What is stars galaxies planets?

Stars are luminous celestial bodies composed mostly of hydrogen and helium that generate energy through nuclear fusion in their cores. Galaxies are vast systems that contain billions to trillions of stars, along with gas, dust, and dark matter, bound together by gravity. Planets are smaller celestial bodies that orbit stars, forming from the residual material in a star's protoplanetary disk. Together, stars, galaxies, and planets constitute the fundamental components of the universe.