
The Milky Way is a barred spiral galaxy that has a diameter usually considered to be roughly 100,000–120,000 light-years in diameter but may be 150,000–180,000 light-years.[25] The Milky Way is estimated to contain 100–400 billion stars, although this number may be as high as one trillion. There are probably at least 100 billion planets in the Milky Way. 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 dynamicsand suggests that much of the mass of the Milky Way does not emit or absorb electromagnetic radiation. This mass has been given the name "dark matter". The rotational period is about 240 million years at the position of the Sun. 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 must have formed shortly after the Big Bang.
Surrounded by several smaller satellite galaxies, the Milky Way is part of the Local Group of galaxies, which forms a subcomponent of the Virgo Supercluster, which again forms a subcomponent of the Laniakea Supercluster.
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