The Astronomy Book

162

STARS ARE DOMINATED

BY HYDROGEN

AND HELIUM

STELLAR COMPOSITION

IN CONTEXT

KEY ASTRONOMER

Cecilia Payne-Gaposchkin

(1900 –1979)

BEFORE

1850s Gustav Kirchhoff

shows that dark lines in the

sun’s spectrum are due to

light absorption by elements.

1901 Annie Jump Cannon

classifies stars by the strength

of the dark lines in their spectra.

1920 Indian physicist Meghnad

Saha demonstrates how

temperature, pressure, and

ionization are linked in a star.

AFTER

1928–29 Albrecht Unsöld and

William McCrea independently

find that hydrogen is a million

times more abundant in the

sun’s atmosphere than any

other element.

1933 Danish astrophysicist

Bengt Strömgren shows that

stars are mainly hydrogen

all the way through, not just

in their atmospheres.

I

n 1923, the consensus among

astronomers was that the sun

and other stars had a similar

chemical composition to Earth.

This belief was based on the

analysis of dark lines (Fraunhofer

lines) in star spectra, which are

caused by the absorption of light

by chemical elements in star

atmospheres. The spectra contain

strong lines for elements that are

common on Earth, such as oxygen

and hydrogen, and metals such

as magnesium, sodium, and iron,

and therefore it was assumed that

Earth and stars were made of the

same chemical elements, in more

or less the same proportions. This

The reward of the old scientist

is the sense of having seen

a vague sketch grow into

a masterly landscape.

Cecilia Payne-Gaposchkin

established view would be

overturned with the arrival that

year of British graduate student

Cecilia Payne at the Harvard

College Observatory (HCO)

in Massachusetts.

Star spectra

Payne set to work analyzing the

HCO’s photographic collection of

star spectra. She wanted to clarify

the relationship between star

spectra and temperatures. Also,

because the pattern of absorption

lines seemed to vary between the

spectra of different classes of star,

she wanted to see what differences

in chemical composition might

exist between these classes.

Since 1901, astronomers at

the HCO had classified stars

into a sequence of seven main

spectral types, and believed that

the sequence was related to the

stars’ surface temperatures. In her

doctoral thesis, however, Payne

applied an equation formulated by

Indian physicist Meghnad Saha in

1920. The equation related a star’s
      spectrum to the ionization (electrical
      charge separation) of chemical
      elements in its atmosphere and the
      ionization of its surface temperature.
      Payne demonstrated a link between