The Great Attractor

The Milky Way over Devil's Tower - courtesy {link}

This post is going to be a bit of a departure from theme parks and movies. I got a question on twitter about an astronomical phenomenon and the best way to discuss it was through a blog post. Stick around though, I'm going to try and blow your mind.


The question asked was "What is The Great Attractor?" In order to explain what it is we need to look around our local neighborhood in the universe.


As I'm sure you know, we live in the Milky Way galaxy. It's an average, run of the mill, spiral galaxy about 120,000 light-years in diameter. Remember, a light-year is how far light can travel in one year, or about 6 trillion miles. To put this in perspective, light travels from the Sun to Earth in 8 minutes and makes it to Pluto at its farthest point in just over 6 1/2 hours. Our fastest spacecraft will take 10 years to make the trip to Pluto. The Milky Way contains roughly 200 billion stars and close to that number of planets. It's a very big place. The Solar System lies about 2/3 of the way out from the center in the Orion Spiral Arm. Essentially we are in the suburbs of a very busy city and when you look towards the Milky Way in the night sky, you are looking in towards the bustling downtown.


The Milky Way galaxy itself is part of larger group of 54 galaxies known as The Local Group. We are the 2nd largest member of the group, The Andromeda Galaxy being the largest member at 2.6 million light-years away from us. The Local Group is actually a suburb itself of a much vaster metropolis known as the Virgo Supercluster. This supercluster is made up of about 100 smaller clusters, like the Local Group, and assorted individual free galaxies. It is 110 million light-years in diameter.


Now we come to The Great Attractor. It is a region of space about 250 million light-years away from us that is thought to contain roughly 10,000 times the mass of the entire Milky Way galaxy. That much mass exerts a fairly significant gravitational pull over a large distance. Large enough that it is affecting thousands of galaxies in the region, including our own Virgo Supercluster. Astronomers can measure the distance to galaxies by using something called redshift. It is similar to what happens the the sound of a car as it passes you. When it is approaching you, the sound waves are piling up and being pushed together, causing them to become higher frequency (pitch seems to go up). As it passes and moves away the sound waves stretch out and the pitch falls. Light does the same thing, except when the frequency goes up, the color changes to blue and when it falls, the color changes to red. By measuring the amount of shift in red or blue we can determine the speed and direction galaxies are moving. By further measuring the variations in the speed and direction of thousands of galaxies, an anomaly was discovered and named The Great Attractor. By knowing the amount it was influencing galaxies by we could calculate its mass.


Recently though, studies have discovered that The Great Attractor is not the object that is pulling galaxies towards it as its mass is far less than we originally thought. As our telescopes have gotten better we refine our estimates and understanding. We know now that the object really pulling us is the Shapley Supercluster which sits beyond The Great Attractor a whopping 650 million light-years away. It is the largest concentration of galaxies in the local universe, though not the largest structure by any means.


The Universe is unimaginably vast and full of incredible wonders which we are just barely beginning to gain glimpses of. There are hundreds of billions of stars in our galaxy and there are trillions of galaxies in the universe. It can be difficult to visualize the extreme scales and sizes involved, but this interactive page does a great job: The Scale of the Universe


Hopefully that answered the question. As with all things science related it probably brought up quite a few more questions in your mind. Feel free to ask any time on twitter (@mycroft16) or via the comments.