The Milky Way is in constant motion, and not just in one direction. Our galaxy spins, falls toward neighboring galaxies, and streams through the cosmos at roughly 600 kilometers per second relative to the faint background radiation left over from the Big Bang. There is no single “speed of the Milky Way” because its motion layers on top of itself at every scale, from the orbital paths of individual stars to the gravitational tug of galaxy clusters hundreds of millions of light-years away.
How the Galaxy Spins
The most intuitive type of galactic motion is rotation. Stars, gas clouds, and dust all orbit the center of the Milky Way, much the way planets orbit the Sun. Our solar system sits about 26,000 light-years from the galactic center and completes one orbit roughly every 225 to 250 million years. That orbital speed has been measured at about 225 km/s using data from the Planck spacecraft and the Gaia star-mapping mission.1arXiv. On the Galactic CMB apex Stars closer to the center orbit faster, while those farther out move at a pace that depends on the distribution of mass, both visible and dark, throughout the galaxy.
Mapping the full rotation curve from the galactic center out to its farthest edges has been a decades-long project. Researchers have constructed a “grand rotation curve” that extends from the very core of the Milky Way out to about a megaparsec (roughly 3.3 million light-years), decomposing the observed speeds into contributions from the central bulge, the stellar disk, and the massive dark-matter halo that envelops everything.2Publications of the Astronomical Society of Japan. Grand Rotation Curve and Dark-Matter Halo in the Milky Way Galaxy That dark halo is the reason the outer rotation speed stays high instead of dropping off: there is far more mass out there than the visible stars alone can account for.
How Astronomers Detect Motion They Cannot Feel
You might wonder how scientists can tell any of this is happening when we are riding along inside the galaxy. The trick is to look at objects outside our system and measure how they appear to shift.
One elegant method uses very distant quasars as fixed reference points. Because the solar system accelerates as it curves around the galactic center, the apparent positions of faraway objects drift by a tiny amount each year. This “secular aberration drift” was detected at high confidence using a catalog of extragalactic radio sources observed with the Very Long Baseline Array. The measured drift matches what you would expect from the Sun’s known orbit around the galactic center.3The Astrophysical Journal Supplement Series. The VLBA Extragalactic Proper Motion Catalog and a Measurement of the Secular Aberration Drift
Closer to home, the Gaia space telescope has revolutionized our view of the Milky Way’s internal motions. A recent kinematic analysis used Gaia’s third data release to map velocity components for 18 million bright stars across the galactic disk, revealing fine structure in how different stellar populations rotate, stream, and wobble relative to one another.4Monthly Notices of the Royal Astronomical Society. A new kinematic model of the Galaxy: analysis of the stellar velocity field from Gaia Data Release 3 The galaxy is not a rigid carousel; its parts move at slightly different speeds and in subtly different directions, carrying imprints of past collisions with smaller galaxies.
Motion Through the Local Group
Zoom out from the Milky Way’s internal spin and you see a second layer of motion. Our galaxy belongs to a small cluster called the Local Group, which includes the Andromeda galaxy (M31), the Triangulum galaxy, and several dozen dwarf galaxies. The Milky Way and Andromeda are the two heavyweights, and they are heading toward each other.
Andromeda’s approach has been known for over a century, ever since astronomers measured the blueshift in its light. More recently, space telescopes have measured the tiny sideways (transverse) component of Andromeda’s motion by tracking the positions of its stars over years, giving a three-dimensional velocity vector.5Nature Astronomy. No certainty of a Milky Way–Andromeda collision Current position and velocity data place Andromeda about 770 kiloparsecs (roughly 2.5 million light-years) away, approaching at a modest speed but with some sideways drift that makes the exact future trajectory uncertain.6Monthly Notices of the Royal Astronomical Society. On stellar migration from Andromeda to the Milky Way
Both galaxies sit within a flattened structure called the Local Sheet, which influences the shapes and orientations of their dark-matter halos. Simulations that reproduce the observed layout of the Local Group find that the sheet tends to align the elongated shapes of the halos but does not strongly control the direction of the galaxies’ spins.7arXiv. Alignment of the Milky Way and M31 with their cosmic environment In other words, the Milky Way’s local environment shapes not just where it is going but how its surrounding dark matter is distributed.
The Milky Way’s Speed Relative to the Universe
The most dramatic number comes from measuring the galaxy’s motion against the cosmic microwave background (CMB), the faint afterglow of the Big Bang that fills all of space. If you were perfectly at rest relative to this radiation, it would look the same in every direction. Instead, it is slightly warmer in one direction and slightly cooler in the opposite direction, a pattern called a dipole. That asymmetry tells us the Local Group is moving at about 600 km/s toward a point in the constellation Hydra.
