The Milky Way: a Growing Galactic Family
Written by Michelle Thaller, Spitzer Science Center
January 3, 2007
|An artist concept of the Milky Way Galaxy
NASA/JPL-Caltech/R. Hurt (SSC)
A newborn child opens its eyes. The first faces it sees are those of its parents, followed by excited grandparents, aunts, uncles, perhaps brothers and sisters. Little by little, the world becomes a bigger place. It takes time, but over the years, each of us comes to understand our part in a larger story of relations and connections, as well as a shared history. This is our family.
To use a metaphor from human experience, our Sun has a family too: the Milky Way Galaxy. We've certainly been aware of the Milky Way for a long time. Even the name we gave to the mysterious white stain across the sky conjures up the image of a parent: the milk from some vast and unseen mother.
Like a Goddess, the Milky Way is truly something to be awed by. We live in a disk of stars so vast, that if our Sun was the size of the dot of an "i" on this page, the Milky Way would stretch from Los Angeles to New York City. Hundreds of billions of stars, our Sun among them, orbit around the dense center of our galaxy, passing in and out of giant dust clouds, the debris of the lives of the stars. Residing in the outskirts of the Milky Way, our Sun takes 226 million years to make one orbit around the dense center of the galaxy. Even so, our motion is far from leisurely. The Sun, along with all our planets, is moving at well over 400,000 miles per hour around a bulging central swarm of millions of stars concealing a supermassive black hole in their midst. And we are part of all this. Some of the stars we see in the sky, although we don't know which ones, formed out of the same birth cloud the Sun did. Other stars, long dead, seeded our solar system with the rich chemicals needed to form life.
But in a way, we're only just beginning to open our eyes and become aware of the larger story. Not only does the Sun belong to a vast extended family, but the Milky Way does as well. Take a fairly basic question: what is the closest galaxy to our own? A lot of people might say the Andromeda Galaxy, which is, in fact, the closest large spiral to us, about two million light-years away. Andromeda is so close, in fact, that it is gravitationally bound to the Milky Way, and is one of the only galaxies in the sky that is moving toward us. Other, somewhat more astronomically savvy people might name the Magellanic Clouds as the closest galaxies. The Large and Small Magellanic Clouds are two small, blobby galaxies that actually orbit the Milky Way, and are about 180,000 and 210,000 light-years away, respectively. And up until the last few years, everyone seemed pretty pleased with this answer; after all, these were the closest galaxies we could see in the sky. But we were wrong to trust our eyes. As it turns out, there are at least two other galaxies that are much, much closer. Part of the reason we missed them, in fact, is that they are so close; they are actually colliding with the disk of the Milky Way. In a very real way, they snuck up from behind us, hiding behind the stars, dust, and gas that fill the volume of the Milky Way's disk.
So how could we have missed two entire galaxies that are colliding with us? The answer has a lot to do with how we know we live in a spiral galaxy in the first place. I mean, you can't look up into the sky and see the spiral structure of our galaxy, so how do we know it's there? Most people are familiar with the dim, blurry band of light that crosses the sky, which ancient people from several cultures likened to a path of milk. This "Milky Way" is actually the combined light of billions of stars in the plane of our galaxy, and people from at least the eighteenth century have realized that our galaxy has a flattened shape.
But what about the spiral arms? That was a bit tricky. The plane of our galaxy is thick with stars, gas, and dark clouds of dust, and our Solar System is embedded inside all that material. There's so much stuff in the Milky Way's disk that we can't actually see very far into our own galaxy. Even the core of the Milky Way, a region where millions of stars are crammed together in tight orbits around a massive central black hole, is invisible to us, blocked by a thick dust cloud that sits inconveniently between us and the center. So, at first, the most we were able to do was map the positions of the nearest and brightest stars to the Sun. When we did, we found that many stars, most notably the brightest, blue-hot, young ones, were distributed in layers of arcs that traced out the boundaries of the spiral arms. We still can't see the entirety of the arms, and we're not even sure how many spiral arms the Milky Way has. But even by mapping the small number of stars we're able to see, we can still deduce our place in the galaxy: we live about 30,000 light-years from the center of the Milky Way, about 20 light-years above the center of the plane of the disk, along a spur of stars called the "Local" or "Orion" arm, which is actually a minor arm that connects two larger spiral arms, the Perseus and the Sagittarius arms.
