The Astronomy Book

112

See also: Analyzing starlight 113 ■ The characteristics of stars 122–27 ■

Refining star classification 138–39 ■ Stellar composition 162–63

I

n 1814, a German maker of

optical instruments named

Joseph von Fraunhofer

invented the spectroscope (see

diagram on p.113). This allowed the

spectrum of the sun, or any other

star, to be displayed and measured

with high precision. Fraunhofer

noticed that there were more than

500 dark lines crossing the sun’s

spectrum, each located at a precise

wavelength (color). These came

to be known as Fraunhofer lines.

By the 1850s, German scientists

Gustav Kirchhoff and Robert Bunsen

had discovered that, if different

chemical elements are heated

in a flame, they emit light at one

or more wavelengths that are

characteristic for that element,

acting like a fingerprint to indicate

its presence. Kirchhoff noticed

that the wavelengths of light given

off by some elements corresponded

to the wavelengths of some

Fraunhofer lines. In particular,

sodium’s emissions at wavelengths

of 589.0 and 589.6 nanometers

exactly matched two Fraunhofer

lines. Kirchhoff suggested that a

hot, dense gas, such as the sun,

will emit light at all wavelengths

and thus produce a continuous

spectrum. However, if the light

passes through a cooler, lower-

density gas, such as the sun’s

atmosphere, some of that light

might be absorbed by an element

(sodium, for example), at the same

wavelengths at which the element

emits light when heated. The

absorption of the light causes

gaps in the spectrum, which are

now known as absorption lines. ■

SODIUM IS TO

E FOUND IN THE B

SOLAR ATMOSPHERE

THE SUN’S SPECTRUM

IN CONTEXT

KEY ASTRONOMER

Gustav Kirchhoff

(1824 –1887)

BEFORE

1802 After creating an image

of the sun’s spectrum by

passing sunlight through

a narrow slit and prism,

English chemist William

Hyde Wollaston notices seven

dark lines in the spectrum.

1814 Joseph von Fraunhofer,

the German inventor of the

spectroscope, discovers 574

of the same dark lines in the

sun’s spectrum. He maps

these in detail.

AFTER

1912 Danish physicist

Niels Bohr introduces a

model of the atom in which

movements of electrons

switching between different

energy levels cause radiation

to be emitted or absorbed

at particular wavelengths.

The path is opened for

the determination of the

chemical composition of

the sun and the fixed stars.

Robert Bunsen