Stars are luminous spheres of plasma held together by gravity, generating energy through nuclear fusion. While stars appear fixed in the night sky from Earth, they are in constant, complex motion through space. This stellar movement is not random; stars engage in gravitational dances on both a massive galactic scale and a more intimate local scale. Stars do orbit, driven by the collective gravity of all matter around them.
The Galactic Orbit
A star’s primary motion is its long orbit around the center of its host galaxy, which for our Sun is the Milky Way. This large-scale path is governed by the combined gravitational pull of all the galaxy’s matter. While the supermassive black hole (Sagittarius A) dominates the orbits of the very closest stars, the majority orbit the galaxy’s overall center of mass, which is distributed across the disk and halo.
Stars in the thin, rotating disk follow nearly circular paths, moving in the same general direction. Stars located in the galaxy’s spherical halo travel on highly elongated and randomly oriented orbits that plunge through the disk plane. Analysis of galactic rotation curves reveals that stars far from the center maintain high, constant orbital speeds, instead of slowing down as predicted by visible matter alone. This observation is attributed to Dark Matter, an invisible substance that forms a massive, spherical halo around the galaxy, providing the extra gravitational influence needed to keep distant stars in orbit.
Mutual Orbits in Binary and Multiple Star Systems
Local orbits occur within gravitationally bound groups of stars, distinct from galaxy-wide motion. A binary star system consists of two stars revolving around a common center of mass known as the barycenter. The barycenter acts as the balancing point for the system, with each star’s distance from it inversely proportional to its mass. If one star is heavier, the barycenter shifts closer to the more massive star.
Binary and multiple star systems are quite common in the Milky Way. Estimates suggest that approximately one-third of all star systems are binaries or multiples, and about half of Sun-like stars have a stellar companion. The local orbit around the barycenter is superimposed upon the entire system’s much larger galactic orbit. Therefore, a star in a binary system simultaneously revolves around its companion and orbits the galactic center.
The Sun’s Motion Through the Milky Way
The Solar System, including the Sun and all its planets, moves together as a single unit on its galactic journey. The Sun is currently located about 27,000 light-years from the Galactic Center, situated within one of the Milky Way’s spiral structures, called the Orion Arm. The orbital speed of the Solar System is approximately 220 to 230 kilometers per second, or 490,000 miles per hour.
Despite this high velocity, the sheer size of the galaxy means the orbit takes a vast amount of time to complete. The period required for the Sun to travel once around the Galactic Center is known as a Galactic Year, estimated to be between 225 and 250 million Earth years. Over its 4.6-billion-year lifespan, the Sun has completed just over twenty such orbits. The Sun’s path is elliptical, and the Solar System periodically moves up and down through the plane of the galactic disk.