201
See also: Space telescopes 188–95 ■ Inside giant molecular clouds 276–79 ■
Ambartsumian (Directory) 338
ATOMS, STARS, AND GALAXIES
were able to peer inside the dusty
cloud and pick out areas of heat.
These areas indicated that Bok’s
hypothesis was right—stars were
indeed forming within.
Bok globules are now seen as
a small and dense type of “dark
molecular cloud,” mostly found in
the spiral arms of the Milky Way.
They are about one light-year
wide and found in H II regions—
vast interstellar spaces filled with
low-density ionized hydrogen
atoms. H II regions form when the
ultraviolet emissions of supergiant
blue stars ionize the surrounding
medium (the matter in interstellar
space)—stripping atoms of their
electrons to create positively
charged ions.
Cold cloud
Bok globules weigh about 50 times
the mass of the sun. They mostly
comprise molecular hydrogen (H 2 ),
but about 1 percent is dust. The
dust comprises particles made of
multiple molecules, and is highly
concentrated. The obscuring
effect of the dust prevents heat
from penetrating the globule,
and the temperatures inside are
among the coldest measured in
the universe—about 10 Kelvin.
The outward pressure of the cold
gas is weaker than the inward pull
of gravity, and a shockwave from
a nearby supernova can make the
cold clouds collapse. They then
become denser and denser until
a hot stellar core forms. ■For many years I have
been a nightwatchman
of the Milky Way galaxy.
Bart BokBart Bok
Bartholomeus Jan Bok was
born near Amsterdam in 1906.
His interest in astronomy
began in scout camps, where
he was able to observe the
stars in clear skies away
from the city. Bok began his
academic career at two Dutch
universities, first at Leiden
and then, as a Ph.D. student,
at Groningen. In 1929, he
opted to transfer his studies
to Harvard and work under
the supervision of Harlow
Shapley. He had fallen in love
with Priscilla Fairfield, one
of Shapley’s researchers, and
married her two days after
arriving in the US. The pair
worked closely together from
then on, although Shapley only
paid Bok, now going by the
Americanized name of Bart.
The Boks worked at
Harvard for 30 years before
they were invited to set up
an observatory in Canberra,
Australia, in 1957. In 1966,
they returned to the US to
run observatories in the
southwest. Priscilla died in- Bart continued working
until his death in 1983.
Key work1941 The Milky Way (with
Priscilla Fairfield Bok)The clouds contain dark regions
where material is collapsing into
dense globules.Stars are made from
material that was originally
spread through space.That material forms
clouds of gas and dust.These globules are the
sites of star formation.