As far as megastructures go, superclusters tend to be pretty big. They’re basically clusters of clusters of groups of galaxies. Whenever you look at a stitched-together map of the entire universe, the threads that form a sparkly, cottony spider-web-like structure are made up of superclusters. This is an effect of the force of gravity, which accounts for the uneven distribution of matter throughout the universe even though it’s the weakest of the four fundamental forces. While there might be some “rogue” galaxies that aren’t part of a supercluster, most known galaxies can be mapped within this spider-web of superclusters.
The Shapely Supercluster is the largest known supercluster and is also one of the most difficult to observe because it’s being obscured by nearly the entire Milky Way Galaxy. In fact, the team of scientists that included its namesake, Harlow Shapely, made some observations of its core but thought it was simply an interesting, yet minor, “cloud” of galaxies in the Centaurus constellation.
Shapely Discovered Due To Gravity
The Shapely supercluster is one of the biggest, with 8,000 galaxies and a mass equivalent to 10,000 Milky Way-sized galaxies. In fact, astronomers only found it because the Hydra-Centauri supercluster, which is basically the next-door neighbor of our own supercluster, named Laniakea, did not have enough mass to account for the gravitational effect on some objects within Laniakea. So a team that included Somak Raychaudhury looked a little farther out into the universe and rediscovered Shapely, which is one of the largest known superclusters. Shapely is so big that it has a detectable gravitational influence on the Virgo Supercluster even though it is 500 million light-years away.
In fact, Shapely’s gravitational pull is so great that some scientists believe that it’s an influencing factor in a gravitational puzzle known as the Great Attractor. The discovery of Shapely has already helped to resolve some of the mysteries surrounding the Great Attractor, which is an extreme mass concentration that supposedly acts like an enormous drain with several galaxies circling it at several hundred kilometers per second. Not even the Milky Way Galaxy’s distance from Shapely makes it immune to the immense gravitational pull in the Centaurus constellation and is being pulled in the general direction of the Shapely Supercluster at a speed of 1.4 million miles per hour.
The Great Attractor’s apparent gravitational effect was difficult to explain on its own when it didn’t have enough mass to have such a strong pull on objects in the Laniakea Supercluster. Studies of the region revealed fewer clusters than expected in the part of the universe that the Great Attractor occupied. There was only enough mass to account for 44% of the attraction felt by the Milky Way. Before Shapely’s discovery, it led led to speculation that the gravitational effect could be evidence of a multiverse or a flaw in the data.
“Fingers of God” Contains Interesting Features
The five most interesting clusters within Shapely make up its core and provide interesting opportunities for astronomers to observe the interactions of clusters within a supercluster. These five clusters are connected by gravity and resemble filaments that scientists sometimes refer to as the Fingers of God. When Harlow Shapley first made observations of the supercluster that bears his name in 1930, he may have been looking at the core clusters. He described them as “a cloud of galaxies in Centaurus that appear to be one of the most populous yet discovered.”
These clusters have cores that contain some of the oldest galaxies in the supercluster, with aging red and orange stars. They contain several thousand galaxies that include some that are still merging with their current cluster. What’s especially interesting (though perhaps unrelated) is the fact that there is a wide gap containing very few galaxies between the core clusters and the surrounding clusters within the Shapely supercluster.
The Shapley 8 supercluster, also known as Abell 3558, shows signs of having collided with a smaller supercluster. This dramatic event released shockwaves that could have been the result of the chaos caused by merging galaxies and created “fragments” and uneven groups releasing telltale emissions around this cluster. Abell 3528 is less broken up and its galaxies form four groups that appear to have paired off in an intergalactic dance that will end with their collision.
Milky Way Is In The Way, and Sometimes Shapely Gets In The Way Too
Our own Milky Way galaxy is between Earth and the Shapely Supercluster. This produces interference in a region known as the Zone of Avoidance, a part of a sky that is obscured by the dense dust, gas, and stars in the center of the Milky Way. As a result, scientists rely on X-ray equipment to make observations of extragalactic objects that are obscured by the Zone of Avoidance.
In an attempt to identify the Great Attractor, scientists did exactly that while mapping galaxies in a project called Clusters in the Zone of Avoidance (CIZA). This is the project that provided a more clear picture of the extent of the the Shapely supercluster, as Hydra-Centauri wasn’t big enough and the Norma cluster – which isn’t a supercluster – contained only 10% of the mass necessary to account for the Great Attractor.
Shapely is big enough to interfere with attempts to map the cosmic microwave background radiation that is believed to be a remnant of the Big Bang. In current maps, the shape of Shapely’s dense core can be clearly made out. Scientists call this the Sunyaev-Zel’dovich effect.
There are some scientists who suspect that Shapely is bigger than we can currently make out and there may be a larger structure on the other side of Shapely. The launch of instruments with greater sensitivity and better resolution, such as the James Webb Space Telescope, may help to pin down many of the mysteries surrounding Shapely and the Great Attractor.