Chapter 16
Live Wires and Dead Stars
Stars die. Their long livesare interesting andcomplex, occasionallyculminating
infantastic explosions(supernovas) thatcan brieflyoutshineentiregalaxies. The
detailsarethesubjectmatterofsomeothercourse. Forus,itissufficienttoknow
thatevery”living”starbalancesitsimmensegravitationalforce,whichtendstocrush
alltheatomsofthestarinwardtoasinglepoint,withanequallyenormousoutward
pressure,duetotheheatproducedbynuclearfusion.Eventually,anystarwillexhaust
itsnuclearfuel,andthenthegravitationalforce,unopposed,crushestheatomsofthe
startoafantasticdensity.Inthecourseofcollapse,forstarsgreaterthanaboutone
solarmass,theatomicelectronsareabsorbedbyprotonsinthenuclei,viatheprocess
e− + p → n + ν (16.1)andthemasslessneutrinos ν, duetotheirweak interactionwithallothermatter,
escapeintospace. Atthisstage,alloftheparticlescomposingthestarareneutrons,
thedensityofthestarapproximatesthatofatomicnuclei,andthedeadstarisknown
asa”neutronstar.”Forstarswithmasseslessthanaboutfoursolarmasses,thatis
theendofthestory: thecolddeadstarremainsasaneutronstaruntiltheendof
time. Butthisbringsupthequestion: whatforcecantherepossiblybe,withinthe
coldneutronstar,thatiscapableofopposingthemightygravitationalforce,which
wouldotherwisecrushallthematterofthestartoasinglepoint?
Itseemsincrediblethatallthisastrophysicaldramashouldhaveanythingatall
todowiththeapparentlymoremundanequestion ofwhy somematerialsconduct
electricity,andsomedon’t.Nevertheless,thephysicsofdeadstars,andthatofquite
ordinarysolids,arerelatedincertainunexpectedways.Bothneutronstars,andthe
electronsincold metals, areexamples of what areknownas degenerate Fermi
gases. Webeginbytakingupthequestionofhowisitpossibleforcertainsolidsto
conductelectricity.