How the Milky Way Works

As we mentioned, Galileo discovered that the Milky Way is made of dim stars, but what about its shape? How can you tell the shape of something if you're inside it? In the late 1700s, astronomer Sir William Herschel addressed this question. Herschel reasoned that if the Milky Way was a sphere, we should see numerous stars in all directions. So, he and his sister Caroline counted the stars in more than 600 areas of the sky. They found that there were more stars in the directions of the band of the Milky Way than above and below. Herschel concluded that the Milky Way was a disk-shaped structure. And because he found about the same numbers of stars in all directions along the disk, he concluded that the sun was near the center of the disk.

Around 1920, a Dutch astronomer named Jacobus Kapetyn measured the apparent distances to nearby and remote stars using the technique of parallax. Because parallax involved measuring the motions of stars, he compared the motions of distant stars with nearby ones. He concluded that the Milky Way was a disc approximately 20 kiloparsecs, or 65,000 light years, in diameter (one kiloparsec = 3,260 light years). Kapetyn also concluded that the sun was at or near the center of the Milky Way.

But future astronomers would question these ideas, and advanced technology would help them dispute the theories and come up with more accurate measurements.

Around the time that Kapetyn published his model of the Milky Way, his colleague Harlow Shapely noticed that a type of star cluster called a globular cluster had a unique distribution in the sky. Although few globular clusters were found within the Milky Way band, there were a lot of them above and below it. Shapely decided to map the distribution of globular clusters and measure their distances using variable star markers within the clusters and the luminosity-distance relationship (see sidebar). Shapely found that globular clusters were found in a spherical distribution and concentrated near the constellation of Sagittarius. Shapely concluded that the center of the galaxy was near Sagittarius, not the sun, and that the Milky Way was about 100 kiloparsecs in diameter.

Shapely was involved in a great debate about the nature of spiral nebulae (faint patches of light visible in the night sky). He believed that they were "island universes," or galaxies outside the Milky Way. Another astronomer, Heber Curtis, believed that spiral nebulae were part of the Milky Way. Edwin Hubble's observations of Cepheid variables finally settled the debate -- the nebulae were indeed outside the Milky Way.

But questions still remained. What shape was the Milky Way, and what exactly existed inside it?

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:

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.

According to Edwin Hubble's classification system, the Milky Way is a spiral galaxy, although more recent mapping evidence indicates that it may be a barred spiral galaxy. The Milky Way has more than 200 billion stars. It's approximately 100,000 light years in diameter, and the sun is located about 28,000 light years from the center. If we look at the structure of the Milky Way as it would appear from the outside, we can see the following parts:

All of these components orbit the nucleus and are held together by gravity. Because gravity depends upon mass, you might think that most of a galaxy's mass would lie in the galactic disk or near the center of the disk. However, by studying the rotation curves of the Milky Way and other galaxies, astronomers have concluded that most of the mass lies in the outer portions of the galaxy (like the halo), where there is little light given off from stars or gases.

The Milky Way's gravity acts on two smaller satellite galaxies called the Large and Small Magellanic Clouds (named after Ferdinand Magellan, the Portuguese explorer). They orbit below the plane of the Milky Way and are visible in the Southern Hemisphere. The Large Magellanic Cloud is about 70,000 light years in diameter and 160,000 light years away from the Milky Way. Astronomers think that the Milky Way is actually siphoning off gas and dust from these satellite galaxies as they orbit.

We mentioned earlier that astronomers have estimated the number of stars in the Milky Way from measurements of the galaxy's mass. But how do you measure the mass of a galaxy? You obviously can't put it on a scale. Instead, you use its orbital motion. From Newton's version of Kepler's Third Law of Planetary Motion, the orbital speed of an object in circular orbit, and a little algebra, you can derive an equation to calculate the amount of mass (M_r) that lies within any circular orbit with a radius (r).

For the Milky Way, the sun lies at a distance of 2.6 x 10²⁰ meters (28,000 light years) and has an orbital speed of 2.2 x 10⁵ meters/second (220 km/s), we get that 2 x 10⁴⁹ kg lies within the sun’s orbit. Since the sun’s mass is 2 x 10³⁰, then there must be 10¹¹, or about 100 billion, solar masses (sunlike stars) within its orbit. When we add the portion of the Milky Way that lies outside the sun’s orbit, we get approximately 200 billion stars.