Everything seems to revolve around something in space. Moons orbit planets. Planets orbit stars. Stars orbit the centers of galaxies. But other than that, things are a little harder to visualize. Make galaxies – and specifically Milky Way – circling something?
To answer that, we first need to know how orbits work. Consider two objects orbiting each other. These two bodies exert a gravitational force on each other that holds them together. Objects orbit around their common center of gravity – if you could shrink the system, the center of gravity would be the point where you could balance it on your finger. But in case Solar System, or the Earth and the Moon, one of the objects is much larger than the other. The center of mass eventually lies inside the larger body, so the larger object doesn’t move much and the smaller object moves in a roughly circular path around the larger one.
On a larger scale, things get a little more complicated. Our galaxy is part of a collection of galaxies called Local group, which includes the Milky Way; the Andromeda Galaxy; a smaller spiral galaxy called Triangulum; and several dwarf galaxies, including the Large and Small Magellanic Clouds. The Milky Way and Andromeda are the two largest objects in the Local Group. Since their masses are comparable, the center of mass lies between the two galaxies Sangmo Tony Sohn, an astronomer at the Space Telescope Science Institute in Maryland. There is nothing significantly larger than the two galaxies nearby, so they both end up orbiting each other.
Related: Scientists have found that the supercluster of galaxies is as massive as 26 quadrillion suns
But the Milky Way’s orbit is not circular or elliptical like the orbits of the planets around the Sun. “It’s going to be weird to say if the Milky Way is orbiting something, because that means there’s a bigger object,” Sohn told Live Science. “But that’s not the point.
Instead, both the Milky Way and Andromeda are in predominantly radial orbits. “Imagine the gravity of two things that attract each other, and they’re not moving in any way other than gravitational force. They’re going to move in a straight line toward each other. That’s a purely radial path,” he said. Chris Mihos, an astronomer at Case Western Reserve University in Ohio. The Milky Way’s orbit is not perfectly radial because there is little lateral movement between the two galaxies, Mihos told Live Science.
Their mostly radial orbits towards each other mean that The Milky Way and Andromeda will eventually collidein about 4.5 billion years. Individual stars are unlikely to collide with each other due to the vast distances between them, so the galaxies will pass each other and separate again – but not for long.
“Galaxy [will] then turn around and get back together. And over the course of hundreds of millions or billions of years, they will actually merge into one big galaxy,” Mihos said.
RELATED MYSTERIES
—The Hubble Space Telescope’s view of a galactic collision turns into a beautiful song
– Galactic archeology reveals that the Milky Way’s neighbor Andromeda has a violent past
—Astronomers weigh the dark matter halos of ancient galaxies for the first time
Gravitational interactions are likely to push the stars in both galaxies together enough to make the combined galaxy an elliptical rather than a spiral like the Milky Way and Andromeda. The merger could also heat the gas along the spiral arms of each galaxy enough to form new stars, Sohn said.
Orbits on scales larger than galaxy groups are even less defined, but “we know for sure that the Local Group is moving,” Mihos said. The local group is drawn towards Cluster Virgowhich contains several hundred galaxies and lies about 65 million light years away. But the Local Group will never get there, Mihos said, because the expansion of the universe it is pulling the Milky Way away faster than the gravitational pull of the Virgo Cluster is pulling it.