Our Milky Way galaxy is vast—100,000 light-years across—but even the Milky Way is part of something larger. As astronomers zoom out to larger and larger scales, they see galaxies bunching up into clusters and these clusters into superclusters.
Astronomers have recently mapped our home supercluster and found that it is five times larger than previously thought. The Milky Way is just one peripheral blip out of 100,000 galaxies. A team led by Brent Tully at the University of Hawaii in Honolulu used a new method to define this supercluster, which they named Laniakea, meaning “immeasurable heaven” in Hawaiian. Laniakea is 500 million light-years across. If the supercluster were as tall as a three-story house, the entire Milky Way would fit inside the head of a pin.
Previous methods could not exactly define which galaxies belong to the supercluster in which we reside, but the new method succeeds by measuring the gravitational pull of galaxies instead of measuring the amount of their matter.
If you stood at the edge of a watershed, you could map its boundary as the farthest place where, if a drop of rain fell, it would flow into the watershed. The team used an analogous method to map the boundary of Laniakea, with galaxies in place of raindrops. All the galaxies in Laniakea are drawn by gravity toward what Tully calls the “downtown area” of Laniakea, known as the Great Attractor. The Milky Way itself is hardly in a central location. If you imagine Laniakea as the continental U.S., the Milky Way would be all the way in the tip of Florida.
Tully is careful to point out that this does not mean the galaxies will collapse together in a heap. Overall, they are still moving away from each other because the universe is expanding at a rate much faster than the gravitational pull of each galaxy.
This method provides better maps than any previous method but is also “much, much harder” because it requires accurate measurements of both a galaxy’s distance and its gravitational pull, says David Schlegel, a physicist who was uninvolved in the study.
The new method gets harder the farther away we try to look, but there is a lot more to see. Laniakea is itself moving toward an even bigger entity called the Shapley concentration. “We’re really only exploring a tiny, tiny bit of the universe,” says Tully.