Geology Reference
In-Depth Information
werehydrogen—morethan90percent ofallatoms—with afewpercent helium andatrace
of lithium thrown in. That mix of elements formed the first stars.
First Light
Gravity is the great engine of cosmic clumping. A hydrogen atom is a little thing, but
take one atom and multiply it by ten to the sixtieth power (that's a trillion-trillion-trillion-
trillion-trillion hydrogen atoms) and they exert quite an impressive collective gravitational
force on one another. Gravity pulls them inward to a common center, forming a star—a
giant gas ball with epic pressures at the core. As an immense hydrogen cloud collapses,
thestar-formingprocesstransformsthekineticenergyofmovingatomstothegravitational
potential energy of their clustered state, which translates into heat once more—the same
violent process that occurs when an asteroid impacts Earth, but with vastly more energy
release.Thecoreofthehydrogensphereeventuallyreachestemperaturesofmillionsofde-
grees and pressures of millions of atmospheres.
Such temperatures and pressures trigger a new phenomenon called nuclear fusion reac-
tions. Under these extreme conditions, the nuclei of two hydrogen atoms (each with one
proton) collide with such force that neutrons are transferred from one nucleus to another,
making some hydrogen atoms more massive than others. After several such collisions, a
helium nucleus with two protons forms. Surprisingly, the resulting helium atom is about
1 percent less massive than the original hydrogen atoms from which it formed. That lost
mass converts directly to heat energy (just as it does in a hydrogen bomb), which promotes
even more nuclear fusion reactions. The star “ignites,” bathing its surroundings with radi-
ant energy, while becoming ever richer in helium at the expense of hydrogen.
Large stars, many of them much bigger than our Sun, eventually used up the prodigious
suppliesofhydrogenintheircores.Butextremeinteriorpressureandheatcontinuedtopro-
mote nuclear fusion. Two-proton helium atoms in a stellar core fused to make carbon, the
vitalelementoflifewithitssixprotons,evenasnewpulsesofnuclearenergytriggeredhy-
drogenfusioninasphericallayerofatomssurroundingthecore.Thencorecarbonfusedto
make neon, neon to make oxygen, then magnesium, silicon, sulfur, and on and on. Gradu-
allythestardevelopedanonionlikestructure,withlayeruponconcentriclayeroffusionre-
actions. Faster and faster these reactions occurred, until the ultimate iron-producing phase
lastednomorethanaday.Bythispointinthefirststars'lifecycles,manymillionsofyears
after the Big Bang, most of the first twenty-six elements in the periodic table had been
brought into existence by nuclear fusion within many individual stars.
Iron is as far as this nuclear fusion process can go. When hydrogen fuses to produce he-
lium, when helium fuses to produce carbon, and during all the other fusion steps, abundant
nuclear energy is released. But iron has the lowest energy of any atomic nucleus. As when
