The Science Book

238 EDWIN HUBBLE

B

y the early 20th century,

ideas about the scale of

the universe divided

astronomers into two schools of

thought—those who believed that

the Milky Way galaxy was, generally

speaking, its entire extent, and

those who thought that the Milky

Way could be just one galaxy

among countless others. Edwin

Hubble was to solve the puzzle, and

show that the universe is much

larger than anyone imagined.

Key to the debate was the

nature of “spiral nebulae.” Today,

a nebula is the term used for an

interstellar cloud of dust and gas,

but at the time of this debate, it

was the name used for any

IN CONTEXT

BRANCH

Cosmology

BEFORE

1543 Nicolaus Copernicus

concludes that Earth is not

the center of the universe.

17th century The changing

view of stars offered by Earth’s

orbit around the Sun gives rise

to the parallax method for

measuring stellar distances.

19th century Improvements

to telescopes pave the way for

the study of starlight and the

rise of astrophysics.

AFTER

1927 Georges Lemaître

proposes that the universe

can be traced back to a single

point of origin.

1990s Astronomers discover

that the expansion of the

universe is accelerating,

driven by a force known

as dark energy.

There is a simple relation

between the brightness of the

variables and their periods.

Henrietta Leavitt

amorphous cloud of light, including

objects that were later found to be

galaxies beyond the Milky Way.

As telescopes improved

dramatically during the 19th

century, some of the objects

catalogued as nebulae began to

reveal distinctive spiral features.

At the same time, the development

of spectroscopy (the study of the

interaction between matter and

radiated energy) suggested that

these spirals were in fact made

up of countless individual stars,

blending seamlessly together.

The distribution of these

nebulae was interesting too—

unlike other objects that clustered

together in the plane of the Milky

Way, they were more common in

the dark skies away from the plane.

As a result, some astronomers

adopted an idea from the German

philosopher Immanuel Kant, who

in 1755 suggested that nebulae

were “island universes”—systems

similar to the Milky Way but vastly

more distant, and only visible

where the distribution of material

in our galaxy permits clear views

into what we now call intergalactic

space. Those who continued to

believe that the universe was far

Edwin Hubble Born in Marshfield, Missouri, in

1889, Edwin Powell Hubble had

a fiercely competitive nature that

manifested itself in his youth as a

gifted athlete. Despite his interest

in astronomy, he followed his

father’s wishes and studied law,

but at 25 years old, after his

father’s death, he resolved to

follow his early passion. His

studies were interrupted by

service in World War I, but after

his return to the United States

he began to work at the Mount

Wilson Observatory. There he

did his most important work,

publishing his study on

“extragalactic nebulae” in

1924–25, and his proof of cosmic

expansion in 1929. In later

years, he campaigned for

astronomy to be recognized by

the Nobel Prize Committee. The

rules were only changed after

his death in 1953 and so he was

never awarded the prize himself.

Key works

1925 Cepheid Variables in

Spiral Nebulae

1929 A Relation Between

Distance and Radial Velocity

among Extra-galactic Nebulae