Milky Way

The Milky Way is the galaxy that contains our Solar System.^{[18][19][20][nb 1]} Its name "milky" is derived from its appearance as a dim glowing band arching across the night sky whose individual stars cannot be 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").^[21][22][23]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 astronomersHarlow Shapley and Heber Curtis,^[24] observations by Edwin Hubble showed that the Milky Way is just one of many galaxies—now estimated to number as many as 200 billion galaxies in the observable universe.^[25][26]

The Milky Way is a barred spiral galaxy that has a diameter usually considered to be about 100,000–120,000 light-years^[27]but may be 150,000–180,000 light-years.^[28] The Milky Way is estimated to contain 100–400 billion stars,^[29][30] although this number may be as high as one trillion.^[31] There are likely at least 100 billion planets in the Milky Way.^[32][33] The Solar System is located within the disk, about 27,000 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.

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".^[34] The rotational period is about 240 million years at the position of the Sun.^[15] 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 likely formed shortly after the Dark Ages of the Big Bang.^[9]

The Milky Way has several satellite galaxies and is part of the Local Group of galaxies, which is a component of the Virgo Supercluster, which is itself a component of the Laniakea Supercluster.