December 21, 2013

The Milky Way - Solar Systems - Nebula - Blackholes - Pulsars

         The Milky Way is the galaxy that contains the Solar System.This name derives from its appearance as a dim un-resolved "milky" glowing band arching across the night sky. The term "Milky Way" is a translation of the Latin for "milky road", Via Lactea, in turn derived from the Greek kyklos galaktikos or "milky circle", "milk" also being the root for the Greek word for galaxy, (galaxias).


      The galaxy has this appearance because of the Earth's position within the galactic plane around two thirds of the way out from the center, on the inner edge of the Orionâ€"Cygnus Arm, with the majority of the galaxy being seen edge on. The concept of this faint band of light being made up of stars was proven in 1610 when Galileo Galilei used his telescope to resolve it into individual stars. In the 1920s observations by astronomer Edwin Hubble showed that the Milky Way was just one of around 200 billion galaxies in the observable universe.
The Milky Way is a barred spiral galaxy 100,000-120,000 light-years in diameter containing 200â€"400 billion stars. The galaxy is estimated to contain 50 billion planets, 500 million of which could be located in the habitable zone of their parent star. Depending on its structure the entire galaxy has a rotational rate of 1 per 15 to 50 million years. The galaxy is also moving at a rate of 552 to 630 km per second depending on the relative frame of reference. It is estimated to be about 13.2 billion years old, nearly as old as the Universe. The Milky Way is part of the Local Group of galaxies.
      All the stars that can be seen in the night sky are part of the Milky Way galaxy, but for the purposes of the astronomical activity of observing the celestial sphere the term "Milky Way" is limited to the hazy band of white light arching around the entire night sky.The light originates from un-resolved stars and other material that lie within the galactic plane. Dark regions within the band, such as the Great Rift and the Coalsack, correspond to areas where light from distant stars is blocked by dark nebulae.
      The Milky Way has a relatively low surface brightness with an apparent magnitude of around +4.5 to +5.[citation needed] Its visibility is greatly affected by how bright the night sky is (limiting magnitude) due to light pollution, stray light from the moon, etc. It becomes visible at limiting magnitudes of +5.1 or higher, showing a great deal of detail above +6.1.[15] This makes the Milky Way difficult to see from any urban or suburban location due to light pollution, but very prominent when viewed from a rural area.
       The center of the galaxy lies in the direction of Sagittarius, and it is here that the Milky Way looks brightest. From Sagittarius, the hazy band of white light appears to pass westward through the constellations of Scorpius, Ara, Norma, Triangulum Australe, Circinus, Centaurus, Musca, Crux, Carina, Vela, Puppis, Canis Major, Monoceros, Orion and Gemini, Taurus, Auriga, Perseus, Andromeda, Cassiopeia, Cepheus and Lacerta, Cygnus, Vulpecula, Sagitta, Aquila, Ophiuchus, Scutum, and back to Sagittarius. The fact that the band divides the night sky into two roughly equal hemispheres indicates that the Solar System lies close to the galactic plane.
      The galactic plane is inclined by about 60 degrees to the ecliptic (the plane of the Earth's orbit). Relative to the celestial equator, it passes as far north as the constellation of Cassiopeia and as far south as the constellation of Crux, indicating the high inclination of Earth's equatorial plane and the plane of the ecliptic relative to the galactic plane. The north galactic pole is situated at right ascension 12h 49m, declination +27.4° (B1950) near beta Comae Berenices, and the south galactic pole is near alpha Sculptoris.

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