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Lecture 18

AST 101 Lecture Notes - Lecture 18: Atomic Nucleus, Current Source, Red Giant


Department
Astronomy
Course Code
AST 101
Professor
Walter Freeman
Lecture
18

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The$Sun
We’ve$explained$the$solar$spectrum$now:
the$hot$innards$of$the$Sun$emit$blackbody$radiation$because$of$their$
temperature
the$cooler$gases$in$the$Sun’s$atmosphere$absorb$those$wavelengths$
corresponding$to$their$energy-level$transitions,$giving$rise$to$the$missing$
wavelengths$(dark$lines)$in$the$spectrum
But$why$is$the$Sun$so$hot?$What’s$the$source$of$the$Suns$energy$in$the$first$
place?
Nuclear$fusion
The$answer$is nuclear(fusion a$nuclear$reaction$involving$building$larger$ nuclei$out$of$
smaller$ones.
The$Sun$is$mostly$made$of$hydrogen.$The$inside$of$the$Sun$is$so$hot,$and$under$such$
terrific$pressure,$that$all$of$the$electrons$are$torn$off$of$their$atoms;$for$this$discussion,$
we’re$going$to$be$thinking$only$about$the$nuclei$of$these$atoms.$Most$hydrogen$nuclei$
are$just$single$protons.
As$you$know,$protons$are$positively$charged.$Since$like$charges$repel,$if$two$protons$
get$near$each$other,$they$will$violently$repel$each$other.$It$takes$a$tremendous$amount$
of$heat$and$pressure$to$overcome$this.$However,$this$amount$of$heat$and$pressure$is$
found$in$the$heart$of$the$Sun,$where$the$temperature$reaches$15$million$Kelvin$(or$
Celsius).
So,$what$happens$if$you$smash$together$two$protons$hard$enough?$Well,$if$you$can$
overcome$the$electric$ repulsion$and$bring$them$very$close$together,$then$there$is$
an attractive nuclear$force$that$will$then$hold$them$together.$In$the$process,$one$of$
them$changes$to$a$neutron$(absorbing$an$electron$in$the$process).$This$gives$us$a$new$
kind$of$atomic$nucleus,$one$made$out$of$a$neutron$and$a$proton$stuck$together.$(It’s$
called$“deuterium”$chemically,$it$is$an$isotope$of$hydrogen,$but$for$nuclear$purposes$
The$Composition$and$Workings$of$the$Sun
Thursday,$November$ 3,$2016
9:06$PM
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