Edwin Hubble studied galaxies and classified them into various types of elliptical and spiral galaxies. The spiral galaxies were characterized by disk shapes with spiral arms. It stood to reason that because the Milky Way was disk-shaped and because spiral galaxies were disk-shaped, the Milky Way was probably a spiral galaxy.
In the 1930s, astronomer R. J. Trumpler realized that the estimates of the size of the Milky Way galaxy by Kapetyn and others were off because the measurements had relied on observations in the visible wavelengths. Trumpler concluded that the vast amounts of dust in the plane of the Milky Way absorbed light in the visible wavelengths and caused faraway stars and clusters to appear dimmer than they actually were. Therefore, to accurately map stars and star clusters within the disk of the Milky Way, astronomers would need a way to peer through the dust.
In the 1950s, the first radio telescopes were invented. Astronomers discovered that hydrogen atoms emitted radiation in the radio wavelengths and that these radio waves could penetrate the dust in the Milky Way. So, it became possible to map the spiral arms of the Milky Way. The key was marker stars like those used in distance measurements. Astronomers found that class O and B stars would work. These stars had several features:
- Brightness: They're highly visible and are often found in small groups or associations.
- Heat: They emit multiple wavelengths (visible, infrared, radio).
- Short life: They live for about 100 million years, so, considering the rate at which stars orbit the galaxy's center, they don't move far from where they were born.
Astronomers could use radio telescopes to accurately map the positions of these O and B stars and use the Doppler shifts of the radio spectrum to determine their rates of motion. When they did this with many stars, they were able to produce combined radio and optical maps of the Milky Way's spiral arms. Each arm is named for the constellations that exist within it.
Astronomers think that the motion of the material around the galactic center sets up density waves (areas of high and low density), much like you see when you stir cake batter with an electric mixer. These density waves are thought to cause the spiral nature of the galaxy.
So, by examining the sky in multiple wavelengths (radio, infrared, visible, ultraviolet, X-ray) with various ground-based and space-based telescopes, we can get different views of the Milky Way.
On the next page we'll look into exactly what's inside the Milky Way.