The Solar System and Galaxies

The Solar System consists of the Sun and all the objects that orbit it. The Sun is a star: a hot ball of gas that releases energy by nuclear fusion in its core.Planets are objects that orbit a star, have enough mass for their own gravity to make them roughly spherical, and have cleared their orbit of most other objects. Our Solar System has eight planets. Dwarf planets, such as Pluto, are spherical but have not cleared their orbits.Planetary satellites are bodies that orbit a planet. They may be natural moons, like the Moon orbiting the Earth, or artificial satellites.Comets are small, irregular lumps of ice and dust that orbit the Sun in highly elliptical orbits. As a comet approaches the Sun, some of its ice vaporises, forming a glowing coma and a tail that points away from the Sun.A solar system (or planetary system) is any star together with the objects orbiting it. Many stars in our galaxy are now known to have planets of their own (exoplanets).A galaxy is a huge collection of stars, dust and gas held together by gravity. Our galaxy, the Milky Way, contains roughly 10¹¹ stars and is about 100 000 light years across. The Sun is about 26 000 light years from its centre.The universe is everything that exists: all of space, time, matter and energy. It contains more than 10¹¹ galaxies.