole at its heartgrow as a result o the merging o galaxies.Everywhere we look, we see signs o galaxies inthe process o colliding, or showing evidence o

collisions in the not-too-distant past. And theurther back we look, the more common thesecollisions seem to be. When galaxies collide,the stars almost never collide: their physicalsize is so small compared to the vast distancesbetween them that they just pass reely pasteach other. However, the enormous gas anddust clouds in both galaxies do collide: thegas is compressed, which triggers more starormation. Meanwhile, some o the gas and

stars are fung out in huge tidal tails, whilesome is sent spiralling towards the centre othe galaxy, where it can eed the black hole.

Figure 7: A series o interacting galaxies observed by the Hubble Space Telescope.

ESA/Hubble


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Our own Milky Way galaxy contains the debriso many dwar galaxies it has swallowed up inthe past, and it is currently in the process odevouring a ew more; the Magellanic Cloudswill be devoured within the next ew hundredmillion years.

Over millions o years the gas cloud getsdenser and colder, and our proton (now ahydrogen atom) nds itsel in a molecule orthe rst time: two hydrogen atoms are boundtogether as molecular hydrogen, H2.

The interstellar gas is the raw material romwhich stars orm. A cloud o gas has a tendencyto collapse under its own gravity, but this in-ward pressure is resisted by the gas pressure.This is sucient to resist the collapse until a

critical threshold is passed, when the collapsebecomes unstoppable. This threshold massdepends on the temperature and density othe cloud, with colder and denser clouds morelikely to collapse. So stars tend to orm in thecoldest, densest regions o gas; these regionsare calledgiant molecular clouds.

A typical giant molecular cloud might be50100 light years across and contain a millionor more solar masses o material. The gas is

now not just the pristine material made in the

rst ew minutes ater the Big Bang; the rststars polluted the interstellar gas with heavyelements when they exploded. So by the timethis second generation o stars begins to orm,the gas cloud already contains small amountso (amongst other things) carbon, nitrogen,oxygen, and other elements. Astronomers canmeasure the proportion o heavy elements inthe spectra o stars, and nd that older starshave signicantly lower proportions thanyounger stars. In our own galaxy, these low-metallicity stars6 are ound primarily in theGalactic bulge and in globular clusters, whileyounger, more metal-rich stars like our Sun areound in the disk o the galaxy.

When the giant molecular cloud starts to col-

lapse, it continues under its own momentum.More and more gas alls inwards as the collapseaccelerates. Multiple clumps develop in thecloud, as denser-than-average regions pull inmore and more gas. Eventually the cloud rag-ments into hundreds o small dense globules,each o which will eventually become a star.The collapse is astest near the centre o eachglobule where, inside a cocoon o gas and dust,the dense core o gas is getting hotter as the ki-netic energy o the accreting matter is convert-

ed into heat. As the density increases the cloudbecomes opaque, trapping the heat within thecloud. This then causes both the temperatureand pressure to rise even more rapidly thecollapsing cloud is now aprotostar.

The cloud containing our proton has beenaccumulating mass, getting denser andmore massive as more gas alls in. Oncethe runaway collapse has started, the gascontaining our proton begins its long all

towards the dense knot that will become thenewborn star. The gas heats up as it alls; rst

6 To an astronomer, every element other than hydrogenand helium is called a metal, so oxygen and carbon aredescribed as metals. Only the rst generation o stars,made rom the primordial gas o hydrogen and helium, wasmetal-free.

Figure 8: The young star cluster NGC 602in the Small Magellanic Cloud, showing

hot young stars just emerged rom theirbirth cloud.NASA, ESA, and the Hubble Heritage Team


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the molecules are stripped apart, then theelectrons are ripped rom the atoms. Whenthe collapse nally ends, our proton ndsitsel near the centre o a young, massive star,about 25 times the mass o our Sun.

The collapse only comes to an end when the

star nds a source o energy to balance theinexorable inward pull o gravity. That sourceis nuclear usion: when the temperature atthe core reaches about 15 million degrees,hydrogen nuclei can begin to use together toorm helium, just as they did in the rst ewminutes ater the Big Bang. Energy is a producto that usion, so as each set o our hydrogennuclei uses into helium via a series o nuclearreactions, energy is produced which increasesthe temperature and pressure o the stars coreenough that it can resist the inward pull ogravity. A star has been born.

