Astronomy
The Dynamics of the Local Group and Its Galaxies
Quick fact
The Andromeda Galaxy is approaching the Milky Way at about 110 kilometers per second, and despite the universe's expansion, the two are gravitationally bound and will eventually merge—likely in about 4 to 5 billion years.
Why this is interesting
You've likely heard that the universe is expanding, but some galaxies are moving toward each other—and our Milky Way and Andromeda are on a collision course. Why don't they just drift apart?
Read the full explanation
Understanding The Dynamics of the Local Group and Its Galaxies
Think of the Local Group as a cosmic island where gravity is the standout force, overriding the universe's expansion on this small scale. Within this group, galaxies orbit a common center of mass, with their velocities reflecting the gravitational tug of all the other galaxies. This is why Andromeda, for example, is not receding but actually approaching us. The Local Group's dynamics are a dance between two opposing effects: the pull of gravity among its member galaxies and the push of cosmic expansion that drives distant galaxies apart. On the scale of the Local Group—about 10 million light-years across—gravity wins. This balance explains why the group remains gravitationally bound while the universe at large expands.
A deeper explanation
The motion of Local Group galaxies is described by 'peculiar velocities'—deviations from the Hubble flow caused by the gravitational influence of neighbors. The Milky Way and Andromeda, the two most massive members, dominate the dynamics. Their mutual attraction is strong enough to overcome the expansion, causing them to approach each other. By measuring their relative velocity and separation, astronomers can estimate the total mass of the Local Group. The result is about 2 to 3 trillion solar masses, far more than the sum of visible stars and gas. This extra mass is attributed to dark matter. The group's internal dynamics, including the orbits of smaller galaxies, are shaped by tidal interactions with the giants. Simulations suggest the Milky Way and Andromeda will merge in about 4 to 5 billion years, a consequence of their gravitational binding. This illustrates how galaxy groups evolve over cosmic time, driven by gravity even as the universe expands.