THE MILKY WAY
The Milky Way or Milky Way Galaxy, or simply the Galaxy, is the galaxy that includes the Solar System, with the name describing the galaxy's appearance from Earth: a hazy band of light seen in the night sky formed by stars in other arms of the galaxy, which are so far away that they cannot be individually distinguished by the naked eye.
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, but following the 1920 Great Debate between the astronomers Harlow Shapley and Heber Doust Curtis, observations by Edwin Hubble in 1923 showed that the Milky Way was just one of many galaxies.
The Milky Way is a barred spiral galaxy with a D25 isophotal diameter estimated at 26.8 ± 1.1 kiloparsecs (87,400 ± 3,600 light-years), but also has a surrounding area of dark matter with a total diameter possibly approaching 613 kpc (2 million ly). The stars in the innermost 10,000 light-years form a bulge, from which the bar radiates. At the Milky Way's center is an intense radio source known as Sagittarius A*, a supermassive black hole with a mass of 4.100 (± 0.034) million solar masses. The galaxy is estimated to contain 100–400 billion stars and at least that number of planets.
The galaxy's oldest stars probably formed shortly after the Dark Ages of the early universe, taking a spherical shape before flattening into a spiral disk, in which later generations of stars formed. The Solar System is located on the inner edge of the Orion Arm at a radius of about 27,000 light-years (8.3 kpc) from the Galactic Center. The Milky Way has several satellite galaxies and is part of the Local Group of galaxies, forming part of the Virgo Supercluster, which is itself a component of the Laniakea Supercluster.
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