Milky Way :Composition and structure

The Galaxy consists of a bar-shaped core region bounded by a disk of gas, dust and stars form four distinct arm structures spiralling external in a logarithmic spiral shape. The mass distribution within the Galaxy intimately resembles the Sbc Hubble classification, which is a spiral galaxy with moderately loosely-wound arms. Astronomers first began to think that the Milky Way is a barred spiral galaxy in the 1990s rather than a normal spiral galaxy. Their suspicions were inveterate by the Spitzer Space Telescope observations in 2005[17] which showed the Galaxy's inner bar to be larger than before suspected. The Milky Way's mass is consideration to be about 5.8×1011 solar masses comprise 200 to 400 billion stars. Its integrated absolute visual magnitude has been predictable to be 20.9. Most of the mass of the Galaxy is thought to be dark substance, forming a dark matter halo of a probable 600 to 3000 billion which is spread out relatively uniformly.