The Astronomy Book

200

SITES OF STAR

FORMATION

DENSE MOLECULAR CLOUDS

IN CONTEXT

KEY ASTRONOMER

Bart Bok (1906–1983)

BEFORE

1927 American astronomer

Edward Barnard catalogs

350 mysterious dark nebulae.

1941 Lyman Spitzer Jr.

proposes that stars form

from interstellar matter.

AFTER

1980s Herbig-Haro Objects

are identified as jets of plasma

released by very young stars

within star-forming regions.

1993 High-frequency radio

astronomers observe protostars

in Bok globules.

2010 The Spitzer Space

Telescope takes infrared

images of the interiors of 32

Bok globules. The images

show warm cores to 26 of the

globules and evidence that

multiple stars are forming

within about two-thirds of

them, each star at a different

stage in its formation.

B

art Bok was an unusual

observational astronomer.

He made his career

studying not that which he could

see, but that which he could not.

In the 1940s, while observing

bright nebulae for evidence of star

formation, Bok noticed many small

regions that were completely dark.

They were surrounded by stars

but appeared to be empty holes

in space. In 1947, in collaboration

with US astronomer Edith Reilly,

Bok proposed that these bodies

were dense clouds of gas and

dust that were in the process of

collapsing under their own gravity,

and that a new star was forming

inside. The dust, formed from

specks of silica, water ice, and

frozen gases, was dense enough to

block out the light of surrounding

stars. As a result, no light came

out of the cloud, and the light from

any stars behind the cloud (from

Earth’s perspective) could not pass

through. Bok and Reilly likened

these clouds to a caterpillar’s

cocoon, from which a new, brilliant

star would one day emerge.

Dark nebula

The dense clouds became known

as “Bok globules.” In visible light,

they appeared only as a silhouette

against the backdrop of stars, with

a little light passing through their

diffuse outer edges, and for many

years it was difficult to observe

them in any great detail. This

meant that Bok and Reilly’s

proposal remained hypothetical

for several decades. By the 1990s,

however, a few years after Bok’s

death, infrared and radio astronomy

The Caterpillar Bok globule in

the Carina nebula is shown here in

a photograph from the Hubble Space

Telescope. Stars are forming behind

the dense veils of dust and gas.