This subsequent formation of new stars.All of this

This lesson will teach you about the spiral arms of the galaxy: namely, the major theory that tries to explain their existence as well as why the spiral arms look so different in various kinds of spiral galaxies.

The Spiral Arms of the Milky Way

The Milky Way Galaxy has some very noticeable spiral arms.

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These spiral arms are like a traffic jam moving through space, but instead of cars, this traffic jam is full of stars, gas, and dust. In a traffic jam, the positions of cars and the moods of the drivers may change with time, but the traffic jam itself remains more or less stable in its shape as a whole as it moves down the highway. And so it is that the arms of the Milky Way Galaxy retain their shape although the components within them change with time.This lesson will point out how astronomers believe the existence of spiral arms can be explained, given all of this, and why they may vary in shape from galaxy to galaxy.

The Spiral Density Wave Theory

As you can imagine, if a driver isn’t paying attention to the fact that there’s a traffic jam in front of them and slams into the traffic jam from behind, lots of sparks and explosions would fly! Well, out in space, gas clouds slam into the gas already located within the spiral arms from behind, resulting in compression of the gas and the subsequent formation of new stars.All of this relates to the spiral density wave theory, the theory that tries to explain the spiral arms of disk galaxies.

Basically, this theory states that the spiral arms of a disk galaxy are regions of the galaxy that are of higher density; and so we call them density waves. They are also areas that are moving more slowly than the galaxy’s stars and gas.What this means is that as faster-moving gas enters a spiral arm, it gets compressed because of the higher density within the spiral arm, and this compression of gas triggers the formation of new stars. The newly formed stars and any remaining gas not used up in the formation of stars eventually make their way out of the slowly moving spiral arm, like cars eventually make their way out of the front of a traffic jam.

Grand Design and Flocculent Galaxies

One of the problems with the spiral density wave theory is that it doesn’t explain very well why some galaxies are galaxies that have two well-defined spiral arms (termed grand design galaxies), some galaxies are galaxies that have many small discontinuous spiral segments (called flocculent galaxies), and some galaxies (like the Milky Way Galaxy) are a mix of the two.Astronomers believe that these differences are explained by the notion that star formation out in space can be contagious, or have a domino-like effect.

I mean, if a car slams into a traffic jam in front of it, it’s not out of the question that this will cause the car it slams into to slam into the car in front of it, and so forth down the line, causing sparks to fly all over the place.Out in space, star formation in one place can lead to star formation in other places nearby. The differential rotation of the galaxy will then separate out these star-forming regions to form discontinuous spiral segments, or spurs in the galaxy.The differential rotation of spiral galaxy is basically the notion that the inner parts of the galaxy move faster than the outer parts of the disk, and this spreads things apart with time.

Overall, spiral density waves are what form two well-defined spiral arms, but self-sustaining star formation and differential rotation is what’s responsible for the production of many discontinuous branches, spurs, and little arms in a galaxy.

Lesson Summary

The existence of spiral arms in a spiral galaxy can be explained, in part, by the spiral density wave theory, the theory that tries to explain the spiral arms of disk galaxies.In essence, this theory says that a galaxy’s density waves are its spiral arms, and they are regions of increased density moving slowly through space. Faster-moving gas clouds inevitably overtake these slowly moving density waves. As they do so, these gas clouds get squeezed by the regions of higher density.

This triggers star formation as the gas clouds collapse, and this new star formation lights up the spiral arms with a lot of light!While spiral density waves can create two well-defined spiral arms, it is self-sustaining star formation combined with differential rotation of the galaxy that may cause lots of branches, spurs, and discontinuous spiral arms to be formed in other galaxies.Galaxies that have two well-defined spiral arms are termed grand design galaxies and galaxies that have many small discontinuous spiral segments are called flocculent galaxies. Some galaxies, like our very own Milky Way Galaxy, are a mix of the two.

Learning Outcomes

Once you are finished with this lesson you should be able to:

  • Discuss the spiral density wave theory
  • Explain why spiral galaxies can look different
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