The Milky Way, our home galaxy, is a massive collection of stars, gas, and dust that is constantly moving. This motion occurs simultaneously on multiple scales, from the smallest internal orbits of individual stars to the enormous drift across the cosmos. Understanding the galaxy’s movement requires acknowledging that all motion is relative, meaning the measured speed depends on the reference point used for the observation. The Milky Way’s journey is a complex interplay of internal rotation, gravitational tugs from neighbors, and the overall flow of mass across the universe.
The Galactic Spin: Internal Movement
The most immediate form of the Milky Way’s motion is the rotation of its internal components around a central point. All stars, stellar remnants, gas clouds, and dark matter within the flattened disk of the galaxy are in orbit around the supermassive black hole, Sagittarius A, at the galactic center. This movement is called galactic rotation, which is not like a solid spinning plate but rather a differentiated flow.
The Sun, located in the outer spiral arm, travels in a nearly circular path with an orbital speed of approximately 220 to 240 kilometers per second. At this velocity, it takes the entire solar system about 220 million years to complete a single orbit, a period known as a galactic year. Stars closer to the center complete their orbits faster, while those farther out move slower, a phenomenon called differential rotation.
The galaxy also includes a spherical stellar halo and an extensive hot gas halo that orbits differently from the disk. Stars in the halo follow highly elongated and randomly oriented paths, contrasting with the nearly circular orbits of the disk stars. The hot gas surrounding the galaxy’s disk rotates in the same direction as the disk, though at a slightly slower speed of approximately 180 kilometers per second. This difference in orbital mechanics between the disk and halo components offers clues about the galaxy’s formation history.
The Milky Way’s Local Journey
Beyond its internal spin, the Milky Way is moving through its immediate cosmic neighborhood, a gravitationally bound collection of galaxies known as the Local Group. This local motion is dominated by the mutual gravitational attraction between the Milky Way and the Andromeda Galaxy (M31), the largest spiral galaxy in the group. The two galaxies are currently approaching each other at an estimated speed of about 110 kilometers per second.
The gravitational pull between the two galaxies is strong enough to overcome the general expansion of space, making a collision or merger an inevitable event. Current calculations estimate this event will begin in about 4 to 4.5 billion years, resulting in the formation of a new, larger elliptical galaxy, often nicknamed Milkomeda. The immense distances between individual stars mean that stellar collisions are extremely unlikely, but the gravitational upheaval will reshape the galaxies.
The merger will eventually scatter the stars into new orbits, potentially flinging our solar system much farther from the new galactic core. Newer observations using data from the Hubble and Gaia space telescopes suggest a slight chance, perhaps 50%, that the two galaxies might miss or only graze each other. The future of the Milky Way is inextricably linked to its Local Group neighbors.
Navigating the Cosmic Web
The Milky Way’s motion extends far beyond the Local Group, as it is pulled along by the largest structures in the universe, known as the Cosmic Web. Our galaxy is part of the Laniakea Supercluster, which translates to “immense heaven” and spans over 500 million light-years. The overall mass distribution of this vast region exerts a gravitational force on all its member galaxies.
This large-scale influence causes the Milky Way and the entire Local Group to stream towards a region known as the Great Attractor. This concentration of matter, located near the Norma and Centaurus clusters, acts as the gravitational focal point of the Laniakea Supercluster. The Milky Way’s velocity resulting from this large-scale gravitational pull is estimated to be around 600 kilometers per second.
This streaming motion is superimposed on the background of the universe’s general expansion, often called the Hubble Flow. While gravity binds local structures like the Milky Way and Andromeda, the universe’s overall expansion is causing structures farther away to recede from us. The Laniakea Supercluster will eventually disperse due to the accelerating expansion driven by dark energy.
Measuring Galactic Velocity
Determining the Milky Way’s speed requires understanding what reference frame is being used. For measuring the motion of local stars and nearby galaxies, scientists use the Doppler Effect, analyzing the light emitted by these objects. Light from objects moving toward us is shifted to shorter, bluer wavelengths (blueshift), while light from objects moving away is shifted to longer, redder wavelengths (redshift).
For measuring the galaxy’s absolute motion through space, astronomers use the Cosmic Microwave Background (CMB) radiation as the universal reference frame. The CMB is the faint afterglow of the Big Bang, providing a nearly perfect “rest frame” for the entire universe. As the Milky Way moves through this field of background radiation, the CMB appears slightly warmer and blueshifted in the direction of motion and slightly cooler and redshifted in the opposite direction.
This subtle variation, known as the CMB dipole anisotropy, is a direct signature of our motion. By measuring this effect, scientists have calculated the velocity of the Local Group relative to the CMB to be approximately 627 kilometers per second. This speed represents the accumulated effect of all gravitational influences acting on the galaxy, from the internal spin to the pull of the distant Great Attractor.