Why Galaxies Have Spiral Arms

Astronomers have found a unique trait of spiral arms that supports a longstanding idea about where these patterns come from.


The galaxy M101 is a "grand design" spiral (meaning it's dominated by prominent, well-organized arms) of type Sc. Of its estimated trillion stars, many thousands of its brightest supergiants are resolved by Hubble.
NASA / ESA / K. Kuntz (JHU) / F. Bresolin (Univ. of Hawaii) / J. Trauger (JPL) / J. Mould (NOAO) / Y.-H. Chu (Univ. of Illinois, Urbana) / STScI

Arguably the prettiest objects in space are spiral galaxies. Young, bright stars trace the arms of these graceful whorls, and dark dust lanes act like galactic eyeliner to dramatically shade them.

In principle it’s easy to make a spiral arm. For various reasons, stuff in the disk sometimes clumps together, but the clump won’t stay a clump for long: stars and clouds near the galactic center circle the galaxy faster than the material farther out does, so over time the clump gets stretched into a spiral.

However, by this reasoning, the arm should quickly wrap itself around the galaxy’s center, destroying the spiral. That generally doesn't happen. Thus for at least half a century, astronomers have debated why these patterns persist. Maybe, many have suggested, stars don’t actually create the pattern — instead, they’re just passing through it. The arms instead would arise thanks to what are called density waves. Now, observations published in the August 10th Astrophysical Journal Letters provide long-looked-for evidence that these waves do exist.

Yield to Oncoming Stars

If you’ve ever been in a slowdown on the highway, you’ve experienced a density wave. Cars whizzing down the road encounter a region where, for whatever reason, they have to decelerate. Once they’ve passed it, they speed up again. Yet even though cars are successfully passing through the jam, the slow stretch persists and keeps propagating along the highway.

The same thing happens (we think) in spiral galaxies. Even as a clump in the disk stretches into a spiral, all the stars and clouds keep moving through that arm, just as cars continue to pass through a highway choke point. Essentially, clouds and stars slow down and speed up again in a chain reaction — a density wave — that moves through the galaxy.

how density waves work

This diagram shows the authors' scenario for how density waves create spiral arms. The green dashed line is the co-rotation radius, where the density wave (brownish curve, labeled "stellar arm") and the stars and gas in the galactic disk travel at the same speed. Within that radius, the stars travel faster than the wave; outside the radius, the stars travel slower. In the scenario above, the density wave compresses the gaseous arm (black), which then forms new stars (blue arm) that age as they travel farther from the density wave. Those newborn stars combine with other, old-and-red stars that were already in the disk and were squeezed closer together by the wave (red). Because arms wind up with time, a galaxy's arms will look tighter or looser depending on which population of stars astronomers observe.
Hamed Pour-Imani et al. / Astrophysical Journal Letters 827:L2, 2016 August 10. © AAS, reproduced with permission

The reason we can see this spiral pattern is because as it passes through the galaxy the density wave compresses gas clouds, triggering star formation. The youngest, brightest stars will thus be nearest the wave and trace out an arm.  As stars move out of the wave and spread out across the disk they will age and these biggest, brightest stars will die off, preventing the arm from totally winding up.

But that doesn’t mean there’s no winding. An important prediction comes out of this scenario: how tightly wound a spiral’s arms appear depends on which population of stars you observe. As time goes on the stars get farther from the wave, and — because the inner stars move faster and the outer stars move slower — their orbital motions do wind the arm they’re tracing, tightening the spiral over time.

But because the hot, bluish, live-fast-die-young ones kick the bucket soon after they encounter the density wave, they’ll only trace loosely wound  arms. Conversely the older, redder stars will trace more tightly wound arms. So if astronomers look at a galaxy in wavelengths that pick up young stars, they’ll see a more relaxed spiral than if they look in wavelengths that pick up old stars.

Density Waves Detected

Until now, astronomers hadn’t conclusively seen this effect. But the new study by Hamed Pour-Imani (University of Arkansas) and colleagues is convincing proof in its favor. The team compiled archival images of 28 spiral galaxies in far-infrared, near-infrared, optical, and ultraviolet wavelengths. The far-infrared and ultraviolet wavelengths pick up star-forming regions, while optical and near-infrared probe older stars.

The team checked its results three ways and sure enough, it found exactly what’s predicted: arms traced by older stars hug the galactic centers more tightly than those traced by star-forming regions. The result is a neat confirmation that density waves exist.


Reference: H. Pour-Imani et al. “Strong Evidence for the Density-Wave Theory of Spiral Structure in Disk Galaxies.” Astrophysical Journal Letters. August 20, 2016.

Learn more about all things astronomy with a subscription to Sky & Telescope.

6 thoughts on “Why Galaxies Have Spiral Arms

  1. heaton

    This appears to be an interesting paper, which I have yet to read. The underlying fundamental question is what causes density waves? For example, what could cause a structured gravitational potential to act over such enormous distances? And what causes that pattern to take on different forms in different galaxies (e.g., 2-arm, 3-arm spirals) or perhaps at different distances (i.e., z)?

    1. Camille M. CarlisleCamille M. Carlisle Post author

      Yes, I took that bit out of the blog — it’s rather confusing, because *why* the waves exist is an open question. Something happens gravitationally to cause clumping in the galaxy’s disk, but that could be the tidal yank of a passing galaxy, the pileup of giant molecular clouds in the disk, or orbital resonances between two lumps of stuff in the galaxy. In the last scenario, if a bit of material completes an orbit around the galactic center in a whole fraction of the time that material farther out does, that’ll build up the gravitational influence one clump has on the other. That in turn would create spots of higher and lower density. Any or all of these might be at play.

  2. Jim-BaughmanJim-Baughman

    Since spiral galaxies often exhibit the same structure as hurricanes and water spiraling down a drain I’d enjoy seeing a study linking these three phenomena.

    Also, it is crucial to remember that a galaxy’s stars are not in orbit around the nucleus of the galaxy—how could the 100 billion solar mass of the Milky Way orbit a (relatively) miniscule 4-million solar mass black hole? Planets orbit the Sun because it contains 98% of the mass of the solar system, and thus it can direct the distribution and motion of its captives. Orbital mechanics do not apply to galactic structures, but those of eddies and cyclones.

  3. Vad-Falcone

    One of the first signs of dark matter was that the edge of galaxies was spinning faster than predicted by Newton’s laws. In this case, shouldn’t dark matter also slow down the wrapping up of spiral arms?

All comments must follow the Sky & Telescope Terms of Use and will be moderated prior to posting. Please be civil in your comments. Sky & Telescope reserves the right to use the comments we receive, in whole or in part, and to use the commenter’s username, in any medium. See also the Terms of Use and Privacy Policy.