This 600 km/s is a “peculiar velocity,” meaning it is on top of the expansion of the universe. And it is not unique to us. Vast regions of the cosmos, often spanning hundreds of megaparsecs, move coherently at hundreds of kilometers per second in what are called bulk flows.8ScienceDirect. Large-scale peculiar velocities in the universe The Milky Way is one participant in a much larger river of galaxies all drifting in roughly the same direction, pulled by the uneven distribution of matter across the universe.
What Is Pulling Us, and What Is Pushing
If the Local Group is barreling through space at 600 km/s, something must be responsible. For decades, the leading suspect was the Great Attractor, a massive concentration of galaxies roughly 150 to 250 million light-years away in the direction of the constellations Centaurus and Norma. Recent distance measurements using surface brightness fluctuations confirm that galaxies in that region are indeed falling inward: those about 40 to 50 megaparsecs away show peculiar velocities of 500 to 1,500 km/s, with the flow converging to near zero at about 70 megaparsecs from us.9arXiv. Return to the Great Attractor: Strong Evidence for a Steradian-sized Flow Converging at ∼70 Mpc within the GA Supercluster and Aligned with the CMB Dipole That is strong evidence for a real gravitational pull.
But the Great Attractor alone does not fully explain our motion. An analysis of mass distribution within about 155 megaparsecs found that it accounts for only about 72% of the Local Group’s velocity, with a directional mismatch of roughly 38 degrees. No single structure, including the Great Attractor, dominates the velocity budget.10The Open Journal of Astrophysics. Revisiting the Great Attractor: The Local Group’s streamline trajectory, cosmic velocity and dynamical fate The remaining portion comes from structures even farther away, such as the Shapley Supercluster (about 650 million light-years out), and possibly from a complementary effect on the opposite side of the sky: a vast, nearly empty region dubbed the Dipole Repeller. Where the Great Attractor pulls, the Dipole Repeller effectively pushes by having less-than-average mass to slow us down. Together, attraction from one side and a relative void on the other add up to produce the full 600 km/s motion we observe.
Why You Do Not Feel Any of This
Six hundred kilometers per second sounds terrifying. That is more than a thousand times the speed of a bullet. But motion at a constant velocity in the vacuum of space produces no sensation at all. You only feel acceleration, not speed. The accelerations involved in galactic motion are extraordinarily gentle. The Sun’s orbital acceleration around the galactic center is roughly one ten-billionth of the gravitational pull you feel standing on Earth’s surface. Everything in your neighborhood, every planet, every nearby star, every grain of interstellar dust, is carried along at essentially the same speed and in the same direction, so there is no relative motion to notice.
Even the eventual approach of Andromeda will be imperceptibly slow on human timescales. The closing speed is modest enough that, spread over billions of years, nothing about it registers locally. The night sky will look essentially the same for millions of years to come. Only on timescales of hundreds of millions of years do the consequences of galactic motion become visible, as the constellations slowly distort and the galactic neighborhood reshuffles.
When the Milky Way Meets Andromeda
The most dramatic consequence of our galaxy’s motion is the eventual encounter with Andromeda. Using fiducial orbital parameters, one simulation finds that the two galaxies will make their closest approach in about 4.3 billion years, with the full merger playing out over roughly 10 billion years.11Astronomy & Astrophysics. Future merger of the Milky Way with the Andromeda galaxy and the fate of their supermassive black holes That timeline is longer than some older estimates, largely because updated measurements of Andromeda’s sideways motion suggest the two galaxies may not be on a perfectly head-on collision course.
Not everyone agrees on the timeline, though. A separate analysis argues the free-fall time is closer to 1.7 billion years rather than 4.5 billion, which would mean the encounter happens much sooner.12International Journal of Astrophysics and Space Science. On the Andromeda-Milky Way Future Encounter: Thrice Faster Over Time The disagreement hinges on assumptions about the total mass of the two galaxies and how much dark matter each carries. And recent proper-motion refinements have even raised the possibility that a direct collision is not guaranteed at all, with a glancing flyby remaining in play depending on Andromeda’s exact transverse velocity.5Nature Astronomy. No certainty of a Milky Way–Andromeda collision
If the merger does happen, “collision” is a bit misleading. The spaces between stars are so vast that almost no individual stars will physically crash into each other. Instead, the two galaxies’ gravitational fields will warp and stretch each other, flinging stars into new orbits and eventually settling into a single, larger elliptical galaxy sometimes nicknamed “Milkdromeda.” The supermassive black holes at each galaxy’s center would spiral inward over billions of additional years before merging themselves.
