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The Milky Way's spiral arms should have disappeared long ago

The Sun, the Earth and the entire solar system are immersed in one of the several arms that form the spiral of the Milky Way. These arms are regions of higher density, where large clouds of gas and dust are concentrated, which give rise to the hundreds and thousands of nebulae that we observe from Earth, but which also contain young and massive stars , emission nebulae and open clusters formed recently . This seems to indicate that the spiral arms of a galaxy like ours are a region of star formation , an area of the galaxy where new stars are constantly being born from these gigantic clouds that populate it. In fact, the powerful brightness of the aforementioned stars is the main reason why the spiral arms of other galaxies can be distinguished despite the enormous distances that separate us.

However, this image of the spiral arms, as composed of large clouds that rotate around the galactic center , poses a problem , since structures like these should not survive long periods of time . The reason why is quite simple: we have studied in detail how the different parts of the galactic disk rotate and we know that not everything rotates at the same speed. The inner parts , closer to the core, rotate faster than the outer parts . We have known this for more than 50 years and in fact these observations were one of the first convincing proofs of the existence of dark matter. But that is a topic for another article.

Given this different speed of rotation, it is clear that the galactic disk does not rotate as a whole, but that the positions of the different objects will change over time. This differential rotation would make it impossible for any galactic-scale structures related to these spiral arms to survive in time. In fact, what should happen is that these spiral arms would wind around themselves in such a way that after several revolutions, after several hundred million years, they would have disappeared. And yet these spiral arms seem to have survived in our galaxy and in many others we observe scattered throughout the universe during the ten billion years that have elapsed since they formed. Whatever these spiral arms are, it’s clear they can’t just be huge clouds of gas and dust swirling around the galactic core.

The most plausible explanation for this apparent paradox tells us that the arms would actually be spiral density waves . That is to say, waves that, as if from the waves that are formed in a pond when throwing a stone, go through the galaxy and create regions where interstellar gas and dust accumulate in their path. The passage of these waves and the consequent compression and accumulation of the clouds would cause the creation of new stars and other structures. Therefore we could observe these waves by the trail they leave behind.

This explanation does not conflict with the differential rotation mentioned above, because the propagation of pressure waves is not tied to a particular gas cloud , in the same way that the waves of the pond do not cause a net movement of matter, they simply transmit a disturbance. This is consistent with our knowledge that the Sun and the rest of the solar system have been passing through different spiral arms of the galaxy throughout their existence. It has even been proposed that these incursions could have altered the Earth’s climate, although the evidence is not conclusive. This can be easily understood by remembering our last experience bathing in the sea. The waves that go to the shore do not stop passing us, without changing our position . They may momentarily make us wobble, and of course they make us go up and down with the crest of the wave but, unless they are very strong, they do not get us out of the water. The same would happen with the waves that circulate around the galactic nucleus forming these spiral structures.

Observations seem to show that stars and other material orbiting the galactic center are moving faster than these waves , leading them ahead of time. When it reaches one of these waves it suffers a certain wobble until it comes back out the other side. You can visualize this by imagining a traffic jam on the road . The crush of cars, due to a very heavy truck traveling at low speed, for example, will move slowly. The rest of the cars will reach the agglomeration, lose some time in it, and end up leaving it ahead, following their path as before.

The initial disturbance , which can be the passage of a satellite galaxy or a series of supernova explosions, thus manages to last over time , for billions of years and define the appearance of the galaxy on a large scale.


Elena D’Onghia et al, 2013, SELF-PERPETUATING SPIRAL ARMS IN DISK GALAXIES, The Astrophysical Journal

Bertin, G. and CC Lin. 1996. Spiral Structure in Galaxies: A Density Wave Theory. Cambridge: MIT Press.

Eric Chaisson, Stephen McMillan, 2017, Astronomy Today, Prentice Hall

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