The Cosmic Cliffs are not cliffs

The image is one of the most reproduced pictures of the last decade: a ridge of
what looks like orange rock against a deep blue sky, with points of light
scattered above it like stars over a desert. Almost everything that visual
reading suggests is wrong, and what is actually happening is stranger.

It is a wall, not a ridge

There is no solid surface anywhere in the picture. What looks like rock is the
edge of a cavity — a bubble being hollowed out of the gas of NGC 3324, a
star-forming region in the Carina Nebula about 7,600 light years away.

The stars doing the hollowing are not in the frame. They sit above the top
edge of the image: massive, hot young stars pouring out ultraviolet radiation
and stellar wind hard enough to push the surrounding gas away from them. The
"cliff face" is simply where that push has got to.

Why the edge is ragged

An erosion front moving through a cloud of uneven density does not stay
straight. Where the gas is thinner it retreats quickly; where a knot is denser
it holds, and the material behind that knot is shaded from the radiation.

What you get is a boundary full of spires and fingers, each one pointing back
towards the stars eroding it, each one sheltering a column of gas that has not
been reached yet. The tallest features in this image are around seven light
years from base to tip. A "mountain" here would take a beam of light seven
years to climb.

What the infrared adds

This was taken with NIRCam, Webb's near-infrared camera, and that choice is
the reason the picture exists at all.

Dust blocks visible light almost completely, so in an optical image this region
is a dark mass with a bright rim. Infrared passes through it. What comes back
are things that were simply invisible before:

• Protostars still inside their cocoons, some only a few hundred thousand
  years old.
• Jets and outflows — narrow beams of gas fired out along the rotation axis
  of a forming star, punching through the cloud around it.
• The shape of the cavity itself, including how far the erosion has
  actually reached.

Two things happening at once

The reason astronomers care about a boundary like this is that it is where two
processes meet and compete.

Radiation from the massive stars is destroying the cloud — stripping gas
away, and eventually it will disperse the whole region. But the same pressure
also compresses the gas ahead of it, and compressed gas is exactly what
collapses into new stars.

So the erosion front both triggers star formation and ends it, and which one
wins depends on details of density and timing that are difficult to model. An
image like this one is a snapshot of that argument in progress.