184 PART 2^ |^ THE STARS
■ FIGURE P-12
In this artist’s conception, the fi rst stars produce fl oods of ultraviolet photons that ionize the gas in expanding bubbles. Such a
storm of star formation ended an age when the universe had expanded in darkness. As star formation began in the universe, the
production of the chemical elements heavier than helium also began. (K. Lanzetta, SUNY, A. Schaller for STScI, and NASA)that is when the fi rst protons and electrons formed. Protons and
electrons, even when they are not attached to each other, are
hydrogen, so the fi rst gas of the big bang was hydrogen. Th e
temperature and density were so high that nuclear fusion reac-
tions could occur, fusing some of the hydrogen into helium.
By the time the universe was a few minutes old, expansion
had cooled its gases, and nuclear fusion stopped. In those fi rst
few minutes, about 25 percent of the mass had been converted
into helium. But because there are no stable atomic nuclei with
masses of fi ve or eight times that of hydrogen, the fusion pro-
cess could not go past helium. Few heavier atoms could be
made.
As the universe expanded and cooled, it became dark.
Gravity drew matter together to form great clouds that con-
tracted to form the fi rst galaxies. Eventually, stars began to form;
and, as those fi rst stars began to shine, they lit up the universe
and ionized the hydrogen gas in great shells around the galaxies
(■ Figure P-12).
Since the formation of the fi rst stars, galaxies have collided
and merged; generation after generation of stars have lived and
died. Mass loss from aging stars, planetary nebulae, and super-
nova explosions has spread heavy elements out into the thin
gas in space where they have become incorporated into new
stars in a continuous gas-star-gas cycle (■ Figure P-13). Except
for hydrogen, all of the atoms of which you are made were
cooked up inside stars before the sun and Earth were born
(■ Figure P-14). When the sun began to form from a cloud of
gas, the atoms now inside your body were there. Th ey were
part of Earth as it formed, and now they are part of you. Every
atom has a history that stretches back through the birth and
death of stars to the fi rst moment in time. You exist because
stars have died.