Milky Way

The Milky Way is the galaxy that contains the Solar System.This name derives from its appearance as a dim un-resolved "milky" glowing band arching across the night sky. The term "Milky Way" is a translation of the Latin for "milky road", Via Lactea, in turn derived from the Greek kyklos galaktikos or "milky circle", "milk" also being the root for the Greek word for galaxy.

The galaxy has this appearance because it is a disk-shaped structure that is being viewed edge-on. Earth is located within the galactic plane of this disk, around two thirds of the way out from the center, on the inner edge of a spiral-shaped concentration of gas and dust called the Orion–Cygnus Arm. The concept of this faint band of light being made up of stars was proven in 1610 when Galileo Galilei used his telescope to resolve it into individual stars. In the 1920s observations by astronomer Edwin Hubble showed that the Milky Way was just one of around 200 billion galaxies in the observable universe.

The Milky Way is a barred spiral galaxy 100,000-120,000 light-years in diameter containing 200–400 billion stars. The galaxy is estimated to contain at least as many planets, 10 billion of which could be located in the habitable zone of their parent star. Depending on its structure the entire galaxy has a rotational rate of once every 15 to 50 million years. The galaxy is also moving at a velocity of 552 to 630 km per second, depending on the relative frame of reference. It is estimated to be about 13.2 billion years old, nearly as old as the Universe. The Milky Way is part of the Local Group of galaxies.