Scientists have redrawn the cosmic map of our corner of the universe, using new tools to define which galaxies interact with our own. The so-called supercluster of galaxies that contains the Milky Way has been named "Laniakea," which means "immense heaven" in Hawaiian.
Defining regions in an infinite universe is tricky business: Clusters of dozens of galaxies, called local groups, are further bound into clusters containing hundreds of galaxies. The Laniakea supercluster, described in a paper published in this week's Nature, is 500 million light-years in diameter and contains 100,000 galaxies - and we sit at the very edge of it. Together, those galaxies carry 100 million billion times the mass of our sun.
How can such a massive number of galaxies be connected? While some areas of space are basically empty, others contain highly concentrated star power. In these areas, the supercluster galaxies are drawn toward each other in intricate ways. According to R. Brent Tully, an astronomer at the University of Hawaii at Manoa and lead author of the study, galaxies in the cosmos can be compared to water on Earth.
"Within a land form, water flows in certain directions," Tully said. "The water knows, even if the land is very flat, which way is downhill." Instead of downhill valleys that attract water flow, our universe has something called the "Great Attractor." This region serves as a gravitational focal point , influencing the motion of galaxies in the supercluster.
In the new study, Tully and his colleagues provide our first clear definition of a supercluster. By mapping the flow of over 8,000 galaxies that surround our own, they figured out where the clusters diverged. In other words, they pinpointed at what point galaxies started to be drawn toward a different "valley" than we are.
It's possible that scientists will eventually map out a cluster that's even more super. "We've found the local region, but we already see that our base of attraction is actually being pulled toward another base of attraction," Tully said. "We don't really understand why."
To solve that mystery, researchers will have to look two to three times farther than they did when they mapped Laniakea. That, Tully said, will be a huge undertaking - they'll have to chart the flow of tens of thousands more heavenly bodies. But while the borders of our cosmological continent may still be unknown, Earth has charted its country in the stars.