Spiral arms
Spiral arms are the characteristic spiral-shaped structures of spiral galaxies, which include our Milky Way. These arms are not rigid structures but density waves that propagate through the galactic disk, compressing the gas and triggering star formation.
What are spiral arms, really?
Contrary to what one might think, stars are not permanently bound to a spiral arm. The arms are actually density waves—regions where matter is temporarily more concentrated. Stars and gas pass through these regions as they orbit the galactic center. As they pass through, the gas is compressed, triggering the formation of new stars.
Why are the arms luminous?
The spiral arms appear particularly bright for two reasons. First, the compression of the gas triggers star formation, including some very massive and very bright stars. These hot blue stars have short lifespans (a few million years) and die before leaving the arm—their concentration therefore highlights the spiral structure. Interstellar dust also accumulates in the arms, creating contrasting dark bands.
The Structure of the Milky Way
Our galaxy has several major spiral arms: the Perseus Arm, the Sagittarius Arm, the Centaurus Arm, and the Outer Arm. The Sun is located in a secondary arm called the Orion Arm (or local arm), situated between the Perseus and Sagittarius arms. Observing our own galaxy “from the inside” makes mapping it complex.
Did you know?
Spiral galaxies account for about 60% of the galaxies in the local universe. We distinguish between classic spirals, such as the Milky Way, and barred spirals, which have a central bar of stars from which the arms extend. Our own Milky Way is, in fact, a barred spiral.
Frequently Asked Questions
Do stars move along with the spiral arms?
No. Stars orbit around the galactic center, passing through the spiral arms as they revolve. The arms are density waves that rotate more slowly than individual stars. The Sun, for example, crosses an arm every 100 to 200 million years.
How many arms does the Milky Way have?
Four main arms according to modern observations: Perseus, Sagittarius, Centaurus, and the Outer Arm. Plus a few secondary arms, such as the Orion Arm, where the Sun is located. The exact number varies depending on the detection method used.
Why aren’t all galaxies spiral galaxies?
A galaxy’s shape depends on its history. Elliptical galaxies often result from major mergers that destroyed pre-existing spiral disks. Irregular galaxies did not have the time or the necessary conditions to develop an orderly structure. The Milky Way itself will likely collide with Andromeda in 4.5 billion years to form a giant elliptical galaxy.