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The Spiral Arms of the Milky Way Galaxy

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  • 0:01 The Spiral Arms of Our Galaxy
  • 0:50 Radio Astronomy Is the Answer
  • 2:33 Detecting the Spiral Arms
  • 4:29 Lesson Summary
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Lesson Transcript
Instructor: Artem Cheprasov
How do we know our galaxy has spiral arms? You can't really see them, but we can infer they're there thanks to radio waves and observations of other galaxies, which you will learn about in this lesson.

The Spiral Arms of Our Galaxy

As you look at an artist's conception of how the Milky Way galaxy looks from afar, the coolest feature has got to be those funky-looking spiral arms. Spiral arms are concentrations of gas and dust extending from the center of a galaxy in a pinwheel shape.

The Milky Way Galaxy
The Milky Way Galaxy

Note a couple of things. I mentioned the image is an artist's conception, and I mentioned the spiral arms contain dust. Clearly, there are two problems. First, we can't see the actual shape of our galaxy because we are inside of it, and second, plenty of what we can see is obstructed from view by the dust in the interstellar medium.

This should naturally make you wonder how we even know our galaxy has spiral arms to begin with. Stick around, because this lesson will explain how we know this to be the case.

Radio Astronomy Is the Answer

The answer lies in radio waves, a type of electromagnetic radiation, with long wavelengths, which can penetrate the interstellar medium.

We know that hydrogen is the most abundant element in our universe and that hydrogen atoms will emit visible light if their electrons are excited. So, to figure out how matter is spread around our galaxy, we need to look for concentrations of hydrogen gas. Unfortunately, the excitation of hydrogen is not likely to occur in the cold reaches of interstellar space, and thus, we are unable to see concentrations of hydrogen with ordinary light telescopes.

I mean, it's hard to get excited, energized, and move around when you're really cold. You sort of freeze up in cold weather. Hydrogen atoms act the same way out in the cold depths of space.

Even if hydrogen were to be excited, the visible light emitted as a consequence of this excitation would be blocked by the interstellar dust. Again, that would render our ordinary telescopes basically useless for this endeavor. The interstellar dust is basically like a really thick cloud. Like a thick cloud blocks out the light of the sun, a thick interstellar cloud of dust and gas would block any glowing hydrogen atoms.

Yet we have a trick up our sleeves when it comes to mapping out the distribution of hydrogen in our galaxy; it is the radio waves I defined before. This is because even the cold hydrogen clouds, containing neutral hydrogen, or H I Hydrogen, will emit radio waves. Another way to think of H I hydrogen is as unexcited hydrogen.

Detecting the Spiral Arms

So how do radio telescopes actually tell us that spiral arms exist in our galactic disk? Astronomers can use the Doppler effect to help map out the hydrogen in our galaxy. The Doppler effect, or Doppler shift, is an apparent change in the frequency of a wave caused by the motion of the observer or source emitting the wave. I encourage you to watch the lesson describing it for more detail.

But to avoid confusion, I'm going to boil everything down to this. Different Doppler shifts throughout our galaxy cause radio waves hitting our radio telescopes to arrive with different wavelengths. This allows us to sort and visualize the gas clouds in our galaxy using a map of neutral hydrogen.

The image on your screen clearly shows you that neutral hydrogen is not spread uniformly in our galaxy's disk, but rather in lanes of sorts that look like arches and spirals that have branches, gaps, and spurs. We see similar features in other spiral galaxies.

Neutral hydrogen dispersion
neutral hydrogen seen through radio waves

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