The Astronomy Book

270

of the planets are plotted against
their distance from the sun, the data
forms a smooth downward “rotation
curve.” It followed that plotting the
orbital speeds of stars at different
distances from the galactic center
should produce a similar curve.
In 1932, Dutch astronomer Jan
Oort was the first person to provide
observational proof that the galaxy
was a single orbital system made
up of a swirling spiral of stars,
in which the sun completed a
225-million-year orbit. However, in
the course of his calculations, Oort
found that the motion of the galaxy
suggested that it was twice as
massive as the total mass of visible
stars. He concluded that there must
be some hidden source of mass.
A year later, the Swiss−American
Fritz Zwicky was studying the
relative motion of galaxies in the
Coma cluster. He found, again,
that their motion suggested the
mass of what could be seen was
not the only stuff there. He named
the missing material dunkle
Materie or “dark matter.”

Oort’s early measurements were

inaccurate, while Zwicky’s initial

assessment was that “dark matter”

was 400 times more abundant than

the matter of visible material—a

huge overestimate. This meant

that their findings were dismissed

as measurement errors. In 1939,

American Horace Babcock again

found anomalies in the rotation of

Andromeda and suggested that there

was some mechanism by which

light from the missing matter was

being absorbed in the galactic core.

Galactic rotation curve

More than 20 years later, Rubin

returned to the problem of galactic

rotation. Like Babcock, she chose

to focus on the rotation of the

Andromeda galaxy, the Milky Way’s

nearest galactic neighbor. She

worked with her colleague Kent

Ford at the Carnegie Institution

of Washington to measure the

velocities of objects in the outer

region of the galaxy. They did this

using a sensitive spectrograph,

which allowed them to detect the

DARK MATTER

redshift and blueshift of objects,

and calculate their relative speeds

away from and toward Earth.

After several years of slow but

careful work, Rubin had enough

data to plot a rotation curve for

the galaxy. Instead of swooping

downward like the curve of the

solar system, the speed data

of the galactic curve stayed

relatively level with distance.

This meant that the outer regions

of Andromeda were moving at the

same speed as the areas nearer to

the center. If the galaxy’s mass was

limited to what could be observed

using telescopes, the outer regions

of Andromeda would be moving

faster than escape velocity, and

they should simply fly off into

space. However, they were clearly

being held in place by the galaxy’s

overall mass. Rubin calculated that

the total galactic mass required

to hold the outer regions in orbit

was about seven times greater

than the visible mass. The ratio

of matter to dark matter is today

thought to be around 1:6.

What is dark matter?

Rubin’s galactic rotation curve,

widely disseminated in 1980, was

the visual proof that dark matter

existed. As further evidence

mounted, the mystery as to what

it might be remained. Dark matter

No observational problem will

not be solved by more data.

Vera Rubin

In the absence of dark matter, the velocities of objects in the outer regions of
galaxies would be slower than the observed values. Here, the observed rotation
curve is plotted against the expected curve from visible matter alone.

DISTANCE FROM CENTER (x 100 LIGHT YEARS)

Rotation curve expected

from visible matter

100

150

50

0

VELOCITY (KM PER SECOND)

Observed rotation curve

Range

of error

in each

observation

Observation

0 10 20 30 40 50 60