The Astronomy Book

89

See also: Messier objects 87 ■ The southern hemisphere 100–01 ■ Properties of nebulae 114–15 ■

Spiral galaxies 156–61 ■ The shape of the Milky Way 164–65

URANUS TO NEPTUNE

brightness of a star indicated its

distance from Earth, with dimmer

stars being more distant. The

even distribution, he believed, must

mean that the solar system was

close to the center of the galaxy.

Herschel’s model was refined by

other astronomers, but was not

replaced until the early 20th century.

New nebulae

Herschel did not limit himself to the

study of stars in his investigation

into a galaxy’s shape. He also

observed nebulae, the fuzzy patches

of light that dotted the sky. Herschel

was a skilled telescope-maker as

well as an astronomer, and used

two large, powerful telescopes

with 49½-in (126-cm) and 18½-in

(47-cm) apertures. From 1782, he

used these instruments to conduct

systematic observations of the

“deep sky,” searching for objects

that were not stars. He listed these

as nebulae or as clusters of stars,

and published details of 1,000

new objects in 1786, with further

catalogs appearing in 1789 and

1802. Herschel classified the objects
      he listed into eight categories,
      depending on their brightness,
      size, or whether they appeared
      to consist of dense or scattered
      clusters of stars. He also conjectured
      that most nebulae were similar in

nature and size to the Milky Way,

decades before it was confirmed

that nebulae were in fact galaxies

in their own right.

The current model of the Milky

Way is a barred spiral galaxy.

Around two-thirds of spiral

galaxies have central bars like

the Milky Way’s. The early idea

of a disk of stars is broadly correct,

but the stars within the disk are

arranged in a series of spiral arms,

with the sun in a sparse area of

the Orion–Cygnus arm. ■

The Milky Way comprises stellar arms spiraling out from the

bulging “bar” at the center. The arms are labeled here. The sun is

located in the Orion–Cygnus arm, 26,000 light-years from the center.

There is a dense

band of stars across

the night sky.

The solar system

is positioned within

a disk of stars.

This suggests that

the solar system

is in the center

of the disk.

Stars of different

magnitudes are

distributed evenly

within this band.

Perseus

Far 3kpc

Norma

Scutum–
Centaurus

New Outer

Carina–Sagittarius

Orion–Cygnus

Near 3kpc

Sun

I have observed stars of

which the light, it can be

proved, must take two million

years to reach the Earth.

William Herschel