A star is a luminous sphere of gas held together by its own gravity. is a star. A planet orbits the star, while a moon orbits the planet. Stars are visible to the naked eye from earth during the night, while the sun is visible during the day. Historically, the most prominent stars were grouped into constellations and asterisms, and the brightest stars were given proper names. Extensive catalogs of stars have been assembled by astronomers throughout the ages. Stars shine on account of nuclear fusion reactions originating inside them. A planet is a body orbiting a star that: - is massive enough to be rounded by its own gravity, - is not massive enough to cause thermonuclear fusion, and - has cleared its neighboring region of planetesimals. There are 8 planets (as of now and including earth) orbiting the sun. The moon is earth's only natural satellite. There are more than 181 moons of the various planets, dwarf planets and asteroids in the solar system.