The Milky Way is moving. Measured against the cosmic microwave background, the afterglow of the Big Bang that fills all of space, our galaxy and its neighbours in the Local Group are travelling at about 620 kilometres per second. Part of that motion comes from a pull towards one region of the sky, in the direction of the southern constellations Norma and Triangulum Australe. Astronomers call it the Great Attractor.
The Seven Samurai
In the 1980s seven astronomers set out to measure how elliptical galaxies move on top of the expansion of the universe: David Burstein, Roger Davies, Alan Dressler, Sandra Faber, Donald Lynden-Bell, Roberto Terlevich and Gary Wegner. Colleagues nicknamed them the Seven Samurai. In 1987 they reported that galaxies across a huge region were streaming towards a single area about 150 to 250 million light-years away. Dressler gave it the name. Early models put its mass at around 10¹⁶ times that of the Sun.
Hidden behind the Milky Way
The Great Attractor lies almost exactly behind the disc of our own galaxy, in a part of the sky astronomers call the Zone of Avoidance. There the stars, gas and dust of the Milky Way block much of the view, so the galaxies there were long hard to count. Near the attractor’s centre lies the Norma Cluster, a rich cluster of galaxies about 220 million light-years away.
The region turned out to be less massive than some of the early estimates. The Great Attractor is not a single object but the densest part of a large concentration of galaxies and dark matter.
Laniakea
In 2014 Brent Tully and colleagues mapped the motions of thousands of galaxies and drew the boundary of our home supercluster, which they named Laniakea, Hawaiian for “immense heaven”. It is about 520 million light-years across and contains the mass of about 10¹⁷ Suns. The Great Attractor sits at its focal point: within Laniakea, galaxies flow towards it like water draining towards the bottom of a valley.
The Great Attractor is not the end of the story. Further away lies the even more massive Shapley Supercluster, and the Great Attractor region itself is moving towards it. In 2017 Yehuda Hoffman and colleagues found the other half of the picture: a large, underdense region on the opposite side of the sky, the Dipole Repeller, which pushes us away as effectively as Shapley pulls.