A look at the night sky at any time of year will reveal a faint band of light stretching across the sky, either through the middle or near the horizon. The ancient Greeks saw this band of light and called it "galaxies kuklos," for "milk circle." The Romans called it the "Milky Way." In 1610, Galileo used the first telescopes and determined that the light of the Milky Way comes from billions of dim stars that surround us.
For centuries, astronomers asked many basic questions about the Milky Way. What is it? What is it made of? What is it shaped like? These questions were difficult to answer for several reasons.
- We live inside the Milky Way. It's like living inside a gigantic box and asking, what is the box shaped like? What is it made of? How do you know?
- Early astronomers were limited by technology. The early telescopes weren't very large, didn't have much range and couldn't magnify great distances or resolve them.
- Early telescopes could detect only visible light. The Milky Way contains a lot of dust that obstructed their views. In some directions, looking at the Milky Way is like looking through a dust storm.
The 20th century brought great advancements in telescope technology. Large optical, radio, infrared, and X-ray telescopes (both ground-based and orbiting space telescopes) allowed astronomers to peer through the vast quantities of dust and far into space. With these tools, they could piece together what the Milky Way actually looks like.
What they discovered was amazing:
- The Milky Way is actually a galaxy -- a large system of stars, gas (mostly hydrogen), dust and dark matter that orbits a common center and is bound together by gravity.
- Our galaxy is spiral-shaped.
- Contrary to popular belief, our solar system is not at the center of the galaxy.
- The Milky Way is but one of billions of galaxies in the universe.
Come follow us on a journey of discovery as we explore the Milky Way. We'll examine how astronomers figured out its shape, size and structure. We'll look at how the stars within it move and how the Milky Way compares to other galaxies.
Early Milky Way Theories
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.
Globular Clusters and Spiral Nebulae
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?
What shape is the Milky Way?
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.
Milky Way Structure
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 (this was estimated from its mass -- see next page). 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:
- Galactic disk: This is where most of the Milky Way's stars are located. The disk is made of old and young stars, as well as vast amounts of gas and dust. Stars within the disk orbit the galactic center in roughly circular orbits. (Gravitational interactions between the stars cause the circular motions to have some up-and-down motion, like horses on a merry-go-round). The disk itself is broken up into these parts: Nucleus: The center of the disk Bulge: This is the area around the nucleus, including the immediate areas above and below the plane of the disk. Spiral arms: These areas extend outward from the center. Our solar system is located in one of the spiral arms of the Milky Way.
- Globular clusters: A few hundred of these are scattered above and below the plane of the disk. Globular clusters orbit the galactic center in elliptical orbits in which the directions are randomly scattered. The stars in the globular clusters are much older stars than those in the galactic disk, and there's little or no gas and dust.
- Halo: This is a large, dim, region that surrounds the entire galaxy. The halo is made of hot gas and possibly dark matter.
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.
So, how many stars does the Milky Way actually contain? We'll show you the formula on the next page.
How many stars are in the Milky Way?
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 (Mr) that lies within any circular orbit with a radius (r).
- Orbital speed of a circular object (v) v=2Πa/p
- Because it's a circular orbit, a becomes radius (r) and M becomes the mass within that radius (Mr). Mr rv2/G
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.
To learn more about the Milky Way, look at the links on the next page.
More Great Links
- A Map of the Milky Way. http://www.atlasoftheuniverse.com/milkyway.html
- A Teacher's Guide to the Universe. http://www.astro.princeton.edu/~clark/teachersguide.html
- American Museum of Natural History. "Our Place in Space the Milky Way Galaxy." http://www.amnh.org/ology/astronomy/milkyway/index.htm
- Arny, T.T. "Explorations an Introduction to Astronomy." Mosby, 1994.
- Bennett, J. et al. "The Cosmic Perspective (third edition)." Pearson, 2004.
- Chaisson, E., McMillan, S. "Astronomy Today." Prentice Hall, 2002.
- Discovery Education. Understanding the Universe: Galaxy Tour. http://school.discoveryeducation.com/schooladventures/universe/galaxytour/index.html
- Henry, J. Patrick et al. "The Evolution of Galaxy Clusters." Scientific American, December 1998. http://atropos.as.arizona.edu/aiz/teaching/a204/darkmat/SciAm98b.pdf
- Kaufmann, W.J. "Universe (fourth edition)." WH Freeman & Co., 1994.
- Multiwavelength Milky Way. http://mwmw.gsfc.nasa.gov/
- NASA Imagine the Universe. "The Hidden Lives of Galaxies" book. http://imagine.gsfc.nasa.gov/docs/teachers/galaxies/imagine/titlepage.html
- NASA Imagine the Universe. "The Hidden Lives of Galaxies" poster. http://imagine.gsfc.nasa.gov/docs/teachers/galaxies/imagine/poster.jpg
- NASA Imagine the Universe. "The Milky Way Galaxy." http://imagine.gsfc.nasa.gov/docs/features/objects/milkyway1.html
- NASA/JPL GALEX. http://www.galex.caltech.edu
- Seeds, M.A. "Stars & Galaxies (second edition)." Brooks/Cole, 2001.
- Windows to the Universe. "The Milky Way Galaxy - Our Home." http://www.windows.ucar.edu/tour/link=/the_universe/Milkyway.html
- WMAP Cosmology 101: The Milky Way Galaxy. http://map.gsfc.nasa.gov/m_uni/uni_101mw.html