1.3 Earth’s Cosmic Address

When you look up at a star-filled sky on a clear night, all the stars visible to the unaided eye are part of a single collection of stars we call the Milky Way Galaxy, or simply the Galaxy. (When referring to the Milky Way, we capitalize Galaxy; when talking about other galaxies of stars, we use lowercase galaxy.). The Sun, which lies at the center of the planetary system we call home—the solar system, is one of hundreds of billions of stars that make up the Milky Way Galaxy. In a very rough sense, you could think of the solar system as your house or apartment and the Galaxy as your town, made up of many houses and buildings. In the twentieth century, astronomers were able to show that, just as our world is made up of many, many towns, so the universe is made up of enormous numbers of galaxies. (We define the universe to be everything that exists that is accessible to our observations.) Galaxies stretch as far into space as our telescopes can see, many billions of them within the reach of modern instruments. When they were first discovered, some astronomers called galaxies island universes, and the term is aptly descriptive; galaxies do look like islands of stars in the vast, dark seas of intergalactic space.

You might think it is easy to study the Milky Way Galaxy because it is so close. However, the very fact that we are embedded within it presents a difficult challenge. Suppose you were given the task of mapping New York City. You could do a much better job from a helicopter flying over the city than you could if you were standing in Times Square. Similarly, it would be easier to map our Galaxy if we could look at it from the outside, somewhere a good distance away from its outermost edge. Instead we are trapped inside, in the galactic equivalent of Times Square. For this reason, we do not have an image of the Milky Way as a whole. However, we can still learn about the size and shape of the Milky Way by: (1) using observations made from the inside by ground and space telescopes and (2) comparing those observations to other galaxies in the universe. This is how we came to know that the Milky way is a barred spiral galaxy! Any full image of the Milky Way that you may is actually an illustration based on such observations (Figure 1.9).

From the side, the Milky Way looks like a giant disk with a small ball in the middle. If we could move outside of the Milky Way and look down on the disk from above,  it would probably resemble the galaxy in Figure 1.10, with its spiral structure outlined by the blue light of hot adolescent stars. The extent of the Galaxy staggers the human imagination. Within a sphere 10 light-years in radius centered on the Sun, we find roughly ten stars. Within a sphere 100 light-years in radius, there are roughly 10,000 (10⁴) stars—far too many to count or name—but we have still traversed only a tiny part of the Milky Way. Within a 1000-light-year sphere, we find some ten million (10⁷) stars; within a sphere of 100,000 light-years, we finally encompass the entire Galaxy.

Our solar system resides in the suburbs of the Galaxy, roughly 25,000 light-years from the center of the Milky Way. We are specifically located in the Orion Spur, which one of the Galaxy’s minor spiral arms (Figure 1.9). From our position inside the Milky Way Galaxy, we cannot see through to its far rim (at least not with ordinary light) because the space between the stars is not completely empty. It contains a sparse distribution of gas (mostly the simplest element, hydrogen) intermixed with tiny solid particles that we call interstellar dust. This gas and dust collect into enormous clouds called nebulas in many places in the Galaxy, becoming the raw material for future generations of stars. Figure 1.11 shows an image of the disk of the Milky Way as seen from our vantage point on Earth.

Typically, the interstellar material is so extremely sparse that the space between stars is a much better vacuum than anything we can produce in terrestrial laboratories. Yet, the dust in space, building up over thousands of light-years, can block the light of more distant stars. Like the distant buildings that disappear from our view on a smoggy day in Los Angeles, the more distant regions of the Milky Way cannot be seen behind the layers of interstellar smog. Luckily, astronomers have found that stars and raw material shine with various forms of light, some of which do penetrate the smog, and so we have been able to develop a pretty good map of the our home galaxy.The nearest galaxy to us, discovered in 1993, is a small one that lies 70,000 light-years from the Sun in the direction of the constellation Sagittarius, where the smog in our own Galaxy makes it especially difficult to discern. (A constellation, we should note, is one of the 88 sections into which astronomers divide the sky, each named after a prominent star pattern within it.) Beyond this Sagittarius dwarf galaxy lie two other small galaxies, about 160,000 light-years away. First recorded by Magellan’s crew as he sailed around the world, these are called the Magellanic Clouds (Figure 1.12). All three of these small galaxies are satellites of the Milky Way Galaxy, interacting with it through the force of gravity. Ultimately, all three may even be swallowed by our much larger Galaxy, as other small galaxies have been over the course of cosmic time.

The nearest large galaxy is a spiral quite similar to our own, located in the constellation of Andromeda, and is thus called the Andromeda galaxy; it is also known by one of its catalog numbers, M31 (Figure 1.13). M31 is a little more than 2 million light-years away and, along with the Milky Way, is part of a small cluster of more than 50 galaxies referred to as the Local Group.

At distances of 10 to 15 million light-years, we find other small galaxy groups, and then at about 50 million light-years there are more impressive systems with thousands of member galaxies. We have discovered that galaxies occur mostly in clusters, both large and small (Figure 1.14).

Some of the clusters themselves form into larger groups called superclusters. The Local Group is part of a supercluster of galaxies, called the Virgo Supercluster, which stretches over a diameter of 110 million light-years. We are just beginning to explore the structure of the universe at these enormous scales and are already encountering some unexpected findings.