Laniakea: Our home supercluster


Somewhere in the Universe
is a small blue planet, the third rock from
a star called the Sun, just one of billions of stars in a spiral
galaxy known as the Milky Way. But where in the
Universe is the Milky Way? A team of scientists gathered data on more
than 8,000 of the galaxies that surround us. They mapped each galaxy’s
position and movement in space. And for the first time, they’ve shown that the Milky
Way is part of a much larger system of galaxies, a supercluster that they
have named Laniakea. The Milky Way is nested in the furthest
reaches of this structure, on the outskirts. The entire Universe can be seen as an
intricate network of galaxies, a cosmic web. Some areas are almost
empty, dark voids. Others are densely packed with
galaxies in regions known as superclusters. Superclusters are the biggest
structures found in the Universe, but scientists have struggled to define
where one ends and another begins. To map our home supercluster, a team led
by Brent Tully at the University of Hawaii studied the motions of the galaxies
around us in unprecedented detail. Even though the entire Universe is expanding rapidly,
gravity is also at work, pulling against this acceleration. By discounting cosmic expansion, the team worked out which galaxies are being pulled towards us (shown in blue) and which are
being pulled away (shown in red). This enabled them to create a map of cosmic
flows, the paths that galaxies migrate along, tugged at a tiny pace by the force of gravity. Using this motion, they came up with a new way
to map the distribution of matter in the Universe. Delving into our home supercluster, you can see that most galaxies are being pulled towards a dense centre. This is known as the Great Attractor. Our galaxy is among those sliding
towards this patch of space which dominates our region of the
Universe. Let’s take a different view. Each circle represents a galaxy. Again, we can see most galaxies being pulled towards the Great Attractor in the direction of the arrows. Between the Great Attractor and us, the Milky Way, there’s a relatively empty area, a blue void, and next to us is Virgo,
a large and dense cluster whose large galaxies have been
observed from Earth for centuries. Until now, astronomers grouped us, the
Milky Way and its surrounding galaxies, with Virgo and nearly 100 other
clusters in a supercluster that stretches 100,000,000 lightyears across. But using this new technique, we can
see that this is just the tip of the iceberg. This cluster of clusters is merely an
appendage of a much larger supercluster, more than 100 times bigger and more massive. But how did the team know how to
redraw the boundaries of this cosmic map? Here’s our supercluster (in black) and a neighbouring
structure, Perseus-Pisces (in red). Scientists defined the boundary as the points
where the flows of galaxies diverge, like water dividing at a watershed. This is where neighbouring
structures shear apart. This is the first clear definition
of a supercluster. Laniakea means ‘immeasurable
heaven’ in Hawaiian, a fitting name for the vast community
of galaxies that we are a part of. So, now we know that on the edge
of a supercluster called Laniakea, in a galaxy called the Milky Way,
around a star we call the Sun, there is a small blue
planet, our home.