Our proton sits near the core but not in it.Here, where our proton sits, a bit less thana quarter o the way out rom the core, thetemperature is lower. Unprotected by theirelectron shells, protons regularly collide, buttheir positive charge means they just bounceo each other. It is only deep in the core,more compressed by the great weight o the

star, that the nuclei are moving ast enoughto collide and orm new elements: rst deu-terium, then helium.

This conversion produces enough heat tosupport the immense mass o the star againstthe inexorable pull o gravity.

As long as the star can continue using hydro-gen to helium in its core, it can maintain itsequilibrium against the pull o gravity. The starstays like this or 6.6 million years, steadilyconverting the hydrogen in its core to helium:the star is a main sequence star. Outside thecore, where our proton sits, no usion is takingplace: the material o the star is still the same asthe original gas cloud rom which it was born:mostly hydrogen, with some helium and sometrace amounts o other elements let over romprevious generations o stars.

For six and hal million years, nothing muchhas changed or our proton. Enormous

quantities o radiation food past everysecond, produced in the core and fowing

through the star to its surace, there to shineinto space.

But this situation cannot last. Eventually thetime comes when the hydrogen in the core isexhausted. When that happens, the core can nolonger support itsel against gravity, so it starts

to collapse. When it does so, it heats up, andhydrogen just outside the core nds itsel hotenough to use into helium or the rst time.This sudden increase in energy orces the outerlayers o the star to swell up dramatically: thediameter o the star increases by a actor o200. The outer layers, being so much larger,also cool dramatically, so the star becomesenormously large and red: a red giant star. Ithis star replaced our Sun, it would stretch past

the orbit o Jupiter.Meanwhile, the helium core is still collaps-ing and heating up. Eventually, it becomeshot enough or helium to use: this takesmuch higher temperatures, about 100 milliondegrees. But eventually the helium is also ex-hausted: then the collapse re-commences. Thecycle repeats: each time a uel is exhausted, thecore begins to collapse once more, which heatsit up even higher, so that even heavier elements

can use, which produces more energy to sup-port the star. The usion o these nuclear uelsgoes aster and aster as the atomic numberincreases, both because there are ewer atomsto use, and less energy is released each time.

At last something changes. The pressurebeneath our proton drops, and it starts toall inwards. The all is stopped as the gasbeneath is more compressed, but now thetemperature is higher. Several times thiscollapse and halting happens, with the tem-perature increasing each time. At last the daycomes when, instead o bouncing o otherprotons, the particles collide and stick: ourproton is now part o a deuterium nucleus.Almost immediately, this uses with two moreparticles to orm rst helium-3 and then he-lium-4. Later, ater more collapse o the core,this helium-4 nucleus uses with two others toorm a carbon-12 nucleus.

Once silicon has used to iron, however, there

is no next step. Iron is the most stable nucleus,and does not release energy when it is used:adding anything else to the nucleus costs


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energy instead o producing it. The star hasreached the end o the line, and can no longersupport itsel. Gravity has won.

The core o the star collapses inwards. Theinwards pressure orces electrons and protons

in the core to combine to orm neutrons. Theseneutrons are squeezed so tightly together thatin less than a second the whole core o the star,weighing about one and a hal times the masso our Sun, is compressed to a sphere only 15km in diameter: it has become a neutron star.

Meanwhile, the outer layers o the star are stillalling, oblivious to what is happening to thecore. When these layers meet the newbornneutron star, they bounce o it so hard that

they are ejected outwards again at a substantialraction o the speed o light. This creates ashock wave which blasts the whole envelope o

the star outwards at tremendous speed. Whenthis blast wave reaches the surace o the star,it becomes visible as an enormously expandingreball: a supernova.

Sitting in the layer o carbon, our proton (in

its carbon nucleus) has no warning o the cat-astrophic events that have taken place deepbelow it in the core o the star. The rst signthat something has changed is that the pres-sure beneath the layer suddenly drops; thestar begins to collapse. With nothing sup-porting it rom beneath, the outer layers othe star, including the carbon atom contain-ing our proton, begin to all inwards. Secondslater, however, the blast wave explodingoutwards through the star roars past, and the

gas is exploded outwards. All around, nucleiare being used with other nuclei to ormheavier elements, and bombarded by a food

Figure 9: Chandra X-ray image o the supernova remnant Cassiopeia A.NASA/CXC/SAO


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o neutrons in the wake o the blast. Withinseconds, elements up to uranium are ormed,in the crucible o a supernova explosion.

