The Astronomy Book

278

The impact of a density wave, or

something more violent such as

a blast from a nearby supernova,

creates turbulence inside a GMC.

However, highly tangled magnetic

fields run through the cloud, and

these stop the turbulence from

ripping the cloud apart. The

magnetism also acts to prevent

the cloud from collapsing in on

itself under its own gravity.

Cloud cores

Over millions of years, the magnetic

pressure and turbulence in the

gases dissipate, creating regions

of calm, where slowly rotating

“cloud cores” form. On closer

inspection, GMCs are not uniform,

but made up of dark fragments

or clumps of denser material,

known as Bok globules. Each

globule is thought to contain

several cloud cores.

Shu’s model supposes that the

core becomes a single isothermal

(equal-temperature) sphere, or

something very close to it. This

means that the gravity pulling the

ball of gas together is balanced by

the outward pressure of the moving

gas and its magnetic forces. Such

a state can never persist for long,

INSIDE GIANT MOLECULAR CLOUDS

and the contracting gravitational

force in the core wins out over

the outward pressure.

The inner region of the cloud

core contracts to form a dense ball

of gas at the center. This is the

protostar. Protostars do not form

in a rapid process, but take millions

of years, and millions more to

grow into a full-fledged star. The

protostar is also surrounded by

a disk of material formed by the

system’s rotation, and wave upon

wave of material is pulled in from

the surrounding envelope of gas.

With each wave, the mass of the

protostar and its more diffuse disk

grows, and its gravity grows with

it. The increasing gravity steadily

pulls in material from farther

away, hence the description of the

process as an “inside-out collapse.”

The star gathers mass

The protostar warms up as it

becomes denser, but it is still too

small and cold to produce energy

by fusing hydrogen in its core.

The force of all of the new material

landing on its surface also adds

to the heat signature given out by

the protostar. At this stage, it is

giving out only faint infrared and

Frank Shu

Born in Kunming, China,

Frank Shu moved to the United

States when he was six to

join his father, an academic

mathematician, who was

beginning research at MIT.

Frank followed his father to

MIT, where he completed

a degree in physics in 1963.

While there, Shu worked

on the density wave theory

of spiral arms. He later moved

to Harvard to complete his

doctorate in astronomy in 1968.

Shu worked on his protostar

model while at Berkeley and

was the head of the astronomy

department there by the time

he presented the full review of

his isothermal sphere model in

1986. Today, Shu holds tenure
      at Berkeley. In recent years he
      has used his knowledge of
      astrophysics to tackle climate
      change. He often works in
      collaboration with his graduate
      students, who are collectively
      known as the “Shu Factory.”

Key work

1981 The Physical Universe

Frank Shu’s inside-out model
describes the four-stage formation
of a star from a giant molecular cloud.

1 Cores form within GMCs as magnetic
forces and turbulence calm.

2 A protostar with a surrounding nebular
disk forms at the center of a cloud core,
collapsing from inside out.

3 A stellar wind breaks out along the
rotational axis of the system, creating
a bipolar flow.

4 The infall of material ends, revealing a
newly formed star with a circumstellar disk.