![]() The youngest, brightest stars will thus be nearest the wave and trace out an arm. 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. ![]() Astrophysical Journal Letters 827:L2, 2016 August 10. Because arms wind up with time, a galaxy's arms will look tighter or looser depending on which population of stars astronomers observe. 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). 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. Within that radius, the stars travel faster than the wave outside the radius, the stars travel slower. 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. This diagram shows the authors' scenario for how density waves create spiral arms. Essentially, clouds and stars slow down and speed up again in a chain reaction - a density wave - that moves through the galaxy. 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. The same thing happens (we think) in spiral galaxies. Yet even though cars are successfully passing through the jam, the slow stretch persists and keeps propagating along the highway. Once they’ve passed it, they speed up again. Cars whizzing down the road encounter a region where, for whatever reason, they have to decelerate. If you’ve ever been in a slowdown on the highway, you’ve experienced a density wave. Now, observations published in the August 10th Astrophysical Journal Letters provide long-looked-for evidence that these waves do exist. The arms instead would arise thanks to what are called density waves. Maybe, many have suggested, stars don’t actually create the pattern - instead, they’re just passing through it. Thus for at least half a century, astronomers have debated why these patterns persist. However, by this reasoning, the arm should quickly wrap itself around the galaxy’s center, destroying the spiral. 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. In principle it’s easy to make a spiral arm. Young, bright stars trace the arms of these graceful whorls, and dark dust lanes act like galactic eyeliner to dramatically shade them. of Illinois, Urbana) / STScIĪrguably the prettiest objects in space are spiral galaxies. ![]() Of its estimated trillion stars, many thousands of its brightest supergiants are resolved by Hubble. The galaxy M101 is a "grand design" spiral (meaning it's dominated by prominent, well-organized arms) of type Sc. Astronomers have found a unique trait of spiral arms that supports a longstanding idea about where these patterns come from. ![]()
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