The Astronomy Book

161

it seems to me, gains favor in the
present observations.” Slipher was
echoing Kant’s suggestion that
some nebulae, in particular the
spiral ones, could be separate
galaxies from the Milky Way.
In 1920, partly prompted by
Slipher’s findings, a formal debate
took place in Washington, D.C., to
discuss whether the spiral nebulae
were separate galaxies outside
the Milky Way. Now referred to
as the “Great Debate,” two eminent
American astronomers advocated
opposing positions—Harlow
Shapley that the spiral nebulae
were part of the Milky Way; and
Heber D. Curtis that they were far
beyond it. Neither astronomer
changed his position as a result
of the debate, but many perceptive
figures were concluding by this
time that the spirals had to be
outside the Milky Way.

Slipher’s legacy
Despite an enthusiastic response
from many in the astronomical
community, some still questioned
Slipher’s findings. For more than
a decade, until others began
to believe Slipher’s ideas and
understand the implications arising
from them, he was virtually the
only person investigating the
Doppler shifts of spiral nebulae.
In 1924, a new paper by American
astronomer Edwin Hubble put a
decisive end to the debate about
the nature of spiral nebulae. Hubble
had observed a class of stars called
Cepheid variables in some nebulae,
including the Andromeda nebula.
As a result of his observations,
Hubble was able to announce that
the Andromeda “nebula” and others

like it were far too distant to be

part of the Milky Way and so must

be galaxies outside it. Slipher’s

suspicions dating back to 1917

had been proved right. By the

time of Hubble’s paper, Slipher had

measured the radial velocities of

ATOMS, STARS, AND GALAXIES

Some 4 billion years into the

future, the night sky will look like

this, as the Andromeda galaxy

collides with the Milky Way.

In the great majority of cases

the nebula is receding; the

largest velocities are positive.

The striking preponderance

of [these positive velocities]

indicates a general fleeing

from us or the Milky Way.

Vesto Slipher

39 spiral nebulae, the majority

of which showed high velocities

of recession—as much as 775 miles

per second (1,125 km per second).

Hubble used Slipher’s measurements

of redshifts in galaxies that he had

proved were outside the Milky

Way to find a relationship between

galaxy redshifts and distances.

By the late 1920s, Hubble

had used this result to confirm

that the universe is expanding.

Thus, Slipher’s work in the years

1912–25 played a crucial role in

what today is often considered the

greatest astronomical discovery

of the 20th century, paving the

way for further investigations

into the motions of galaxies and

cosmological theories based on

an expanding universe. As for the

Andromeda galaxy, it is expected

to collide with the Milky Way in

about 4 billion years, and together

the two are likely to form a new

elliptical galaxy. ■