Spiral Arms Explained: The Science Behind the Milky Way’s Structure

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In this video, we’ll dive into the fascinating realm of the Milky Way’s spiral arms, uncovering how they formed, what they look like, and how we measure them. The Milky Way, like other spiral galaxies, has these majestic spiral arms, but because we’re inside it, observing them isn’t straightforward. To understand the spiral arms, we first need to recognize the challenges. Nobody can travel thousands of light years to view the Milky Way from above. Instead, we rely on measuring gas and dust clouds to trace the spiral structure. Spiral arms are dotted with hot, young stars, type O and B stars, which illuminate these regions. These stars are short-lived, so their locations indicate ongoing massive star formation. These arms can’t rotate along with the galaxy because their structures are not fixed entities but rather patterns where star formation is primarily happening. The sun, for instance, takes about 260 million years to orbit the galaxy, completing approximately 50 orbits before it dies, whereas O and B type stars live for only about 1 to 10 million years. Hence, when we see an O star, it hasn’t moved far from its birthplace. Walter Baade, a pioneering astronomer, described spiral arms as “beads on a string,” highlighting regions packed with intense star formation activity. These arms function as density waves, places where gas and dust compress, leading to star formation, akin to a traffic jam of stellar material. The stars then continue their journey around the galaxy, giving the illusion of a rotating spiral. By examining nested elliptical orbits, we see that overlapping streams of gas create these stellar traffic jams, forming the spiral arms. This concept explains why we see bright star clusters and pink hydrogen glows concentrated in specific regions—these are the result of compressed gas clouds forming new stars. We have impressive visual evidence from galaxies like the Whirlpool Galaxy (M51). Here, you can see dark lanes of gas where star formation is happening, followed by bright, hot stars moving out of these dense regions. The spiral structure becomes evident when gas compresses in these lanes, triggering star formation. Closer to home, we look at the Spitzer Space Telescope’s infrared images of our own galaxy. Images of regions like M17 show how star formation progresses from dark dust clouds to bright H2 regions, with gas compressing and forming new stars. These observations help us understand the spiral arms’ structure in great detail. Radio astronomy, particularly the study of the 21-centimeter radiation, plays a crucial role in mapping hydrogen gas in the Milky Way. This hydrogen emission helps us map where stars are forming, as hydrogen is the primary building block of stars. By measuring redshifts and Doppler shifts, we can determine the locations and movements of these gas clouds, creating a map of the spiral arms. In fact, by examining the neutral hydrogen map and the carbon monoxide emissions, we can trace the spiral arms and understand their structure. This has been confirmed by large-scale surveys like those conducted by the Very Large Array (VLA), which showed spiral structures in nearby galaxies similar to the Milky Way. Our journey today uncovers the methods and evidence used to map and understand the Milky Way’s spiral arms. These arms aren’t just beautiful structures but active sites of star formation, shaped by complex interactions of gas, dust, and gravitational forces. So sit back and enjoy as we explore the intricate dance of stars and gas within the spiral arms of our home galaxy.

This is part of my complete intro Astronomy class that I taught at Willam Paterson University and CUNY Hunter.

0:00 Introduction
3:10 Spiral Arms are Sites of Active Star Formation
4:38 Galactic Spiral Arms
6:10 What are Spiral Arms?
8:23 Nested Elliptical orbits of stars...
14:03 M17 SWex Dark Star-Forming Cloud (younger)
15:57 Many eras of star formation
16:21 All-sky Milky Way in Hydrogen emission 21cm
17:18 21 cm radiation in our Milky Way Hydrogen atom
19:21 21cm Radiation of Neutral Hydrogen (HI)
22:23 Neutral Hydrogen Map of the Milky Way
23:17 All molecular clouds are found in HI regions
25:59 REVIEW QUESTIONS