The Astronomy Book

140

See also: Supernovae 180–81

A

ustrian-born physicist

Victor Hess made a series

of dangerous high-altitude

ascents over eastern Germany in a

hydrogen balloon in the years 1911

and 1912. His goal was to measure air

ionization at a height of 3 miles (5 km).

Ionization is the process by

which electrons are stripped from

atoms. In the early years of the

20th century, scientists were

puzzled by the levels of ionization

in Earth’s atmosphere. After the

discovery of radioactivity in 1896,

it was suggested that ionization

was caused by radiation emitted

by substances in the ground,

meaning that air ionization should

decrease with altitude. However,

measurements made at the top

of the Eiffel Tower in Paris in

1909 indicated a higher level

of ionization than expected.

Hess’s results showed that

ionization decreased up to an

altitude of about half a mile

(1 km), and then increased above

that point. He concluded that

powerful radiation from space

was penetrating and ionizing the

atmosphere. This radiation later

became known as cosmic rays.

In 1950, scientists found that

cosmic rays consisted of charged

particles, some possessing very

high energies. They smash into

atoms in the atmosphere, creating

new subatomic particles that may

themselves create collisions, which

in turn cause a cascade of collisions

called a cosmic ray shower. ■

PENETRATING

RADIATION

IS COMING

FROM SPACE

COSMIC RAYS

IN CONTEXT

KEY ASTRONOMER

Victor Hess (1883–1964)

BEFORE

1896 French physicist Henri

Becquerel detects radioactivity.

1909 German scientist

Theodor Wulf measures air

ionization near the top of

the Eiffel Tower. Levels are

higher than expected.

AFTER

1920s American physicist

Robert Millikan coins the

term “cosmic ray.”

1932 American physicist

Carl Anderson discovers the

positron (antiparticle of the

electron) in cosmic rays.

1934 Walter Baade and Fritz

Zwicky propose the idea that

cosmic rays come from

supernova explosions.

2013 Data from the Fermi

Space Telescope suggest that

some cosmic rays come from

supernova explosions.

In 1951, the Crab nebula was found

to be a major source of cosmic rays.

Since then, supernovae and quasars

have also been identified as sources.