Milky Way Galaxy

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Milky Way Galaxy

Milky Way Galaxy

The Milky Way is the galaxy that contains our Solar System. Its name “milky” is derived from its appearance as a dim glowing band arching across the night sky in which the naked eye cannot distinguish individual stars. The term “Milky Way” is a translation of the Latin via lactea, from the Greek γαλαξίας κύκλος (galaxías kýklos, "milky circle"). From Earth, the Milky Way appears as a band because its disk-shaped structure is viewed from within. Galileo Galilei first resolved the band of light into individual stars with his telescope in 1610. Up until the early 1920s, most astronomers thought that all of the stars in the universe were contained inside of the Milky Way. Following the 1920 Great Debate between the astronomers Harlow Shapley and Heber Curtis, observations by Edwin Hubble definitively showed that the Milky Way is just one of many billions of galaxies.

Type - SBc (barred spiral galaxy)Diameter - 100–120 kly (31–37 kpc)Thickness of thin stellar disk - ≈2 kly (0.6 kpc)Number of stars - 200–400 billion (3×1011 ±1×1011)Oldest known star - >13.6 GyrMass - 0.8–1.5×1012 M☉Sun's distance to Galactic Center - 27.2 ± 1.1 kly (8.34 ± 0.34 kpc)Sun's Galactic rotation period - 240 MyrSpiral pattern rotation period - 220–360 MyrBar pattern rotation period - 100–120 MyrSpeed relative to CMB rest frame - 552 ± 6 km/s

Stars and planets

The Milky Way contains at least 100 billion planets and between 200 and 400 billion stars. The exact figure depends on the number of very low-mass, or dwarf stars, which are hard to detect, especially at distances of more than 300 ly (90 pc) from the Sun. As a comparison, the neighboring Andromeda Galaxy contains an estimated one trillion (1012) stars. Filling the space between the stars is a disk of gas and dust called the interstellar medium. This disk has at least a comparable extent in radius to the stars,[44] whereas the thickness of the gas layer ranges from hundreds of light years for the colder gas to thousands of light years for warmer gas. Both gravitational microlensing and planetary transit observations indicate that there may be at least as many planets bound to stars as there are stars in the Milky Way and microlensing measurements indicate that there are more rogue planets not bound to host stars than there are stars. The Milky Way Galaxy contains at least one planet per star, resulting in 100–400 billion planets, according to a January 2013 study of the five-planet star system Kepler-32 with the Kepler space observatory. A different January 2013 analysis of Kepler data estimated that at least 17 billion Earth-sized exoplanets reside in the Milky Way Galaxy.[50] On November 4, 2013, astronomers reported, based on Kepler space mission data, that there could be as many as 40 billion Earth-sized planets orbiting in the habitable zones of Sun-like stars and red dwarfs within the Milky Way Galaxy. 11 billion of these estimated planets may be orbiting sun-like stars. The nearest such planet may be 12 light-years away, according to the scientists. Such Earth-sized planets may be more numerous than gas giants. Besides exoplanets, "exocomets", comets beyond the Solar System, have also been detected and may be common in the Milky Way Galaxy.


The Milky Way consists of a bar-shaped core region surrounded by a disk of gas, dust and stars. The gas, dust and stars are organized in roughly logarithmic spiral arm structures (see Spiral arms below). The mass distribution within the Milky Way closely resembles the type SBc in the Hubble classification, which represents spiral galaxies with relatively loosely wound arms.[1] Astronomers first began to suspect that the Milky Way is a barred spiral galaxy, rather than an ordinary spiral galaxy, in the 1990s.[59] Their suspicions were confirmed by the Spitzer Space Telescope observations in 2005[60] that showed the Milky Way's central bar to be larger than previously suspected.


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