Our carbon nucleus is swept outwards as parto an expanding shell o gas. As it expands,the shell cools, and ater about a hundred

thousand years stops glowing. The remnanto the star, now light years behind, is visibleor a ew million years as a pulsar, then it tooades. The gas rom the explosion, no longerdiscernible as a shell, mingles with the inter-stellar medium.

Without supernova explosions, there wouldbe no heavy elements rom which to orm(amongst other things) rocky planets. Recallhydrogen, helium and a tiny bit o lithiumwere the only elements ormed in the Big Bang,and until the rst stars ormed nearly a bil-lion years later, they were the only elementsin the universe. Once the rst stars had beenborn, heavier elements like carbon and oxygenwere created in their cores, but these elementswere locked up, inaccessible beneath layers ounburnt hydrogen. Supernova explosions notonly liberate these elements into interstellarspace, but are also responsible or creating allthe elements heavier than iron in the explosion

itsel7. All the stars in our Galactic neighbour-hood, including the Sun, have about 1% otheir mass composed o elements heavier thanhelium. All these elements must have comerom earlier generations o massive stars whichlived their lives, then exploded as supernovae,seeding the surrounding gas with the new ele-ments. This gas, now enriched with heavy ele-ments, can then be incorporated into new stars.

Eventually, our carbon atom nds itsel in

another cloud o cold, dense gas. A nearbysupernova triggers the collapse o this cloud.Again, the gas is drawn inwards to wherethe densest regions are pulling in ever morematerial, eventually to reach high enoughtemperatures that nuclear usion begins andstars are born.

7 There is a tiny number o elements that are ormedin dierent ways, like beryllium, ormed when cosmic

rays split heavier nuclei in the interstellar medium, ormolybdenum, ormed in the atmospheres o red giantstars. Every other element in the periodic table is madeinside stars.

Figure 10: Protoplanetary disks aroundstars in the Orion Nebula, rom HST.

NASA, ESA, and the Hubble Heritage Team


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This time, our carbon atom is not in thedense centre o the cloud, so by the time thenew star begins to shine, it is trapped in anicy body at the outer edges o a giant diskswirling around the star.

As the centre o the cloud collapses, the outerregions fatten into a disk surrounding the pro-tostar. The Hubble Space Telescope has takenpictures o these disks in regions like the OrionNebula, showing that they are common aroundyoung stars. It is rom a disk like these that theSolar System ormed.

In the inner regions o the disk, dust andparticles condense out, then stick together,bumping and colliding and growing in sizeuntil the ragments, now calledplanetesimals,have grown to about 1 km in size. Now theirgravity starts assisting with their growth: thelarger they grow, the more matter they attract.The largest planetesimals sweep up all the mat-

ter within their reach, until we are let with ahundred or so proto-planets, each the size oour Moon or larger, orbiting the inant Sun.Over the next hundred million years or so, thisnumber was reduced to the current handul oplanets as the proto-planets cross orbits andcollide in giant impacts. Every old surace inthe Solar System bears witness to having beenbattered by impacts o all sizes.

Orbiting beyond the outermost planets were

the let-over planetesimals, which never coa-lesced into a planet. We still see this disk olet-over bodies as the Kuiper Belt, a regionbeyond the orbit o Neptune containing prob-ably 70,000 or more small icy bodies morethan 100 km in diameter. Pluto and the newly-discovered dwar-planets like Haumea andMakemake are just the largest members o theKuiper Belt.

Six hundred million years ater the planets

ormed, something is happening in the end-less cold at the edge o the Solar System.As they orbit, the icy bodies eel new and

Figure 11: Every old surace in the SolarSystem bears witness to having beenbattered by impacts o all sizes. From topto bottom: Callisto, Mercury and Mimas.


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dierent pulls; orbits that were previously sta-ble are perturbed. Chaos ensues; some bod-ies are fung outwards, but many others arehurled inwards, towards the inner solar sys-tem. The icy rock containing our carbon atomis one o them. It hurtles towards a smallrocky world circled by one large moon...

