Adding a Line to our Cosmic Address

Welcome to Laniakea!

The Laniakea Supercluster as modeled by a computer. The white lines trace the motions of galaxies toward a "Great Attractor". SDvision interactive visualization software by DP at CEA/Saclay, France.

Where are you in the cosmos? Do you know your cosmic address? Where is it? Interesting questions, and it turns out astronomy has some good answers for them! It's not as simple as saying, "the center of the cosmos", since we're not really central to the universe. The true address for us and our planet is a bit more complicated.

If you had to write down your full address, you'd include your street, house or apartment number, city, and country. Send a message to another star, and you add on "the Solar System" to your address. Write a greeting to someone in the Andromeda Galaxy (some 2.5 million light-years away from us), and you'd have to add "Milky Way" to your address. That same message, sent across the universe to a distant cluster of galaxies would add another line that said "The Local Group".

Finding Our Local Group's Address

What if you had to send your greetings across the universe? Then, you'd need to add the name "Laniakea" to the next address line. That's the supercluster our Milky Way is part of — a huge collection of 100,000 galaxies (and the mass of a hundred quadrillion Suns) gathered together in a volume of space 500 million light-years across. The world "Laniakea" means "immense heaven" in the Hawaiian language and is meant to honor Polynesian navigators who used their knowledge of the stars to go voyaging across the Pacific Ocean. It seems like a perfect fit for humans, who are also voyaging the cosmos by observing it with ever-more-sensitive telescopes and spacecraft.

The universe is full of these galaxy superclusters that make up what is known as the "large-scale structure". Galaxies aren't scattered randomly in space, as astronomers once thought. They're in groups, such as the Local Group (home of the Milky Way). It contains dozens of galaxies, including the Andromeda Galaxy and the Magellanic Clouds (irregularly shaped galaxies that can be seen from the Southern Hemisphere). The Local Group is part of a larger collective called the Virgo Supercluster, which also contains the Virgo Cluster. The Virgo Supercluster itself is a small part of Laniakea.

Laniakea and the Great Attractor

Inside Laniakea, galaxies follow paths that all seem to be directed toward something called the Great Attractor. Think of those paths as acting like streams of water descending a mountainside. The region of the Great Attractor is where the motions in Laniakea are directed. This region of space lies about 150-250 million light-years away from the Milky Way. It was discovered in the early 1970s when astronomers noticed that the rate of expansion of the universe was not as uniform as theories suggested. The presence of the Great Attractor explains localized variations in the velocities of galaxies as they move away from us. The rate of a galaxy's motion away from us is called its recession velocity, or its redshift. The variations indicated something massive was influencing the galaxy velocities. 

The Great Attractor is often referred to as a gravity anomaly — a localized concentration of mass tens or thousands more than the mass of the Milky Way. All that mass has a strong gravitational pull, which is shaping and directing Laniakea and its galaxies. What's it made of? Galaxies? No one is sure yet. 

Astronomers mapped Laniakea by using radio telescopes to chart the velocities of the galaxies and clusters of galaxies it contains. The analysis of their data shows that Laniakea is headed toward the direction of another large collection of galaxies called the Shapley Supercluster. It may turn out that both Shapley and Laniakea are part of an even larger strand in the cosmic web that astronomers have yet to map. If that turns out to be true, then we'll have yet another address line to add below the name "Laniakea".