136 PART 2^ |^ THE STARS
As a child in England, Cecilia Payne
(1900–1979) excelled in classics, languages,
mathematics, and literature, but her fi rst love
was astronomy. After fi nishing Newnham
College in Cambridge, she left England, sens-
ing that there were few opportunities there for
a woman in science. In 1922, Payne arrived at
Harvard, where she eventually earned her Ph.D.
(although her degree was awarded by Radcliff e
because Harvard did not then admit women).
In her thesis, Payne attempted to relate
the strength of the absorption lines in stellar
spectra to the physical conditions in the
atmospheres of the stars. Th is was not easy; a
given spectral line can be weak because the
atom is rare or because the temperature is
either too high or too low for it to absorb
effi ciently. If you see sodium lines in a star’s
spectrum, you can be sure that the star con-
tains sodium atoms, but if you see no sodium
lines, you must consider the possibility that
sodium is present but the star is too hot or
too cool for that type of atom to produce
detectable spectral lines.
Density is another factor. If a gas is
dense, the atoms are packed tighter together,
collide more often, distort the electron orbits,
and thereby change the strength of spectral
lines (Focus on Fundamentals 4). If
the gas is low density, the atoms collide less
often and the spectral lines are less aff ected.
Payne’s problem was to untangle these
factors and fi nd both the true temperatures of
the stars and the true abundance of the atoms
in their atmospheres. Solving this puzzle
required some of the most recent advances in
atomic physics and quantum mechanics. About the time Payne
left Cambridge University, Indian physicist Meghnad Saha pub-
lished his work on the ionization of atoms. Drawing from such
theoretical work, Payne’s calculations showed that over 90 percent
of the atoms in stars (including the sun) must be hydrogen and
most of the rest are helium (■ Table 7-2). Th e heavier atoms like
calcium, sodium, and iron seem more abundant only because
they are better at absorbing photons at the temperatures of stars.
At the time, astronomers found it hard to believe Payne’s
abundances of hydrogen and helium. Th ey especially found such
a high abundance of helium unacceptable. After all, hydrogen
lines are at least visible in most stellar spectra, but helium lines
are almost invisible in the spectra of all but the hottest stars.
Nearly all astronomers assumed that the stars had roughly the
same composition as Earth’s surface; that is, they believed that
the stars were composed mainly of heavier atoms such as carbon,The Composition of the Stars
It seems as though it should be easy to learn the composition of
the sun and stars just by looking at their spectra, but this is actu-
ally a diffi cult problem that wasn’t well understood until the
1920s. Th e story is worth telling, not only because it illustrates
the temperature dependence of stellar spectral types, it is also the
story of an important American astronomer who waited decades
to get proper credit for her work.
1000
Wavelength (nm)L3 1950KIntensityL5 1700KL9 1400KT0 1300KT4 1200KT9 700KFeH H 2 OH 2 OCH 41500Absorption by iron
hydride is strong
in L dwarfs.Absorption by
methane is strong
in T dwarfs.Water vapor absorption
bands are very strong in
cooler stars.■ Figure 7-10
These six infrared spectra show the dramatic differences between L dwarfs
and T dwarfs. Spectra of M stars show titanium oxide bands (TiO), but L and
T dwarfs are so cool that TiO molecules do not form. Other molecules, such as
iron hydride (FeH), water (H 2 O), and methane (CH 4 ), can form in these very
cool objects. (Adapted from Thomas R. Geballe, Gemini Observatory, from a graph
that originally appeared in Sky and Telescope Magazine, February 2005, p. 37.)