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Vol.18, no.1
February/March 2015


The Super-Dupercluster 
Story By: Rachel Davies

For as long as we’ve existed, humans have stared up at the night sky in wonder. There’s nothing like being reminded of how tiny we are in comparison with the vastness of space to put life into perspective, and over the last hundred years we’ve learned much more about the nature of what we’re seeing and our place in the universe—more than at any other time in human history. It wasn’t until the 1920s that we learned that the galaxies we spied in the heavens aren’t part of our own Milky Way, but separate island galaxies. Around the same time, we found evidence that the universe is expanding and proposed the Big Bang Theory. In the 1930s we noticed that the detectable mass of a galaxy can’t generate enough gravity to keep itself together and hypothesized the existence of an unseen mass called dark matter; today dark matter, along with dark energy, is believed to make up the lion’s share of the mass of the universe, with all visible matter weighing in at a puny 5 percent of what’s out there: Apparently everything the human eye can actually see is just a fraction of a massive, mysterious entity that physicists are working night and day to understand. 

We’ve also observed that there are more stars in the universe than grains of sand on Earth and studied how these stars—along with clouds of gas and dust, planets, exoplanets, moons, asteroids, supernovas and black holes—form galaxies. These galaxies in turn group together into “superclusters”—the largest structures in the known cosmos. And in September of last year, thanks to the work of modern space explorer Professor R. Brent Tully and his cohorts in France and Israel, we were presented with a map of our home supercluster. It’s called Laniakea, which translates to “immeasurable heavens,” and it’s much bigger than anyone imagined.

“I wanted for sure to give it a Hawaiian name,” explains Tully. He is sitting in his tiny office in the Institute for Astronomy at the University of Hawai‘i at Manoa, where he has been on the faculty since 1975. Every surface is covered with stacks of papers. On the edge of a desk teeters a much-thumbed edition of The Nearby Galaxies Catalog, a tome he authored in 1988 that describes the locations of the three thousand galaxies closest to our Milky Way. Glorious color images of heavenly bodies are pinned to the walls. Use a little imagination and Tully’s tiny office could be the captain’s quarters of a ship. Certainly he is a pioneering explorer and mapmaker. Here in modern times, he’s at the vanguard of understanding what’s out there in the oceanic immensity of space.

Tully embraces the metaphor of a seafaring explorer, noting that the name Laniakea was chosen to recognize the achievements of the Polynesian voyagers who discovered the Hawaiian Islands in the middle of the vast Pacific Ocean. “I don’t have to risk my life,” he says, “but I feel a kinship with the ancient explorers. And philosophically this kind of work has that kind of import. It’s about where we live. It’s about trying to understand our environment and make it familiar.”

The known universe has the structure of a vast, interconnected web. In some parts of the web there are large voids, in others dense clusters of galaxies. Tully’s work involves discovering what defines and separates those galaxies and clusters. Mapping the precise boundaries of Laniakea involved a solid decade of calculating the distances to eight thousand galaxies, measuring farther out than ever before and then tracking the movements and flow of those galaxies in relation to each other. 

Previously, superclusters had been defined simply as areas where there were lots of galaxies, but the data Tully and his group collected revealed a new and more precise way of defining superclusters: by the movement of galaxies within them. The group discovered that all of the galaxies in the region we now call Laniakea flow toward each other, the way water on one side of a mountain will flow into one watershed. The edge of a supercluster is the place where gravitational flow lines diverge; when this happens galaxies drift in a different direction and into other, neighboring superclusters. To give me a visual sense of it all, Tully pushes a button on his office computer, and on his screen the Laniakea Supercluster springs to life. We watch a video that shows what Tully calls Laniakea’s “basin of attraction,” toward which galaxies are flowing, including our own Milky Way, which is a barely discernible dot way out in the suburbs near the boundary of Laniakea and its neighboring super-cluster, Perseus-Pisces. Seen in 3-D, Laniakea is shaped slightly like a hat. “Yes,” Tully agrees, “I always think it looks like one of those old warrior’s helmets.”

Laniakea is vast. Tully and his team determined that Laniakea comprises a region one hundred quadrillion (ten to the seventeenth power) times the mass of our sun and five hundred million light-years across. It contains more than one hundred thousand galaxies. It’s a hundred times larger than the Virgo Cluster (which was previously considered our home supercluster), and Laniakea also encompasses the Hydra-Centaurus and Pavo-Indus clouds.