The Milky Way Galaxy, commonly referred to as just the Milky Way, or sometimes simply as the Galaxy, is the home galaxy of the Solar System, and of Earth. The Solar System is located in the Milky Way galaxy around two thirds of the way out from the center, on the inner edge of the Orion–Cygnus Arm. The Sun orbits around the center of the galaxy in a galactic year—once every 225-250 million Earth years. The "Milky Way" is a translation of the Latin Via Lactea, in turn translated from the Greek Γαλαξίας (Galaxias), referring to the pale band of light formed by stars in the galactic plane as seen from Earth. It is agreed that the Milky Way is a barred spiral galaxy, with observations suggesting that it is a spiral galaxy. It contains 200-400 billion stars and is estimated to have at least 50 billion planets, 500 million of which could be located in the habitable zone of their parent star. New data suggests there may be up to twice as many free-floating planets in the Milky Way as there are stars. The Milky Way is part of the Local Group of galaxies and is one of around 200 billion galaxies in the observable universe.
The stellar disk of the Milky Way galaxy is approximately 100,000 light-years (30 kiloparsecs, 9×1017 km) in diameter, and is considered to be, on average, about 1,000 ly (0.3 kpc) thick. It is estimated to contain at least 200 billion stars and possibly up to 400 billion stars, the exact figure depending on the number of very low-mass, or dwarf stars, which are hard to detect, especially more than 300 ly (90 pc) from the Sun, and so current estimates of the total number remain highly uncertain, though often speculated to be around 250 billion. This can be compared to the one trillion (1012) stars of the neighbouring Andromeda Galaxy. The stellar disc does not have a sharp edge, a radius beyond which there are no stars. Rather, the number of stars drops smoothly with distance from the centre of the galaxy. Beyond a radius of roughly 40,000 ly (12 kpc), the number of stars drops much faster with radius, for reasons that are not understood.
In 2007, a star in the Galactic halo, HE 1523-0901, was estimated to be about 13.2 billion years old, nearly as old as the Universe. As the oldest known object in the Milky Way at that time, it placed a lower limit on the age of the Milky Way.
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