For centuries, people assumed stars were fixed because they saw the same familiar patterns in the night sky. This perception is a deep-seated misconception, true only from the limited perspective of a human lifetime. All stars are in motion, but their speed and direction are only discernible when considering the proper frame of reference and the immense timescales of the cosmos. Accurately measuring this constant movement is essential to understanding the universe, from nearby stellar neighbors to the largest galactic structures.
Apparent Motion Versus True Motion
The movement most people observe every night is actually an illusion caused by our planet’s spinning and orbiting. This common observation is known as apparent motion, where the entire sky seems to rotate from east to west. This nightly sweep is due to the Earth’s rotation on its axis, which makes distant stars appear to rise and set.
The seasonal shift in visible constellations is also a form of apparent motion. As the Earth completes its yearly revolution around the Sun, our perspective on the background stars changes. This orbital motion causes different parts of the starry background to become visible or obscured over the months.
In contrast, true motion is the star’s actual, intrinsic movement through space. This velocity is independent of the observer’s location or the planet’s movements. True motion combines a star’s trajectory relative to our solar system and its larger-scale drift as part of the galaxy. Determining a star’s true motion requires separating it from the apparent motions caused by our constantly moving terrestrial platform.
Measuring Individual Star Movement
Astronomers break down a star’s true motion into two components to measure its total movement through space. The first component is proper motion, which is the star’s movement across the sky, or its tangential velocity, as viewed from Earth. This tiny angular shift is measured in arcseconds per year; for most stars, it is only a few thousandths of an arcsecond annually.
Because stars are so far away, decades must pass between observations for a reliable measurement of proper motion. Barnard’s Star, a nearby neighbor, holds the record for the fastest proper motion at 10.25 arcseconds per year. Even so, it takes about 180 years for it to move an angular distance equal to the Moon’s diameter in the sky. The vast distance makes constellations appear fixed over a human lifetime, as proper motion only becomes noticeable over thousands of years.
The second component is radial velocity, the speed at which a star is moving directly toward or away from Earth along the line of sight. Astronomers measure this motion using the Doppler effect, which analyzes changes in the star’s light spectrum. If a star is approaching, its light waves are compressed, shifting the spectral lines toward the blue end (blueshift).
Conversely, if a star is receding, its light waves are stretched, causing the spectral lines to shift toward the red end (redshift). Radial velocity is measured in kilometers per second and can be determined with a single spectral observation, unlike proper motion, which requires years of positional data. Combining a star’s radial velocity and proper motion allows astronomers to calculate its total three-dimensional space velocity relative to the Sun.
The Movement of the Milky Way
All localized stellar movements exist within the larger, organized movement of the Milky Way galaxy. Our Sun and the entire solar system orbit the center of the galaxy, a motion known as galactic rotation. The Sun is located approximately 28,000 light-years from the galactic center and travels at an average speed of about 230 kilometers per second.
Even at this speed, it takes the solar system an immense amount of time to complete one full orbit around the Milky Way. This orbital period, sometimes called a cosmic year, is estimated to be between 225 and 250 million Earth years. The stars within the galaxy move together, with their individual proper motions being small deviations from this massive, shared orbital path.
Expanding the frame of reference reveals that the Milky Way itself is moving through the cosmos. Our galaxy is part of the Local Group cluster, and this entire group is gravitationally influenced by structures far beyond. The Local Group, including the Milky Way, is being pulled toward a concentration of mass known as the Great Attractor, which lies some 150 to 250 million light-years away in the direction of the Centaurus constellation.
The Milky Way’s velocity, relative to the local co-moving frame of reference in the universe, is estimated to be around 600 to 630 kilometers per second. This cosmic drift places stellar movement within the largest possible context, showing that stars are not only moving locally within their galaxy but are also participating in the collective flow of matter across the universe.