Astronomy - USA (2022-06)

Collapsing

molecular cloud

Magnetic

field lines

Accretion disk

Protostar

Magnetic

field lines

Plasma

PLASMA IN THE WIND

even dark matter. Gravity makes

matter accrete. And when matter

accretes, it forms objects. Thus,

accretion and formation are very

closely related in astronomy:

The former can be considered

an aspect of the latter.

The Soviet scientist Otto

Schmidt devised the first accre-

tion model of planetary formation

in 1944, and his countryman

Viktor Safronov f leshed out the

mathematics of accretion in 1969.

The underlying principles of

gravitational attraction have since

been applied to the formation of

stars and even galaxies. The dis-

covery of quasars and compact

X-ray sources in the 1960s, using

optical, radio, and X-ray observa-

tions, set the field in motion.

But the true renaissance began

a decade ago, when the Atacama

Large Millimeter/submillimeter

Array (ALMA), an array of 66

radio telescopes in Chile, came

online. With the ability to study

distant, cool objects in detail

came the data necessary to

understand the process of accre-

tion in a variety of circumstances.

Seeing accretion in action prom-

ised to be a game-changer.

To d a y, A L MA a n d o t h e r

advanced telescopes are observ-

ing numerous objects of different

sizes and stages of evolution:

galaxy groups, molecular clouds,

stellar nurseries, protostars,

planetary disks, black holes, and

many more. We now know that

whether on scales of kilometers

or light-years, accretion operates

on the same broad principles.

The particular mechanisms

remain mysterious, but the veil

is beginning to lift.

Stellar nurseries

at the crossroads

The largest structures in the

universe are groups of galaxies

that are gravitationally bound

to each other. We are not yet

able to see them, but peculiar

arrangements of objects have

been interpreted as evidence of

such superstructures. They are

given various names according

to their observed shapes, such

as arcs, rings, or walls.

The observable portions of

superstructures consist mostly of

molecular clouds of gas millions

of light-years across. Over the

eons, these diffuse regions are

perturbed by a variety of effects:

the chaotic motions of the galax-

ies within them, the winds

thrown by quasar jets, the passing

wakes of rotating black holes, and

blasts from supernovae. Here and

there, a conf luence of gas and

dust will become dense enough

that gravity takes over and a

domino effect begins, as more

and more mass is drawn into a

conglomeration, where a star-

forming region is born.

The mechanics of these stellar

nurseries, from which hundreds

or thousands of stars are created,

are not completely understood.

Sometimes a region containing

a few hundred solar masses of

dense gas and dust will form 100

Sun-like stars. Other times a few

massive stars will also appear.

This difference is of particular

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