The Astronomy Book

165

See also: Spiral galaxies 156–61 ■ Beyond the Milky Way 172–77 ■ The Oort cloud 206 ■ Dark matter 268–71

ATOMS, STARS, AND GALAXIES

Kapteyn, who had described
a phenomenon he called star
streaming. Stars did not move in
random directions, Kapteyn said,
but appeared to move in groups,
going either one way or in the
opposite direction. Lindblad
himself was a leading expert in
measuring the absolute magnitude
of stars from their spectra, and was
able to calculate their distances
from Earth. He combined this data
with his observations of the motion
of globular clusters and made an
interesting discovery.

Spinning in subsystems
Lindblad saw that stars move in
subsystems, and each subsystem
moves at a different speed. From
this, he deduced that Kapteyn’s star
streaming was in fact evidence of
the galaxy rotating, which meant
that the Milky Way’s stars were
all moving in the same direction
around a central point. Stars that
streamed ahead of the solar system
were nearer to the center, and stars
that were farther from it appeared to
stream in the opposite way because

they were lagging behind. As

Shapley had predicted, Lindblad

placed the galactic center in

Sagittarius. He supposed that

subsystems farther from the galactic

center orbited more slowly than the

ones closer in. This was confirmed

in 1927 by the observations of Jan

Oort, one of Kapteyn’s students.

The Milky Way was revealed to

be a swirling disk that spun, albeit

very slowly, taking 225 million years

to complete one orbit. Although

Bertil Lindblad Bertil Lindblad grew up in Örebro,

Sweden. He did his undergraduate

degree at Uppsala University,

north of Stockholm, and became

an assistant at the observatory

there. It was while working at

Uppsala that Lindblad made his

observations of the motion of

globular clusters that led to his

theory of galactic rotation, which

was published in 1926. The

following year, still barely into

his 30s, Lindblad was offered

the directorship of the Stockholm

Observatory and became the chief

astronomer for the Royal Swedish

Academy of Sciences. He held

that position until his death,

overseeing many improvements.

In later years, he was a leading

organizer for the European

Southern Observatory, which

has been located in the high

desert of Chile since 1962, and

president of the International

Astronomical Union.

Key works

1925 Star-Streaming and the

Structure of the Stellar System

1930 The Velocity Ellipsoid,

Galactic Rotation, and the

Dimensions of the Stellar System

Lindblad had offered no evidence of

bodies lying outside the Milky Way,

his disk-shaped galactic model with

a bulging core gave credence to the

idea that similar-looking objects

were also galaxies. However, Oort’s

observations would also reveal a

new puzzle. The galaxy appeared

to be rotating faster than could be

accounted for by the mass of its

visible matter. Here was the first

hint of a mystery that endures

today: dark matter. ■

Stars in the same subsystem appear to move

in the same direction and at the same speed.

The galaxy is shaped like a spiraling disk with

outer regions moving more slowly than inner ones.

If stars in other subsystems move in the

opposite direction, it is because they are lagging behind,

but they are all moving in the same direction.