The Astronomy Book

164

OUR GALAXY

IS ROTATING

THE SHAPE OF THE MILKY WAY

IN CONTEXT

KEY ASTRONOMER

Bertil Lindblad (1895 –1965)

BEFORE

1904 Jacobus C. Kapteyn

shows how stars can be

divided into two streams

moving in opposite directions.

1917 Vesto Slipher shows that

spiral nebulae are moving

faster than any star.

1920 Harlow Shapley predicts

that the center of the galaxy is

in Sagittarius, estimating it at

50,000 light-years away (now

known to be 26,100 light-years).

AFTER

1927 Jan Oort confirms that

the galaxy is rotating and

proposes that a large mass of

stars forms a bulge at its core.

1929 Edwin Hubble shows

that other galaxies lie far

beyond the Milky Way.

1979 Vera Rubin uses galactic

rotation to show that galaxies

contain invisible dark matter.

I

n the 1920s, there were two

opposing views of the universe.

Some astronomers thought

the Milky Way was itself the entire

universe. Others argued that the

spiral nebulae observed were not

cloudy masses on the edge of the

Milky Way, but galaxies in their

own right a vast distance away.

In 1926, a Swedish astronomer

named Bertil Lindblad considered

the likely shape of the Milky Way,

concluding that it took the form

of a rotating spiral. Lindblad was

building on the work of two other

astronomers. The first was the

American Harlow Shapley, who

believed that the Milky Way made

up the whole universe. Shapley

suggested that the edge of the

galaxy could be plotted using the

many globular clusters of stars that

had been observed, and that the

center lay in Sagittarius. The second

was the Dutchman Jacobus C.

The solar system is orbiting the

center of the Milky Way at a speed

of 140 miles/s (230 km/s). Stars closer

to the center orbit at a faster speed.