What To Know
- In systems like Earth and the Moon where one object is much larger than the other, the center of mass is often inside the larger body, making the orbit of the smaller object circular around the larger one.
- Our Milky Way is part of a group of galaxies known as the Loca Groupwhich includes the Andromeda Galaxy, a small spiral galaxy called the Triangulum, and several dwarf galaxies, such as the Grand and Small Magellanic CloudsThe Milky Way and Andromeda are the two largest galaxies in this group and their comparable masses place the center of mass somewhere between them.
- In short, the Milky Way does not orbit a specific object, but revolves around a common center of mass with the Andromeda Galaxy.
It seems like everything in space is orbiting something else. Moons orbit planets, planets orbit stars, and stars move around the centers of galaxies. But what about galaxies themselves, and specifically our own galaxy, the Milky Way?
Galaxies in the Local Group
To answer this question, it is essential to understand how orbits work. When two objects orbit each other, they exert a gravitational attraction on each other that keeps them connected. These objects orbit around their common center of mass, a point that could balance on a finger if the system could be made smaller. However, in systems like Earth and the Moon where one object is much larger than the other, the center of mass is often inside the larger body, making the orbit of the smaller object circular around the larger one. On a larger scale, orbits become more complex. Our Milky Way is part of a group of galaxies known as the Loca Groupwhich includes the Andromeda Galaxy, a small spiral galaxy called the Triangulum, and several dwarf galaxies, such as the Grand and Small Magellanic CloudsThe Milky Way and Andromeda are the two largest galaxies in this group and their comparable masses place the center of mass somewhere between them.
The future collision
Unlike planetary orbits, the Milky Way’s orbit is neither circular nor elliptical. According to Sangmo Tony Sohn, an astronomer at the Space Telescope Science Institute, said the two galaxies are on mainly radial orbitsmeaning that they are moving directly toward each other under their gravitational pull. This dynamic means that these two galaxies will eventually collide in about 4.5 billion years. They will initially pass through each other without their stars colliding, due to the immense distances between them. Eventually, this interaction will result in the two objects merging into one big galaxy.
The Andromeda Galaxy (M31). Credits: Adam Evans
Orbits on a larger scale
At even larger scales, the orbits become less defined. The Local Group, of which the Milky Way is a part, moves toward Virgo’s clustera gigantic collection of several hundred galaxies located about 65 million light-years away. However, due to the expansion of the Universe, the Local Group will never join this clusterbecause this expansion pushes galaxies apart faster than gravitational attraction brings them together. In short, the Milky Way does not orbit a specific object, but revolves around a common center of mass with the Andromeda Galaxy. It is also part of a complex dynamic of gravitational attractions with other members of the Local Group.
