Shape of the Milky Way

Have you ever seen the Milky Way in the night sky? Watch the teacher video Appearance of the Milky Way to see what it looks like in Stellarium.
Explore 25.1 The Architecture of the Galaxy to understand the components and structure of the Milky Way Galaxy, including its disk, halo, bulge, and central black hole.
Astronomers estimate that the Milky Way galaxy contains some 300 billion stars. It’s difficult to understand how much that really is. Check out the video 300 Billion Things, which is my attempt to show you just how big of a number “300 billion” really is. In addition, you should check out this webpage, which shows one billion dots. (Keep in mind that our galaxy has 300 times as many stars as dots on this page! It may take a minute to load and be sure to scroll all the way down and to the right). Imagine that every one of those dots is a star like our sun - and each could have its own solar system of planets!
Not all stars are found by themselves like our star is. Many are formed as binary stars - two stars orbiting one another. And many others are formed in large clusters containing anywhere from hundreds to hundreds of thousands of stars. Explore these star cluster images from the Hubble Space Telescope.
Our Sun moves through this mess of stars, clusters, and nebulae without ever running into any of it. While it seems our galaxy is full, there is still an enormous amount of empty space to move through. This does not mean, however, that the sun is isolated from potential threats nearby in our galaxy. Watch the BBC clip Our Perilous Journey for a demonstration of how a star passing nearby could cause a very bad day on earth.
Read 25.2 Spiral Structure to learn about the spiral arms and differential rotation of the Milky Way Galaxy, as well as the formation and development of spiral structure in galaxies.
The book suggests that the formation of spiral arms is a natural byproduct of the evolution of a galaxy. Watch the video Spiral Arm Formation to see a simple simulation of this process in action.