Simulations o the ormation o the planetssuggest that Uranus and Neptune probablyormed much closer to the Sun than they are

now. The our largest planets interacted witheach other and with the disk o icy planetesi-mals which circled beyond them. The orbitsslowly expanded, until ater about 700 millionyears, the orbit o Saturn came into 2:1 reso-nance with Jupiter, which means that Jupiterorbited the Sun exactly twice or every oneo Saturns orbit.

This made the orbits o Uranus and Neptuneunstable, and their orbits expanded outwardsinto the disk. Planetesimals were scattered inall directions; some were fung outwards, andsome were sent careening into the inner SolarSystem. This late heavy bombardment let scarson most planets and satellites in the solar sys-tem. It produced the great basins o the mariaon the Moon, and may have contributed to theatmospheres o the inner planets. None o therocks brought back rom the Moon were olderthan 3.9 billion years, which is 600 million

years younger than the age o the Solar System.

Any atmospheres the terrestrial planets hadat the beginning would have been lost duringthe worst o the heavy bombardment. As therate o impacts began to ease, however, theplanets began to cool. Gases trapped in the hotrocks were gradually released, and combinedwith the volatiles (water, carbon dioxide etc.)brought by comets and asteroids rom theouter solar system to gradually build up an

atmosphere. On Earth, once the temperaturedropped below 100 C, water vapour couldcondense out and the oceans begin to orm.

Figure 12: The near side o the Moon,taken by the Galileo spacecrat on its way

to Jupiter. The dark areas are the maria:impact basins lled with lava rom ancient

volcanic eruptions.

Only inside stars can transmutation o ele-ments take place, because only inside stars dowe nd the enormous temperatures required.Outside a star, a proton in the nucleus o anatom is almost certain to stay there orever.However, atoms can orm an enormous varietyo molecules with other atoms, and molecularbonds can orm and break at much lower tem-peratures. Carbon in particular orms bonds

with many elements, and can orm moleculeso great complexity.

We dont know how long it was ater the lateheavy bombardment beore conditions wereright or lie to orm, but the evidence sug-gests it wasnt long. The earliest evidence orlie comes rom the study o isotope ratioso carbon-12 to carbon-13 in rocks rom 3.8billion years ago, suggesting that lie arose amere 100 million years ater the late heavy

bombardment stopped.

The cataclysm o the impact that deliveredour carbon atom to the Earth has subsided;the vaporised material rom the icy planetesi-mal has mixed with the existing atmosphere.Sometime later, our carbon atom joins withtwo oxygen atoms to orm carbon dioxide,the principal component o the atmosphereo the young Earth. Soon it nds itsel dis-solved in the newly-ormed oceans. The en-

ergetic UV radiation encourages many dier-ent compounds to orm and re-orm, so ourcarbon atom goes through a continual cycle


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Here are some suggestions or popular-level books covering some o these ideas.

The First Three Minutes: A Modern View O The Origin O The Universe by Steven Weinberg(Basic Books, 1993)

Big Bang by Simon Singh (Fourth Estate, 2004)

The Birth o Stars and Planets by John Bally and Bo Reipurth (Cambridge UP, 2006)

Cosmic Catastrophes: Exploding Stars, Black Holes, and Mapping the Universe by J. CraigWheeler (Cambridge UP, 2007).

The Story o the Solar System by Mark A. Garlick (Cambridge UP, 2002)

Table 1: Redshit and lookback time

Redshift Fraction of current ageTime since Big Bang(in Gy = 10

9

years)

1100 0.0028% 380,000 y CMB

20 1.3% 0.2 GyReionisation

10 3.5% 0.5

5 8.8% 1.2Peak o galaxy

ormation2 24% 3.3

1 57% 5.9

0.5 63% 8.6

0.2 82% 11.3

0 100% 13.7 Now

o new congurations: methane, ammonia,simple amino acids.

One day, a completely new type o moleculeemerges, built around our carbon atom. Thismolecule and its descendants will eventu-ally transorm the planet itsel, changing

its atmosphere, its surace, its oceans. It is

a molecule that can reproduce itsel. Ourproton has made the next step in its longvoyage, rom galaxies to genes...

Further reading:

the private life of a proton

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