Simply observing the numbers, distribution, and motion of the stars has gotten us a long way, and that's how the first of the new galaxies was found back in 1994. Astronomers were studying the distances and velocities of stars near the central region of our galaxy, when they noticed something strange. There was a big clump of stars (about a billion of them) seen along the line of sight toward the center of our galaxy, but they were too far away to be part of our galaxy, and they were moving in the wrong direction. Instead of orbiting with the rest of the stars in the disk, they were passing through at an angle, still holding together against the overwhelming gravity of the Milky Way. We realized we were seeing a completely separate galaxy trying to pass through the plane of the Milky Way, and getting ripped apart in the process. This galaxy was dubbed the Sagittarius Dwarf galaxy (as it, and the core of the Milky Way, lie in the constellation Sagittarius), and until recently, it held the record for the nearest galaxy, at about 70,000 light-years away.
Again, in 2003, we sighted another galaxy colliding with the Milky Way, this one even closer. The reason this one eluded us for so long was that it was completely blocked by one of the thick dust clouds in our own galaxy. Even now, we can't detect any of the visible light from the galaxy at all, only infrared light (which we commonly think of as heat) reaches us through the shroud of cold, dusty material that drifts between the stars in the Milky Way. The 2-Micron All-Sky Survey (2 microns, or millionths of a meter, is the wavelength of the infrared light observed) recently completed a map of the entire sky in infrared light. Infrared light is able to pass through obscuring clouds of dust, allowing astronomers to peer into regions of space where visible light is completely blocked. This gives us a huge advantage when looking through our galaxy's disk, and for the first time, we're getting a clear view of how stars are distributed around our entire galaxy, not just the closer regions we can easily see. Infrared light is also a great way to observe cooler, smaller stars, and that's just what the new galaxy seems to be made up of.
So, using infrared light, astronomers have just found a small galaxy of about a billion stars in the constellation Canis Major, that is -- astonishingly -- only about 25,000 light-years away from the Sun. That's closer to us than the center of our own Milky Way! The Canis Major dwarf galaxy is not faring well in its gravitational battle with the Milky Way, and there are streamers of stars being pulled off the smaller galaxy onto the disk of our own. Some of these cannibalized stars are drifting down to become part of the Milky Way's disk, and others are even heading in the direction of the Sun. It's a pretty weird thought that some of the stars around us may not come from our galaxy at all, but were pulled off the Canis Major dwarf galaxy many millions of years ago. But maybe that's not as strange as it sounds. If the Milky Way is currently engulfing two smaller galaxies, how many has it swallowed in its many-billion-year history? Is that, in fact, how large galaxies like the Milky Way come into being, by pulling in and eating any smaller galaxies that get too close?
This idea has certainly gotten the attention of astronomers, who have had some significant problems figuring out how the Milky Way, as a whole, works. Take the lovely spiral arms, which are actually a great mystery to us. Spiral arms are not rigid structures, but variations in the density of stars inside the galactic disk. The spiral arms move at a different rate than the stars, and our own Sun has existed both in and out of spiral arms during our history. The best analogy I can think of is a traffic jam: it's really just a density wave in a distribution of cars. Individual cars move in and out of the jam, but the jam persists (especially if you live in Los Angeles, like I do). Spiral arms in a galaxy are just like that. Until now, we couldn't figure out what was causing the traffic jam of stars that form the spiral arms -- our gravitational simulations of stars, all orbiting a common center, just couldn't explain it. But then people started looking at what would happen if a smaller galaxy slams into a larger one, and viola, the equations started working. The combined gravity of a billion stars going the wrong direction is more than enough to induce density waves in a large galaxy, and it looks like the mystery is solved. Instead of a single system, the Milky Way makes a lot more sense when viewed as a group of interacting galaxies, all merging into an elegant, if somewhat perturbed, whole. There's no reason to think that we've found the last hidden galaxy, either. As we get better at mapping the true location of the stars, who knows what other galactic relatives we'll find playing hide and seek with us out there? After all, we've only just opened our eyes.