127
they make up 3 percent of the main
sequence and are a little larger than
the sun. The spectra of these stars
contain medium-intensity hydrogen
lines and strengthening lines for
iron and calcium.
The sun’s class
Type-G yellow dwarfs, of which
the sun is one, make up 8 percent
of the main sequence. They are
between 5,200 and 6,000 K on the
surface and have weak hydrogen
lines in their spectra, with more
prominent metal lines. Type-K
dwarfs are orange and make up
12 percent of the main sequence.
They are between 3,700 and
5,200 K on the surface and have
very weak hydrogen absorption
lines but strong metallic ones,
including manganese, iron, and
silicon. Type-M are red dwarfs.
These are by far the most common
main sequence stars, making up
76 percent of the total, although
no red dwarf is visible to the naked
eye. They are just 2,400–3,700 K
on the surface and their spectra
contain absorption bands for oxide
compounds. The majority of the
yellow, orange, and red dwarfs are
believed to have planetary systems.Extended classification
Stellar spectral classes now cover
even more types of stars. Class W
are thought to be dying supergiant
stars. Class C, or carbon stars, are
declining red giants. Classes L, Y,
and T are a diminishing scale of
colder objects, from the coolest red
dwarfs to the brown dwarfs, which
are not quite large or hot enough to
be classed as stars. Finally, white
dwarfs are class D. These are the hot
cores of red giant stars that no longer
burn with fusion and are gradually
cooling. Eventually they should fade
to black dwarfs, but it is estimated
it will take a quadrillion years for
that to happen. ■THE RISE OF ASTROPHYSICS
Annie Jump Cannon
Born in Delaware, Annie Jump
Cannon was the daughter
of a state senator, and was
introduced to astronomy
by her mother. She studied
physics and astronomy at
Wellesley College, an all-
women’s college. Graduating
in 1884, Cannon returned to
her family home for the next
10 years. On the death of her
mother, in 1894, she began to
teach at Wellesley and joined
Edward C. Pickering’s Harvard
Computers two years later.
Cannon suffered from
deafness, and the ensuing
difficulties in socializing led
her to immerse herself in
scientific work. She remained
at Harvard for her entire
career, and is said to have
classified 350,000 stars over
44 years. Subject to many
restrictions over her career
due to her gender, she was
finally appointed a member
of the Harvard faculty in 1938.
In 1925, she became the
first woman to be awarded
an honorary degree by
Oxford University.Key work1918–24 The Henry Draper
CatalogueA white dwarf sits at the heart of the
Helix planetary nebula. When its fuel
ends, the sun will become a white dwarf.