It may be hard to imagine what you have in common with a star or even a galaxy, but all things in the Universe have one thing in common, we are all constantly moving. Even when you are sitting down you are moving because you are on the Earth which is spinning about its axis.
We do not feel this movement because gravity keeps us securely on the surface of the Earth. This movement does not stop there. Our Sun is also moving as it orbits the center of our Milky Way galaxy, pulling us and the rest of our solar system with it. Even our galaxy is being pulled through space along with all our neighboring galaxies.
What is large enough to have a gravitational pull on multiple galaxies? This massive gravitational pull originates from a point in space that astronomers refer to as The Great Attractor.
What is The Great Attractor?
The Great Attractor is a gravitational anomaly in space. Astronomer do not know for sure what has the gravitational power to not only attract our Milky Way but also 100,000 of our neighboring galaxies along with it. Our galaxy is moving towards The Great Attractor at 1,342,162 miles per hour.
If our Earth was orbiting the Sun that fast, our year would be 18 days long. The Great Attractor is about 220 million light years away from our galaxy. To have the gravitational pull to attract so many galaxies The Great Attractor is believed to have a mass of a quadrillion Suns and have a diameter of 300 million light years. Our Milky Way galaxy is approximately 100,000 light years across.
Despite its assumed size, we cannot see The Great Attractor, because it is hidden from us by our own galaxy. If you look at the night sky away from city lights you can often see a white milky streak across the sky, this is an arm of our Milky Way galaxy.
While it is beautiful to look at, the light from all the stars, gas, and dust in this arm blocks our view of anything in space behind it. This is where The Great Attractor lies. If we cannot see it, how do we know about the Great Attractor?
To understand how we found the Great Attractor, we need to look back, all the way back to the origin of the Universe, the Big Bang. The Universe originated in what is called the Big Bang, a massive explosion where everything in our Universe was created. Since the Big Bang the Universe has been expanding.
This means our Universe is growing at 2.2 million km per hour. This would lead us to assume that all galaxies in space are constantly moving away from each other at the same rate, however, understanding that this is not always the case led to discovering The Great Attractor.
The mapping of the Cosmic Microwave Background in the 1970’s gave us great insights into our Universe. The Cosmic Microwave Background is the afterglow of the heat and energy created by the Big Bang. With the Cosmic Microwave Background, we discovered that one side of our Milky Way galaxy is colder than the other.
This difference in temperature is less than one one-hundredth of a degree Fahrenheit, but it is enough of a difference to inform us that our galaxy is moving at 370 miles per second (600 km/s) towards the Centaurus constellation. This 370 miles/s is our peculiar velocity, the movement of our galaxy other than what is accounted for in the growing of space through the expansion of the Universe.
Later we discovered that everything within hundreds of millions of light years of us was also moving in the same direction. The only force that is strong enough to oppose the expansion of the Universe over such large scales is gravity. Initially scientist looked towards the Virgo Supercluster in which our Milky Way resides on the outskirts of, as a possible source of this gravity.
The Virgo Supercluster is made up of 1,300 galaxies, yet it is still not massive enough to cause such a gravitational anomaly to result in our peculiar velocity. While we are being pulled further into the Virgo Supercluster, it is due to the natural process of galaxy clusters being pulled together. This realization led to the understanding that there is an even larger unseen source of gravity, The Great Attractor.
The Great Attractor is something in the Universe with more gravitational force than can be exerted by the Virgo Supercluster, and we can’t see it. As discussed previously, our own Milky Way hides parts of the Universe from us, approximately 20%. The part of the Milky Way that hides this 20% is called the Zone of Avoidance.
This Zone is so full of interstellar dust that is blocks visibility of the Universe behind it. We can attempt to look through the Zone of Avoidance by using X-rays, however this does not provide clear pictures of what lays beyond it. The Great Attractor just so happens to reside in this 20%.
Beyond the Zone of Avoidance
We have discovered the supercluster known as the Norma Cluster behind the Zone of Avoidance. The Norma Cluster is the closest massive galaxy cluster to the Milky Way. This cluster is in the area of the Great Attractor approximately 150 million lights years away.
There is an even more massive supercluster 650 light years past the Norma Cluster, that has the mass of 10,000 Milky Way galaxies. This super cluster is known as the Shapely Supercluster and is one of the largest objects in the observable Universe. The Shapley Supercluster was discovered in the 1930’s by American astronomer Harlow Shapley.
This supercluster resides in the Centauras constellation and is home to more than 8,000 galaxies, making it a great candidate for The Great Attractor.
What Could it Be?
A supercluster is not just a collection of galaxies but is a region of space where all the galaxies in that area are heading towards a common center. The Great Attractor may not be a massive physical body in space but a location where everything around it collects, like a funnel.
With this definition we find that the Virgo Supercluster, which we are on the outskirts of, is an arm of an even larger structure, the Laniakea Supercluster. The center of this supercluster is also a great candidate for The Great Attractor. While a supercluster is currently the most agreed upon answer The Great Attractor, there are other theories as to what it may be.
Some suggest that The Great Attractor is an area of over-density that has an enormous gravitational pull. Others think it may be a super dense area of dark energy. Some scientists believe that The Great Attractor is the beginning of the end of the Universe, referred to as the Big Crunch.
The Big Crunch is the process that will occur when the Universe stops expanding and begins to contract. Eventually, a supermassive black hole would be created at the center of this contraction and consume everything, including itself.
Dangers of The Great Attractor
While it seems like a mysterious unseen gravitational monster pulling hundreds of thousands of galaxies towards it would be considered a threat, it is not. The most likely scenario is that we will not be pulled into the depths of The Great Attractor. Likely, the expansion of the Universe will win against its gravitational force.
Most things in the Universe larger than a small galaxy cluster tend to fall apart, this will most likely happen to the Laniakea Supercluster, the most likely culprit for The Great Attractor. When the supercluster falls apart its gravitational pull will weaken, and so will its hold on us.
The peculiar velocity at which we are moving toward The Great Attractor at is only about 20% of what it would need to be to guarantee we fall into The Great Attractor. Our expansion with the Universe is fighting against the Attractor’s pull at a rate that will prevent us from being consumed by it.
While we know more about what The Great Attractor may be than ever before, there are many unanswered questions. As our technology improves, we will hopefully have a chance at looking through the Zone of Avoidance clearly enough to determine what The Great Attractor is.
Until then it is still mainly a mystery.