The Astronomy Book

238

Bell showed the signal, dubbed
“Little Green Man 1” (LGM-1),
to Hewish. His initial reaction
was that a pulse occurring every
1.33 seconds was far too fast for
something as large as a star, and
the signal must be due to human
activity. Together, Bell and Hewish
ruled out various human-related
sources, including radar reflected
from the moon, Earth-based radio
transmissions, and artificial
satellites in peculiar orbits. A
second telescope was also found
to pick up the pulses, which proved
that they could not be due to an
equipment fault, and calculations
showed that they were coming
from well outside the solar system.
Hewish had to revise his
opinion that the signals had a
human origin. The possibility
that they were being sent by extra-
terrestrials could not be ruled out.
The team measured the duration
of each pulse and found it was only
16 milliseconds. This short duration
suggested that the source could be
no larger than a small planet. But
a planet—or an alien civilization
living on a planet—was unlikely,

since the signal would show slight

changes in frequency, called

Doppler shifts (p.159), as a planet

orbited its star.

Publishing dilemma

Hewish, Bell, and their colleagues

were unsure how to publish their

findings. While it seemed unlikely

that the signals were being sent

by an alien civilization, no one had

any other explanation. Bell returned

to her chart analysis, and soon

found another “scruff” in a different

part of the sky. She discovered

it was due to another pulsating

signal, this time slightly faster,

with pulses every 1.2 seconds.

Now she was reassured that the

pulses must have some natural

explanation—two sets of aliens

in different places would surely

not be sending signals to Earth

at the same time and at nearly

the same frequency.

QUASARS AND PULSARS

By January 1968, Hewish and

Bell had found four pulsing sources

in total, which they decided to

call “pulsars.” They wrote a paper

describing the first source, suggesting

that it might be due to pulsed

emissions from a theoretical type

of superdense collapsed star called

a neutron star. Objects of this type

had been predicted as long ago as

1934, but up to that time had never

been detected.

Explaining the pulses

Three months later, Thomas Gold,

an Austrian−American astronomer

at Cornell University in the US,

published a fuller explanation

for the pulsed signals. He agreed

that each set of radio signals was

coming from a neutron star, but

proposed that the star was rapidly

spinning. A star like this would

not need to be emitting pulsed

radiation to account for the pattern

My eureka moment was in

the dead of night, the early

hours of the morning. But

when the result poured out

of the charts ... you realize

instantly how significant this

is—what it is you’ve really

landed on—and it’s great!

Jocelyn Bell Burnell

A pulsar is a spinning

neutron star with an

intensely strong

magnetic field.

It emits beams

of radiation from

its north and

south poles.

Rotation axis

Neutron

star

Radiation

beam

Magnetic

field

N

S