Spiral Structure

Core Concepts

What traces the spiral arms

The arms are traced by the young inhabitants of the disk: OB associations (loose groups of massive, hot, blue O and B stars), H II regions (clouds of ionized hydrogen glowing around hot young stars), open clusters, and giant molecular clouds (cold, dense clouds where stars form). All are short-lived by cosmic standards — massive stars last only millions to tens of millions of years — which is why they are found in the arms: that is where new stars are being made. Older stars are spread throughout the disk and do not outline the arms at all. This is why photographs of spiral galaxies show blue arms against a redder disk: the arms shine because of brilliant young stars, not because they contain most of the galaxy's mass.

The winding dilemma and differential rotation

The Galaxy does not rotate like a solid plate. Stars and gas follow individual orbits, and orbits closer to the center have shorter periods — differential rotation. The Sun, at about 26,000–27,000 light-years from the center, takes roughly 230 million years per orbit; inner stars complete their circuits faster. If arms were fixed material, differential rotation would shear them into ever-tighter coils in a few orbits — far less than the Galaxy's age. Since arms have persisted for billions of years, they cannot be material; they must be density waves: regions of slightly higher star and gas density that rotate at their own (slower) pattern speed while individual stars and gas clouds move through them.

Density wave theory

In the density wave picture, a spiral-shaped wave of compression sweeps slowly around the disk, like a stadium wave: the wave moves around the stands while each fan stays roughly in place. As gas clouds enter the wave, they are compressed; compression triggers collapse and star formation; the newborn massive stars and their glowing H II regions light up the arm. The stars then continue on their orbits and drift out of the wave. The wave persists, fed by fresh gas flowing into it, and because the pattern speed is slower than the stars' orbital speeds, stars repeatedly catch up to and pass through the arms without ever "belonging" to them. The theory also explains why arms are prime sites of star formation. (Alternatives exist — arms may also be strengthened or triggered by the central bar and by encounters with satellite galaxies — but density waves are the standard explanation for grand-design spirals.)

Seeing through the dust: the 21-cm line

Mapping the arms is hard because we sit inside the dusty disk. The breakthrough came from radio astronomy. Neutral hydrogen atoms (H I) emit a radio line at a wavelength of 21 cm (frequency ≈ 1420 MHz) when the electron's spin flips relative to the proton's — a transition so rare that each atom emits only about once per 10 million years, but hydrogen is so abundant that the Galaxy is bright at 21 cm. Radio waves pass through dust essentially unhindered, so 21-cm observations map hydrogen across the entire Galaxy. Because the gas shares the Galaxy's differential rotation, the Doppler shift of the line gives the gas's velocity along the line of sight; from the rotation curve, that velocity converts into a distance. This velocity-to-distance trick — applied to hydrogen (21 cm) and carbon monoxide (CO, tracing molecular clouds) — is how the arms and bar of the Milky Way were mapped.

The Sun's neighborhood

Our region of the disk is the Orion Arm (also called the Local Arm or Orion spur), a modest arm segment between two major arms: the Perseus Arm farther out and the Sagittarius Arm closer in; other named features include the Scutum–Centaurus and Norma arms. The Sun's position — in a minor arm of a barred spiral, roughly halfway out in the disk — is a standard exam fact, and it explains why our night sky is rich in nearby stars and nebulae (like the Orion Nebula, in the arm segment named after it) without being dominated by a major arm's star-forming complexes.

Common Confusions