The Monoceros Ring is a long, complex, ringlike filament of stars that wraps around the Milky Way three times. This is proposed to consist of a stellar stream torn from the Canis Major Dwarf Galaxy by tidal forces as part of the process of merging with the Milky Way over a period of billions of years, although this view has long been disputed. The ring contains 100 million solar masses and is 200,000 light years long.
The stream of stars was first reported in 2002 by astronomers conducting the Sloan Digital Sky Survey. It was in the course of investigating this ring of stars, and a closely spaced group of globular clusters similar to those associated with the Sagittarius Dwarf Elliptical Galaxy, that the Canis Major Dwarf Galaxy was discovered.
In 2006, a study using 2MASS data cast doubts on the nature of the "Ring", arguing that the data suggests that the ring is actually part of the warped galactic disc of the Milky Way. However, observations using the Anglo-Australian Telescope published in 2007 suggest that a warped disc cannot create the observed structure, which must therefore be formed either by a flare of the Galactic Disc or have an extra-Galactic origin.
Several members of the scientific community recently restated their position affirming the Monoceros structure is nothing more than an over-density produced by the flared and warped thick disk of the Milky Way.
The Corrugated Galaxy
In 2015, a study by an international team of scientists suggests, after sorting and sifting through galactic data from the Sloan Digital Sky Survey, that the Milky Way is actually 50 percent larger than previously thought. It turned out that the disk of the Milky Way isn't just a disk of stars in a flat plane - it's corrugated. As it radiates outward from the sun, there appears to be at least four ripples in the disk of the Milky Way. Scientists assume that this pattern is going to be found throughout the disk.
One way of thinking about this is to imagine being on the ocean when the waves are very high, or standing in hilly terrain. The next rise in the waves or the hills blocks the view of what's beyond. In the same way, the next rise in the galactic structure is blocking our view of what's beyond, which apparently is a significant portion of the galactic disk. Based on the distance of the Monoceros Ring, the diameter of the Milky Way increases from 100,000-120,000 light years to somewhere around 150,000-180,000 light years across. Because of this, the solar system moves from two-thirds of the way out to right about in the middle between the core and the edge.
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