A star system, also known as a solar system, planetary system, or simply a system, is a group of celestial bodies orbiting a star. Star systems usually consisted of one or more planets, and the debris remaining from the formation of the primary planets and central star, such as comets and asteroids. Most stars in the Milky Way had their own planetary system.
The standard approach to naming star systems was to base the name off either the system's central star or a significant planet within the system. However, some systems were unnamed, and were only known by a numerical designation.
Most star systems usually begin as nebulae, dark clouds of gas and dust. Often, these clouds are created out of the remnants of previous stars which have gone supernova. Gravity urges the material in the nebula to coalesce and condense, causing the cloud to begin spinning and flattening. As more and more elements, usually hydrogen and helium, are drawn into the nebula's middle, a bulge known as a protostar is formed. If the protostar is unable to begin a process of nuclear fusion, it will not transition into becoming a full-fledged star, and is known as a brown dwarf. Otherwise, the process of fusion begins, and a new star is born.
Eventually the disk surrounding the star starts to cool, and ever larger lumps of dust begin to form into planetesimals. Over millions of years, these growing bodies continue to collide and consolidate to form the system's planets, all of which travel along their own slightly elliptical pathways, or orbits, around their parent star or stars. This process is how most star systems form.
Other star systems are formed when one large star attracts another into its gravity field, and the objects begin orbiting one another. When this happens, sometimes the planets in one of the stars' systems are flung out into interstellar space, where they become rogue planets. A system with two stars is known as a binary star system, and a system with three stars is a trinary star system. Quaternary, quintenary, sextenary, and septenary systems, composed of four, five, six, and seven stars, respectively, are also known to exist, but are very rare.
Systems which are centered around non-stellar objects, such as protostars, brown dwarfs, neutron stars, pulsars, or black holes, are still sometimes referred to as "star systems" in common vernacular.