An orbit describes the regular, repeating trajectory an object in space follows around another, dictated by a continuous gravitational balance. This celestial motion is a constant tug-of-war between an object’s forward momentum, or inertia, and the pull of gravity from a massive body. The answer to what a star orbits changes dramatically based on whether we look at a star’s immediate neighborhood or its place within the vast structure of a galaxy. Stellar movement ranges from the intimate pairing of two stars to the collective motion of billions across the cosmos.
Orbiting a Shared Center of Mass
The common image of a star sitting still while a planet orbits it is technically inaccurate. All bodies in a system actually orbit a shared point in space called the barycenter, which is the center of mass for two or more orbiting bodies. Its location is determined by the mass and distance between the objects. Since the Sun is vastly more massive than its planets, the barycenter of the Sun and Earth lies deep within the Sun’s interior.
A star’s movement becomes more noticeable when dealing with larger companions. The barycenter of the Sun and Jupiter, for instance, sits just outside the Sun’s visible surface, causing the Sun to execute a small, approximately 12-year wobble. In binary systems, the masses are often comparable, positioning the barycenter in the empty space between them. Both stars then trace elliptical paths around this unseen gravitational focus.
Movement Within the Galactic Core
Stars not part of an immediate binary or planetary system find their primary orbit governed by the collective gravity of their home galaxy. Stars within the Milky Way do not orbit a single, central object but rather the common center of mass of the entire galaxy, which includes billions of stars, gas, and a significant halo of dark matter. This center of mass is anchored by the supermassive black hole, Sagittarius A\ (Sgr A\), estimated to be over four million times the Sun’s mass.
The orbits of stars depend heavily on their location within the galactic structure. Stars in the flat disk of the galaxy follow nearly circular orbits, but they also oscillate vertically, bobbing up and down through the disk plane. Stars much closer to the galactic center, such as the cluster of S-stars, are intensely influenced by the gravity of Sgr A\. These stars whip around the black hole in highly elliptical orbits at speeds reaching thousands of kilometers per second.
Stars That Do Not Orbit
Not every star is gravitationally bound to a system or a galaxy; these individual exceptions are known as rogue or intergalactic stars. The most extreme examples are hypervelocity stars (HVSs), which travel so quickly that the gravitational pull of their home galaxy is not strong enough to keep them in orbit. These stars are traveling fast enough to be completely ejected into intergalactic space.
One common mechanism for this ejection is a close gravitational encounter with the supermassive black hole at a galaxy’s core, known as the Hills mechanism. If a binary star pair gets too close, the black hole can capture one star while slingshotting the other away. A second cause involves a supernova explosion within a tightly bound binary system, where the sudden loss of mass from the detonating star accelerates the surviving companion out of the galaxy.
How Galaxies Themselves Move
Scaling up the cosmic perspective, galaxies are in motion, orbiting the common gravitational centers of larger structures. Our Milky Way is part of the Local Group, a collection of over 50 galaxies, which includes the Andromeda Galaxy. The galaxies within this Group are gravitationally bound, orbiting a shared barycenter located between the two largest members, the Milky Way and Andromeda.
The Local Group is situated on the outskirts of the Virgo Supercluster, an even larger structure containing thousands of galaxies and spanning hundreds of millions of light-years. This supercluster is not a fully bound system, but its immense mass exerts a gravitational influence, causing the Local Group to move toward its center. This local movement is superimposed on the overall expansion of the universe, where space itself is stretching, but local gravity is strong enough to keep galaxies within clusters and superclusters moving toward one another.