The Astronomy Book

ATOMS, STARS, AND GALAXIES 199

heavier elements such as iron

and carbon could form: the cores

of stars were not considered hot

enough for such elements to form

through nuclear fusion processes.

Hoyle, however, thought there could

be processes that would sufficiently

raise the core temperature of a

large enough star.

In 1946, Hoyle showed that

in the cores of massive stars where

the temperature soars to billions

of degrees heavier elements could

be made in circumstances called

nuclear thermal equilibrium. Such

a star would eventually explode

as a supernova and eject its heavy

elements. In 1954, Hoyle went on to

describe how in a massive star that

has exhausted its hydrogen fuel

its core would contract and heat up

before exploding, and helium atoms

would start fusing to create carbon.

At the end of this phase, carbon

atoms would fuse to create heavier,

more stable, elements. This could

account for the creation of several

elements up to iron, which has the

most stable of all atomic nuclei.

Building nuclei heavier than iron

would be more problematic, since

it is an energy-consuming process,

whereas creating elements lighter

than iron releases energy.

Further developments

There was a flaw in Hoyle’s stellar

element-building scheme, however.

A key step known as the triple

alpha process, in which three

helium nuclei fuse to make carbon,

appeared to be too slow. Hoyle

insisted that there must be a

mechanism allowing it to happen

at a faster speed and, in 1953, a

certain property of carbon was

discovered that explained it.

Hoyle also explored other

processes by which many more

elements might be forged in stars.

Some of these processes could only

occur in the violence of a supernova

explosion at the end of a giant

star’s life (pp.180–81). Hoyle’s work

thus explained not only where

chemical elements came from but

also how they came to be dispersed

throughout the universe. ■

See also: Energy generation 182–83 ■ The primeval atom 196–97

Fred Hoyle

Fred Hoyle was born in

Yorkshire, England, in 1915.

He attended the University of

Cambridge from 1933, gaining

a degree in mathematics.

During World War II, he

worked on radar systems

for the British Admiralty. In

1957, Hoyle joined the staff

of the Hale Observatories in

California and the following

year became professor of

astrophysics at Cambridge

University. Aside from his work

on the origin of elements in

stars, Hoyle is best known as a

proponent of the Steady State

theory. This claims that as the

universe expands, its average

density is kept constant as

new matter is continuously

created. Ironically, Hoyle

coined the phrase “Big Bang”

for the main rival theory,

during a popular radio talk

show. From the 1960s, the

Steady State theory fell out

of favor. In later life, he took

particular interest in the

presence of organic molecules

in comets, which he believed

had brought life to Earth.

Key works

1946 The Synthesis of the

Elements from Hydrogen

1950 The Nature of the Universe

Hoyle showed

how in high mass

stars several

elements ranging in

mass from carbon

to iron could be

created by fusion

reactions occurring

at the same time in

shells around the

core of the star. The

number of shells

would increase

as the star aged.

This diagram

shows shells with

element-building

reactions in

an aging red

supergiant.

Hydrogen fusing

to helium

Helium

fusing to

carbon

Carbon

fusing to

neon

Neon fusing
to oxygen and
magnesium

Oxygen fusing to

silicon, sulfur, and

other elements

Nonfusing hydrogen