The Astronomy Book

110

I

n the early 19th century,

astronomy was mainly

concerned with cataloging

the positions of stars and planets,

and understanding and predicting

the movements of the planets. New

comets continued to be discovered,

and there was a growing awareness

of assorted distant phenomena, such

as variable stars, binary stars, and

nebulous objects. However, there

seemed little scope for learning

more about the nature of these

remote objects—their chemical

composition or temperature, for

example. The key that unlocked

these mysteries was the analysis

of light using spectroscopy.

Decoding starlight

A glowing object gives out light

over a range of wavelengths, which

we perceive as a rainbow of colors

from the longest wavelength (red)

to the shortest (violet). When a

spectrum is examined in close

detail, a multitude of fine variations

can emerge. A typical star

spectrum appears crossed by

numerous dark lines, some fine

and faint, some broad and black.

Such lines in the sun’s spectrum

had been noted as early as 1802,

but the first physicists to examine

the physics behind particular kinds

of spectra were Gustav Kirchhoff

and Robert Bunsen. Importantly, in

about 1860, Kirchhoff showed that

different patterns of dark lines are

the spectral fingerprints of different

chemical elements. Here was a

way to investigate the composition

of the sun and stars. It even led to

the discovery of the previously

unknown element, helium.

This new branch of astronomy

was enthusiastically taken up by

the British astronomer William

Huggins and his wife, Margaret,

who also pioneered photography

as a way of recording observations.

They did not restrict themselves

to stars, but studied the spectra

of nebulae as well.

INTRODUCTION

1854

1862

1880

1888

1863

1868

Pioneering American

astrophotographer Henry

Draper takes the first

photograph of the

Orion nebula.

Scottish physicist James

Clerk Maxwell produces

a set of equations that

describe the wavelike

behavior of light.

Italian priest Angelo

Secchi starts a project to

classify stars according

to their spectra.

British astronomer

Joseph Norman

Lockyer discovers a

new element in the sun,

which he calls helium.

Germans Gustav
Kirchhoff and Robert
Bunsen investigate
the physics behind
spectral lines.

Using long-exposure

photography, British amateur

astronomer Isaac Roberts

reveals the structure of the

Andromeda nebula.

Light is to us the sole

evidence of the existence

of these distant worlds.

James Clerk Maxwell