The Milky Way is the galaxy thatincludes our Solar System, with the name describing the galaxy'sappearance from Earth: a hazy band of light seen in the night skyformed from stars that cannot be individually distinguished by thenaked eye. The term Milky Way is a translation of the Latin vialactea, from the Greek γαλακτικός κύκλος (galaktikoskýklos), meaning "milky circle." From Earth, theMilky Way appears as a band because its disk-shaped structure isviewed from within. Galileo Galilei first resolved the band of lightinto individual stars with his telescope in 1610. Until the early1920s, most astronomers thought that the Milky Way contained all thestars in the Universe. Following the 1920 Great Debate between theastronomers Harlow Shapley and Heber Curtis, observations by EdwinHubble showed that the Milky Way is just one of many galaxies.
The Milky Way is a barred spiral galaxywith an estimated visible diameter of 100,000–200,000 light-years,but only about 1000 light years thick at the spiral arms (more at thebulge). Recent simulations suggest that a dark matter area, alsocontaining some visible stars, may extend up to a diameter of almost2 million light-years. The Milky Way has several satellite galaxiesand is part of the Local Group of galaxies, which form part of theVirgo Supercluster, which is itself a component of the LaniakeaSupercluster.
It is estimated to contain 100–400billion stars and at least that number of planets. The Solar Systemis located at a radius of about 27,000 light-years from the GalacticCenter, on the inner edge of the Orion Arm, one of the spiral-shapedconcentrations of gas and dust. The stars in the innermost 10,000light-years form a bulge and one or more bars that radiate from thebulge. The galactic center is an intense radio source known asSagittarius A, a supermassive black hole of 4.100 (± 0.034) millionsolar masses. Stars and gases at a wide range of distances from theGalactic Center orbit at approximately 220 kilometers per second. Theconstant rotational speed appears to contradict the laws of Kepleriandynamics and suggests that much (about 90%) of the mass of the MilkyWay is invisible to telescopes, neither emitting nor absorbingelectromagnetic radiation. This conjectural mass has been termed"dark matter". The rotational period is about 240million years at the radius of the Sun.
The Milky Way as a whole is moving at avelocity of approximately 600 km per second with respect toextra-galactic frames of reference. The oldest stars in the Milky Wayare nearly as old as the Universe itself and thus probably formedshortly after the Dark Ages of the Big Bang. On 12 May 2022,astronomers announced the image, for the first time, of SagittariusA, the supermassive black hole at the center of the Milky Way galaxy.
Etymology and mythology
In the Babylonian epic poem EnūmaEliš, the Milky Way is created from the severed tail of the primevalsalt water dragoness Tiamat, set in the sky by Marduk, the Babyloniannational god, after slaying her. This story was once thought to havebeen based on an older Sumerian version in which Tiamat is insteadslain by Enlil of Nippur, but is now thought to be purely aninvention of Babylonian propagandists with the intention to showMarduk as superior to the Sumerian deities.
In Greek mythology, Zeus places his sonborn by a mortal woman, the infant Heracles, on Hera's breast whileshe is asleep so the baby will drink her divine milk and thus becomeimmortal. Hera wakes up while breastfeeding and then realizes she isnursing an unknown baby: she pushes the baby away, some of her milkspills, and it produces the band of light known as the Milky Way. Inanother Greek story, the abandoned Heracles is given by Athena toHera for feeding, but Heracles' forcefulness causes Athena to rip himfrom her breast in pain.
