Adapted for publication and use by the Big History Project from the essay “Galactic Position System,” by Sarah Scoles, published on aeon.co.
Extended learning opportunity: Curious to dig even deeper into this big topic? Resident astrophysicist Cameron Gibelyou, educator at the University of Michigan and BHP’s support for all “star stuff” held an exchange October 8–12. You can check out the archive of this conversation here and learn more!
Note from the BHP Team: In the Big History Project curriculum, we discuss how our understanding of the origin and structure of the Universe has changed over time. This topic necessarily involves asking: Where are we in the Universe? That is, we humans. How do we fit into the bigger picture? How do we know? And does this matter?
We’re thrilled to add to the course this article on the galactic body we call home – the Milky Way. Among the questions the article raises: Do we know where we are in the Milky Way? How have new scientific insights and discoveries informed this knowledge? Does it matter that we can point to the Milky Way and locate Earth’s position?
We hope you enjoy the read.
If you’re eager for more, you can read one BHP teacher’s response to the article (in which he argues that yes, it matters that we know where we are in the Milky Way)..
Do You Know Where You Are in the Milky Way?
The artist Jon Lomberg kneels next to a plant in the Galaxy Garden. He points to the cheap earring that pierces a speckle on one of the plant’s leaves. This, he says, pointing to the stud, represents the Sun. He is standing 30 feet from the middle of the 100-foot-wide garden in Hawaii. The garden is an accurate scale model of the Milky Way galaxy, where every step takes you 2,000 light years from where you stood before. Here, the speckles on the leaves of the plants stand in for stars. The plants are arranged in spiral arms that reach out from the garden’s center, where a fountain plays the part of an enormous black hole. Along the spiral limbs are colorful plants where nebulae, gas, and dust would be.
- The Galaxy Garden resides in the Paleaku Peace Sanctuary in Honaunau, Hawaii. Photo courtesy and © Simon Bell.
- The gold sphere represents a magnified Solar System. Photo courtesy and © Simon Bell.
When Lomberg stands up, he looks down at our yellow star. “If you put a basketball-sized sphere around where Earth is, everything you would be able to see in the night sky would be in that sphere,” he says, adding that other stars are too distant and dimmed by dust to be seen. “When you see the whole garden,” Lomberg continues, “without any math, or any need for explanation, the scale becomes apparent.”
It is difficult to picture ourselves in the Milky Way. We have no problem placing ourselves somewhere like California, or the United States. Even second-graders can point to a spot on a globe and say: “I live there.” And by third grade, kids can tell you where Earth is in relation to other planets, and that the planets orbit the Sun. Beyond that? Everything else is so distant that we often lump it all into the “out there” category, so that Pluto and Polaris might as well be in the same spot. Perhaps that’s why, as Lomberg says, many people think a galaxy is the same as a solar system.
Only in the past few decades have we become scientifically and technologically advanced enough to know where we are located in our 100,000-light-year-wide galaxy. But making sense of the galaxy isn’t easy for anyone, including astronomers. We were born and will live our lives inside the Milky Way. Imagine being trapped inside a building and trying to determine what the building looks like from the outside.What shape is the building? When was it last painted? What kind of roof does the building have? You can count the number of bathrooms in the building, peer out of a window and describe what the building is made of. But putting those pieces together into an entire picture is like trying to understand your brain by using your brain. You’ll never have a complete view of what it looks like from the outside.
Now, imagine you’ve never even heard of something called a building. That’s the situation astronomers found themselves in until the 1920s. The Milky Way has always streaked our sky, a stripe spreading from horizon to horizon. Its name comes from its fuzzy, lane-like appearance. But it wasn’t until Galileo Galilei trained his telescope on it 400 years ago that we knew it was made of stars.
As centuries passed, telescopes grew larger, allowing astronomers to detect fainter objects. Looking deeper into space than ever before, astronomers were able to see spiral-shaped clouds. Thinking that they were just clouds of gas within our own galaxy, astronomers called them spiral nebulae. After all, if you don’t know an object’s true size, it might be small and nearby, or distant and huge. And in the late nineteenth century, as far as we knew, our cosmos was the whole cosmos. We had learned long ago that the Earth is not the hub of the Solar System, but it wasn’t until we were able to see the spiral clouds that we understood that our galaxy was one of many.
The Milky Way looks and behaves like any other spiral galaxy, and the Universe contains tens of billions of those. Wherever we look in the Universe, we see the laws of physics at work. These laws cause gas to be molded into stars, and stars into galaxies. Peering out of our cosmic window, we can observe these structures to learn about our own.
A Hubble Space Telescope image of galaxy UGC 12158, which is thought to resemble the Milky Way in appearance. Photo by ESA/Hubble & NASA. Public domain.
