Mystery of the Milky Way’s Spiral Arms –“Dark Matter May Be the Clue”

The Milky Way is filled with interstellar matter, gas and dust clouds that absorb the light from the stars that obscures the view of the central region and makes it impossible to see the overarching structure. For this reason, stellar statistics can never encompass the system as a whole, but only the region around the sun up to a radius of about 10,000 light-years. The breakthrough did not come until the middle of the 20th century, when astronomers learned to look at the sky with different eyes using radio telescopes.

Hydrogen is the most common element in the universe. As part of interstellar matter, neutral hydrogen (H1) fills the space between the stars, and thus also fills the Milky Way. This means that the distribution of clouds of hydrogen gas trace the shape of the whole system, similar to the way in which bones shape the human body.


But how can these cosmic “bones” be made visible? The answer is provided by the nanouniverse: in the ground state of hydrogen, the direction of spin of the atomic nucleus and the electron that orbits around it are antiparallel. If two hydrogen atoms collide, the direction of spin of the nucleus and the electron may be flipped to end up parallel to each other – and after a certain time, they return to their basic antiparallel state.

This process releases energy, which is radiated as an electromagnetic wave. This line lies in the radio range of the electromagnetic spectrum. Despite the extremely low density of interstellar matter, atoms are constantly colliding, causing the H 1 areas to glow in the light of this hydrogen line.

This radiation penetrates the dust curtains almost unobstructed and can be picked up by radio telescopes. Thanks to this new window into the universe, astronomers have been able to discover the spiral structure of the Milky Way. However, in the 1970s, researchers found that hydrogen alone was not sufficient as an indicator of the galaxy’s morphology because, for example, it is less concentrated in the spiral arms than expected. The search began anew.

The most important indicator turned out to be clouds of interstellar molecules; they emit radiation in the light of carbon monoxide (CO). Now it was gradually becoming possible to refine the portrait of the Milky Way. Accordingly, the galaxy (from the Greek word gala: milk) is a bent wheel, 100,000 light years in diameter and with a thickness of just 5,000 light years. The wheel hub with its black hole is surrounded by a spherical bulge of stars with an embedded cigar-shaped structure – a kind of bar.

Around 15,000 light years from the center, a ring extends that also consists of dust and gas clouds, as well as stars. The galaxy is characterized by several arms. Most of them bear the names of the stellar constellations in which we observe them: the Sagittarius and Perseus Arms, the Norma and Scutum-Crux Arms, the 3-Kiloparsec Arms and the Cygnus Arm.

Our solar system is located in the Orion Arm, 26,000 light-years from the center and almost on the main plane. The system, which contains around 200 billion suns, is surrounded by a spherical halo containing thousands of globular star clusters and a spherical region consisting of very thin hydrogen plasma. The entire galaxy rotates, with objects closer to the center rotating faster, and those further from the center rotating more slowly. The curve of this differential rotation shows irregularities that cannot be explained by visible mass alone.

Here, it is likely that invisible dark matter plays a role. And the astronomers face yet another problem: despite the rotation, the spiral arms do not unwind, but have maintained their shape for billions of years. One explanation for this is shockwaves that propagate throughout the whole system and compact the matter in the spiral arms like a traffic jam on the motorway. Researchers are still puzzling over what causes these density waves.

Image credit: Top of page, Amanda Smith, University of Cambridge. Astronomers have looked back to a time soon after the Big Bang, and have discovered swirling gas in some of the earliest galaxies to have formed in the Universe. These ‘newborns’ – observed as they appeared nearly 13 billion years ago – spun like a whirlpool, similar to our own Milky Way.

The Daily Galaxy via Max Planck Society