![]() ![]() Its integrated absolute visual magnitude has been estimated to be −20.9. Īs of 2006, the Milky Way's mass is thought to be about 5.8 M ☉ comprising 200 to 400 billion stars. In 1970 Gérard de Vaucouleurs predicted that the Milky Way was of type SAB(rs)bc, where the "rs" indicates a broken ring structure around the core region. This argues for a classification of type SBbc (loosely wound barred spiral). It was only in the 1980s that astronomers began to suspect that the Milky Way is a barred spiral rather than an ordinary spiral, which observations in 2005 with the Spitzer Space Telescope have since confirmed, showing that the Galaxy's central bar is larger than previously suspected. The mass distribution within the Galaxy closely resembles the Sbc Hubble classification, which is a spiral galaxy with relatively loosely-wound arms. Within the disk region are several arm structures that spiral outward in a logarithmic spiral shape. It consists of a bar-shaped core region surrounded by a disk of gas, dust and stars. Observations by the Spitzer Space Telescope in 2005 backed up previously collected evidence that suggested the Milky Way is a barred spiral galaxy. (See also nucleocosmochronology.) Based upon this emerging science, the Galactic thin disk is estimated to have been formed between 6.5 and 10.1 billion years ago. By including the estimated age of the stars in the globular cluster (13.4 ± 0.8 billion years), they estimated the age of the oldest stars in the Milky Way at 13.6 ± 0.8 billion years. This allowed them to deduce the elapsed time between the rise of the first generation of stars in the entire Galaxy and the first generation of stars in the cluster, at 200 million to 300 million years. The team used the UV-Visual Echelle Spectrograph of the Very Large Telescope to measure, for the first time, the beryllium content of two stars in globular cluster NGC 6397. ![]() This estimate is based on research done in 2004 by astronomers Luca Pasquini, Piercarlo Bonifacio, Sofia Randich, Daniele Galli, and Raffaele G. It is extremely difficult to define the age at which the Milky Way formed, but the age of the oldest stars in the Galaxy is now estimated to be about 13.6 billion years, nearly as old as the Universe itself. See Benchmark distances to help put the galaxy's size into perspective. New discoveries indicate that the disk extends much farther than previously thought. The Galactic Halo extends outward, but is limited in size by the orbits of the two Milky Way satellites, the Large and the Small Magellanic Clouds, whose perigalacticon is at ~180,000 light-years. As a guide to the relative physical scale of the Milky Way, if it were reduced to 130 km (80 mi) in diameter, the Solar System would be a mere 2 mm (0.08 inches) in width. It is estimated to contain 200 billion, and up to 400 billion, stars (if small-mass stars predominate). The disk of the Milky Way galaxy is approximately 100,000 light years in diameter, and about 1,000 light years thick. ![]()
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