In fact, we know that from the side, the Milky Way is a thin disk with a bulge in the middle. From above or below, it resembles a hurricane, with spiral arms where rain bands would be. Four of these arms curl from the eye of the storm. The Perseus Arm and the Sagittarius Arm wrap around our side of the Milky Way, while the Scutum-Centaurus and Norma-Cygnus arms form a near-mirror-image on the other side.
A stream of stars forms a curved path between Perseus and Sagittarius. It is on this 15,000-light-year-long branch, called the Orion Arm, that our Sun sits. For a long time, astronomers believed the Orion Arm was unimportant. Recently, they discovered our arm is a major arm in its own right, like Perseus and Sagittarius.
A detailed annotated artist’s impression showing the structure of the Milky Way. Photo by NASA/JPL-Caltech/ESO/R. Hurt. Public domain.
That’s a pretty basic discovery to make so late in the game. But nearly all knowledge of galactic limbs comes from invisible parts of the light spectrum. Unlike the study of visible astronomical objects, which began with Galileo’s telescope, the study of these invisible parts is just beginning. Scientists didn’t discover the spiral arms until the 1950s, with the first radio surveys of hydrogen gas.
All spiral arms lead to the Milky Way’s center. There, a bar of stars in long, looping orbits forms a tapered shape, like the center of a cat’s eye. This eye is 27,000 light years across – nearly as wide as the distance between Earth and the center of the galaxy – and pokes out of the galaxy’s plane at a 45-degree angle. Above and below this midsection, nearly 200 spheres orbit. Each sphere is packed with hundreds of thousands of ancient stars.
At the very center of the galaxy, 27,190 light years from Earth, sits the Milky Way’s most significant object. Our fastest spacecraft would take 475 million years to arrive at its boundary.Although this supermassive black hole, called Sagittarius A*,seems to be making every atom its satellite, this is an illusion. Everything orbits the center of mass, which is the point where the gravitation from all directions is the same. Just as the Earth spins on its axis and revolves around the Sun, the whole Milky Way whirls around the center of mass. Even Sagittarius A*, very slightly offset from the galaxy’s true center, does a delicate jig.
The region around the black hole is dense with dust, which blocks and scatters visible light. Because no optical telescopes could see to the center, astronomers did not find evidence of Sagittarius A* until 1974, when radio telescopes became powerful enough to pinpoint the precise location of a strangely compact source of radiation at the center of the galaxy.
The center of the Milky Way galaxy, with the supermassive black hole Sagittarius A*located in the middle, is revealed in these images. Photos by: X-ray: NASA/UMass/D.Wang et al., IR: NASA/STScI. Public domain.
If you zoom out of the Milky Way entirely, you’ll find 26 known dwarf galaxies orbiting us. It’s likely that there are 1,000 more out there, some nearly invisible, made mostly of dark matter or cold hydrogen gas. Beyond them lie the other members of the Local Group. Fifty-four of these galaxies are known. They travel together through space, and their gravitational center lies between our galaxy and its nearest large neighbor, the Andromeda galaxy. Around the Milky Way and Andromeda, 12 other large galaxies form a protective circle known as the Council of Giants.
The Local Group is part of the Virgo Supercluster, which contains more than 100 galaxy groups and clusters. But even this structure is cosmically insignificant. Just as Earth is one of many planets, the Sun is an average star.Just as the Milky Way is a predictable spiral galaxy, the Virgo Supercluster has 10 million massive relatives.
That number is so large that it hardly means anything to most of us. However, imagine that the Galaxy Garden really is the galaxy. The visible Universe would be the size of Earth, and it would be immaculately landscaped with hundreds of billions of Galaxy Garden clones, all congregating in clusters and superclusters, with long filaments of gas and dark matter connecting the superclusters.
An illustration of the location of Earth in the Universe, in a series of nine frames. Image by Andrew Z. Colvin – own work, CC BY-SA 4.0.
Even if we can’t imagine the scale of the Universe, we’re still able to situate ourselves within it. Lomberg envisions a world in which every third-grader knows where the “You are here” label goes on a galactic map and can give you directions from there to the Scutum-Centaurus Arm. ”The Milky Way is as important to your self-identification as the fact that you live on planet Earth, or you live in the Solar System,” he says. “We may be insignificant, but we figured out that we’re in a galaxy. For insignificant nothings, that’s pretty good.”
About the author: Sarah Scoles writes about space science for a variety of publications, including Slate, Popular Science, and Discover. She lives in San Jose.
Cover image: Milky Way Panorama – The Pinnacles Desert, Western Australia, by Trevor Dobson. CC BY-NC-ND 2.0