<Tr> <Td_colspan="2"> See also: Galaxy, List of galaxies </Td> </Tr> <P> The Milky Way is the galaxy that contains our Solar System . The descriptive "milky" is derived from the appearance from Earth of the galaxy--a band of light seen in the night sky formed from stars that cannot be individually distinguished by the naked eye . The term Milky Way is a translation of the Latin via lactea, from the Greek γαλαξίας κύκλος (galaxías kýklos, "milky circle"). From Earth, the Milky Way appears as a band because its disk - shaped structure is viewed from within . Galileo Galilei first resolved the band of light into individual stars with his telescope in 1610 . Until the early 1920s, most astronomers thought that the Milky Way contained all the stars in the Universe . Following the 1920 Great Debate between the astronomers Harlow Shapley and Heber Curtis, observations by Edwin Hubble showed that the Milky Way is just one of many galaxies . </P> <P> The Milky Way is a barred spiral galaxy with a diameter between 100,000 and 180,000 light - years . The Milky Way is estimated to contain 100--400 billion stars . There are probably at least 100 billion planets in the Milky Way . The Solar System is located within the disk, about 26,000 light - years from the Galactic Center, on the inner edge of one of the spiral - shaped concentrations of gas and dust called the Orion Arm . The stars in the inner ≈ 10,000 light - years form a bulge and one or more bars that radiate from the bulge . The very center is marked by an intense radio source, named Sagittarius A *, which is likely to be a supermassive black hole . </P> <P> Stars and gases at a wide range of distances from the Galactic Center orbit at approximately 220 kilometers per second . The constant rotation speed contradicts the laws of Keplerian dynamics and suggests that much of the mass of the Milky Way does not emit or absorb electromagnetic radiation . This mass has been termed "dark matter". The rotational period is about 240 million years at the position of the Sun . The Milky Way as a whole is moving at a velocity of approximately 600 km per second with respect to extragalactic frames of reference . The oldest stars in the Milky Way are nearly as old as the Universe itself and thus probably formed shortly after the Dark Ages of the Big Bang . </P>

What's the diameter of the milky way