How Early Observations Revealed Galactic Motion
The idea that galaxies move was not obvious. For most of human history, the “spiral nebulae” visible through telescopes were thought to be gas clouds inside our own Milky Way. The breakthrough came in the early twentieth century when Vesto Slipher measured the Doppler shifts of dozens of spiral nebulae and found velocities far larger than anything observed among nearby stars. Those unprecedentedly large Doppler speeds pointed the way toward a new physical interpretation: these objects were entire galaxies, far outside our own, moving through space at enormous speeds.13arXiv. Vesto Slipher and the First Galaxy Redshifts Slipher’s redshifts became a crucial ingredient in the velocity-distance plots that Edwin Hubble later used to establish that the universe is expanding.
Slipher’s work is a useful reminder that the motions discussed in this article are layered. Some of what he measured was the expansion of the universe carrying galaxies apart. Some of it was the peculiar velocities of those galaxies moving through their local gravitational landscape. Separating the two took decades and is, in many ways, still being refined today.
The Expansion of Space and Peculiar Velocity
A common misconception is that all galaxy motion is simply due to the expansion of the universe. Expansion does carry distant galaxies away from us, and the farther away a galaxy is, the faster it appears to recede. But the Milky Way’s 600 km/s peculiar velocity is something on top of that expansion. It represents real movement through space, driven by gravitational attraction to nearby and not-so-nearby concentrations of mass.
For galaxies close to us (within a few hundred million light-years), peculiar velocities can be comparable to or even larger than the expansion speed at that distance. That is why Andromeda is approaching rather than receding: its peculiar velocity toward us overwhelms the expansion of the space between us. Farther out, expansion dominates and peculiar velocities become a relatively minor perturbation. This is why cosmologists care so much about mapping bulk flows: the pattern of peculiar velocities across the sky is essentially a map of how mass is distributed in the universe, including the dark matter that cannot be seen directly.
Stacking Up the Layers of Motion
It can help to think of the Milky Way’s motion as a set of nested movements, each operating at a different scale:
- Galactic rotation: The Sun orbits the galactic center at about 225 km/s, completing one circuit every quarter-billion years or so.
- Local Group drift: The Milky Way and Andromeda are slowly converging, closing the gap between them at a combined rate that will lead to a close encounter in a few billion years.
- Bulk flow: The entire Local Group is streaming at roughly 600 km/s relative to the CMB, pulled by large-scale structures like the Great Attractor and the Shapley Supercluster and nudged by the relative emptiness of the Dipole Repeller.
- Cosmic expansion: On the largest scales, the space between galaxy clusters is stretching, carrying everything apart in a way that compounds with distance.
These velocities do not simply add up like arrows on a map, because they operate in different reference frames and at different scales. But conceptually, every point in the Milky Way participates in all four motions simultaneously. When an astronomer quotes a single speed for “how fast the Milky Way is moving,” they are usually referring to one specific layer, most often the peculiar velocity relative to the CMB.
Dark Matter’s Role in Setting the Pace
Roughly 85% of the mass in the universe is dark matter, and it dominates the gravitational forces that shape galactic motion at every scale. The Milky Way’s own rotation curve, the speed of stars at various distances from the center, stays flat far beyond where the visible disk ends. That flatness is the classic signature of a dark-matter halo extending well past the stars.2Publications of the Astronomical Society of Japan. Grand Rotation Curve and Dark-Matter Halo in the Milky Way Galaxy Without the halo, the outer parts of the galaxy would be orbiting far too slowly to stay bound, and the Milky Way would essentially fly apart.
At larger scales, dark matter is the scaffolding that determines where galaxy clusters form and how strongly they pull on their neighbors. The Great Attractor’s gravitational influence is not just from the visible galaxies sitting there; it is from the enormous dark-matter overdensity underlying them. Similarly, the void behind the Dipole Repeller is a region where dark matter is sparse, creating a gravitational deficit. The Milky Way’s trajectory through the cosmos is, in a very real sense, dictated by the invisible majority of the universe’s mass.
What Happens to the Night Sky
On the timescale of a single human life, galactic motion changes nothing you can see. But over millions of years, the consequences accumulate. The constellations are already deforming, though so slowly that the patterns have been recognizable for all of recorded history. The Sun’s orbit around the galactic center will carry us through different spiral arms over hundreds of millions of years, potentially changing the density of interstellar dust and cosmic rays in our vicinity. Some researchers have speculated that passages through dense spiral arms might correlate with episodes of increased cosmic-ray flux reaching Earth, though this connection remains debated.
On a much longer timeline, the Andromeda encounter will rearrange the night sky dramatically. As the two galaxies interact gravitationally, tidal tails of stars will stretch across the sky, and the eventual merger will replace the familiar band of the Milky Way with the diffuse glow of an elliptical galaxy. Our Sun, if it is still shining at that point (it should be, barely, in 4 to 5 billion years), would simply be reassigned to a new orbit in the merged system. No violent event would mark the transition for any hypothetical observers. Stars are so far apart that the merger would look like two swarms of fireflies passing through each other, gradually blending into one cloud.