166
A SLOW PROCESS
OF ANNIHILATION
OF MATTER
NUCLEAR FUSION WITHIN STARS
IN CONTEXT
KEY ASTRONOMER
Arthur Eddington
(1882–194 4)BEFORE
1890s Briton Lord Kelvin
and German Hermann von
Helmholz suggest that the sun
gets its energy by shrinking.1896 Physicist Henri Becquerel
discovers radioactivity.1906 Karl Schwarzschild
shows that energy can travel
through a star by radiation.AFTER
1931 Robert Atkinson sets out
the process by which protons
can combine to release energy
and build new elements.1938 German physicist Carl
von Weizsäcker discovers that
protons can combine into
helium in stars by the carbon-
nitrogen-oxygen (CNO) cycle.1939 Hans Bethe details how
the proton-proton chain and
CNO cycle processes work.I
n the 1920s, British astronomer
Arthur Eddington was the first
person to explain the processes
at work inside stars. He championed
the idea that their source of energy
is nuclear fusion.A stable sun
When looking at the sun from Earth,
what can actually be seen is the
gaseous surface layer in the top
300 miles (500 km) or so, which
has a temperature of about 9,900°F
(5,500°C). The sun appears to be
in equilibrium, meaning that, over
the centuries in which astronomers
have observed it (a tiny fraction of
time in the lifespan of the sun), it
has always appeared to be the same
size and show the same luminosity.
Eddington realized that the
gravitational force pulling inward
would be balanced not only by the
gas’s tendency to expand outward
but also by the pressure produced by
the radiation pouring out of the star.
Eddington was able to show
convincingly that all stars are giant
balls of hot gas. He calculated how
luminous stars of different masses
would appear if the gas in their
centers, where the temperature and
density are very high, followed the
same physical laws as cooler, lessThe sun is composed
mostly of hydrogen gas.At its center, the sun
is hot and dense.Stars are fueled
by a slow process
of annihilation
of matter.Conditions are right for
nuclear fusion, slowly
turning mass into
energy according to
the equation E = mc^2.