We all know that we here on planet Earth orbit the sun once per year. We also see that the moon orbits our planet every 29 days.
As we scale up, you might know that the whole solar system orbits the center of our galaxy, the Milky Way (technically, it orbits the Milky Way’s center of gravity, but we’ll come back to that later). It takes 230 million years to complete one lap!
What about on larger scales than that?
Does our home galaxy orbit something? And, if it does, what is it?
We’ll answer these questions and more in this article.
Are Galaxies Orbiting Something?
At the galaxy group level (e.g. our Local Group, which includes the Milky Way), individual galaxies are orbiting their common center of gravity, or barycenter. At larger scales, i.e. superclusters and galaxy filaments, gravity is too weak to form a common barycenter, so galaxy groups are not in orbit.
How Gravitational Centers Work
I mentioned above that our solar system orbits the Milky Way’s gravitational center, not the center of the galaxy.
At the Milky Way’s heart is a supermassive black hole called Sagittarius A* (say “a-star”). Astronomers believe that most large galaxies have a similar black hole in the middle of their bright central bulge.
Yet, despite their enormous size – Sagittarius A* has the mass of over four million suns – they are tiny compared to the whole galaxy. Sagittarius A* makes up just three 100,000ths of one percent of the mass of the Milky Way.
This is why we can be sure that stars, including our sun, are not orbiting the black hole at the galactic center. Instead, they are orbiting a point known as the galaxy’s center of mass or barycenter.
The center of a galaxy’s mass is the point that has an equal amount of mass all around it. If you could shrink the galaxy to the size of a frisbee, it would be the point that you could balance on a pencil and it wouldn’t tip over.
A Quick Word on Orbits
When we talk about orbiting, we mean to move along a fixed path around a point in space.
In the case of the sun and Earth, their masses are so different that their barycenter is inside the sun, which is why it looks like we orbit directly around it.
When scientists look at whether galaxies orbit anything, they are looking for a barycenter that the galaxies share with another object (or group of objects) that would cause an orbit to happen.
Whether or not galaxies are orbiting anything depends on the scale you examine. Let’s begin with galactic clusters, specifically the Local Group of galaxies.
The Local Group of Galaxies
The Milky Way is part of the Local Group of Galaxies (LG).
The LG has a diameter of about 10 million light-years and contains around eighty different galaxies. Its two largest members are Andromeda Galaxy and the Milky Way. It also houses the Large and Small Magellanic Clouds. Most of the remaining members are dwarf galaxies.
In the image below you can see that the LG is split into two distinct groups, one with our Milky Way at its heart and the other with Andromeda Galaxy (M31).
Within the LG, Andromeda and the Milky Way are orbiting their common center of gravity, as are all the other galaxies in the cluster. Since they are both about the same mass, their barycenter is midway between the two
They are gravitationally bound and can’t pull away from each other. Eventually, in a few billion years, their gravitational pulls toward each other will cause them to collide and merge.
We know that, in this sense, our galaxy is orbiting something: the center of mass of the Local Group of Galaxies.
In the final section, we’ll step the scale up another level to the supercluster. Our Local Group is part of the Virgo Supercluster
At this magnitude of scale, we believe that galaxy clusters are not orbiting anything, and that’s due to how gravity works.
Two factors affect the influence of gravity: mass and distance. More massive = more gravity, more distance = less gravity.
The Virgo Supercluster, of which our Local Group is part, contains over 100 clusters and groups of galaxies. The masses involved are truly astounding. It is also huge, spanning 110 million light-years of space.
And it is the enormous distances involved that cause the gravitational attractions to become too weak to bind the groups together within the supercluster.
If we could take a huge step back and look at the universe on the scale of superclusters and galaxy filament, we would see that it essentially looks the same in all directions. Its fine structure is like that of a sponge, with lines of galaxy filaments and massive voids or holes between them.
Because they are the same everywhere (on the macro level) they are not gravitationally attracted to each other or orbiting each other. Instead, they just ‘float’ in space which is constantly expanding between them. Although the galaxies within them are moving.
Before leaving this scale of the universe, we should acknowledge the impact of space expansion.
Universal Expansion’s Impact
You may have heard that the expansion of space is pushing galaxies further away from each other and that the farther they are, the faster they recede.
This is true – space is expanding – so you may wonder how galaxies can possibly stay bound to each other.
The answer is that expansion does not have an impact where the force of gravity is stronger. At the galactic group level, galaxies are gravitationally bound together and the space between them is not expanding.
The expansion comes into play at intergalactic distances where general relativity says that gravity is basically gone. At the supercluster scale, as we just saw, mass is not localized enough to create a center of gravity because it is too spread out and homogenous.
That’s why some 97% of the galaxies in the universe are moving away from us and will never be within our reach, even if we could travel at light speed.
One object orbiting another is an illusion. Instead, all objects orbit their common center of mass, called the barycenter. It looks like we orbit the sun because our common center of gravity is beneath the sun’s surface.
When it comes to galaxies, we can say that each one in a cluster, such as our local group, is gravitationally bound and orbiting a common barycenter. That means the Milky Way is orbiting a point in space roughly midway between here and the Andromeda galaxy.
At the scale of superclusters, however, the mass is too spread out and homogenous for centers of gravity to form. With no common center of gravity, clusters of galaxies have nothing to orbit and so don’t.
Instead, they are steadily pushed apart by the expansion of space.
Written by Adam Kirk