interest to astronomers because
massive stars can alter the evolu-
tion of a galaxy. What guides the
seemingly random accretion pro-
cess on these vast scales?
One theory posits that there are
“filaments of f lowing gas, which
thread through these clusters,”
says Todd Hunter, an astronomer
at the National Radio Astronomy
Observatory. Astronomers are
starting to see evidence of these
filaments. “They fragment and
intersect at certain places, espe-
cially in the center of clusters,”
says Hunter. “And where they
meet is where protostars have
access to a lot of gas on a short
timescale” — which feeds the for-
mation of massive stars.
These objects are called infra-
red dark clouds, and they are verylarge, very cold objects that were
difficult to detect and resolve
until recently. The first observa-
tions were made with the Infrared
Space Observatory in 1996. It was
a serendipitous find, made during
the first detailed survey of stellar
populations in the galactic plane.
Nowadays these regions are stud-
ied in detail with ALMA and the
Submillimeter Array in Hawaii,
which are more sensitive and have
higher resolution at submillimeter
wavelengths where cold molecular
gas is easiest to detect.
These facilities have allowed
astronomers to map the gas f lows
they believe provide the necessary
supply for the growth of massive
stars. One survey, published in
The Astrophysical Journal in 2019,
identified hundreds of protostel-
lar and prestellar core candidates
in a particular region and studied
how they affect each other. The
researchers suggest a kind of
“competitive accretion” process
takes place, alongside what they
call “global hierarchical collapse”
— where chaotic gravitational
forces cause a series of collapses
within collapses, with small-scale
events happening later and faster
than large-scale events. ThisEscaping plasma
(MHD wind)THE REASON THAT the gas in a molecular cloud can
accrete into a star may be due to magnetohydrodynamics
(MHD), the physics of how magnetic fields interact with hot
ionized gas.
Interstellar space and everything in it is permeated by a
weak magnetic field. Normally, this background magnetic
field has no effect on a cold, dense cloud of gas and dust.
But this changes when a collapsing cloud heats up and
begins to generate plasma. Because plasma is electrically
charged, it is linked to the magnetic field: As it moves, it
drags the magnetic field lines with it. As the cloud
collapses further and begins to form an accretion disk, the
magnetic field becomes wound up by the disk’s rotation.
The magnetic field also becomes stronger as the field lines
bunch together.
All of these magnetic field lines then act as highways for
plasma to escape the strong magnetic field: Following the
field lines, the charged particles zip away from the
accretion disk into space. This MHD wind carries angular
momentum away from the disk — and this, astronomers
suspect, helps the cloud collapse into a star.
ASTRONOMY:^ROENKELL
YWWW.ASTRONOMY.COM 19
The Giant
GRB Ring
is a suspected
superstructure — a
collection of nine
gamma-ray bursts
(immensely powerful
stellar explosions)
arranged in a loose ring
that spans 5.6 billion
light-years, as shown in
this artist’s concept.
PABLO CARLOS BUDASSI/WIKIMEDIA
COMMONS/CC BY-SA 4.