Four nebulae that revealed the shape of our galaxy

The Eagle, the Omega, the Trifid and the Lagoon are four of the most
photographed objects in the sky. They are also, historically, four of the most
useful — because measuring their distances in the 1950s produced the first real
evidence that the galaxy we live in has spiral arms.

The problem with mapping from the inside

Other galaxies are easy to classify: you look at them from outside and see the
shape. Our own is the hard case. We sit inside the disc, roughly two-thirds of
the way out from the centre, looking through it in every direction. Everything
overlaps.

Without distances, the Milky Way is a band of light with no depth information.
Getting the shape required knowing how far away things are, and until the
mid-twentieth century that was possible for very few objects.

What the 1950s measurements did

A team of astronomers made rough distance estimates to stars inside these four
star-forming regions. The estimates were crude by modern standards. They were
enough.

The four turned out to lie at similar distances, strung along a line rather
than scattered at random — the signature of a spiral arm rather than a
uniform disc. That structure is now called the Sagittarius Arm, and its
identification was among the first direct evidence that the Milky Way is a
spiral galaxy at all.

The logic is worth appreciating. Star-forming regions are good tracers of
spiral arms because arms are density waves: gas is compressed as it passes
through, star formation is triggered, and the young bright regions light up the
wave. Find where the nurseries are, and you have found the arm.

Modern measurements, and a wrinkle

Distances today come from parallax — the tiny apparent shift of a star
against the background as the Earth moves around the Sun — measured by the Gaia
spacecraft to a precision that was unimaginable in the 1950s.

Those measurements confirmed the arm and then complicated it. The Sagittarius
Arm turns out to contain a long, nearly straight structure of star-forming
regions that does not follow the arm's expected curve — a kind of splinter
sticking out at an angle. Similar features show up in images of other spiral
galaxies, where they are visible from outside as feathers and spurs.

What the four are

• The Eagle Nebula (M16) — home of the Pillars of Creation.
• The Omega Nebula (M17) — also called the Swan, one of the brightest
  star-forming regions in the sky.
• The Trifid Nebula (M20) — split into three by dust lanes; unusual in
  containing emission, reflection and dark nebulae together.
• The Lagoon Nebula (M8) — one of the few star-forming regions faintly
  visible to the naked eye from a dark site.

All four are in Sagittarius, which is to say towards the galactic centre. That
is not a coincidence: looking that way means looking along the disc, through
more of the galaxy and more of its arms.