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Concept
Sagittarius Dwarf Spheroidal Galaxy
Summary
The Sagittarius Dwarf Spheroidal Galaxy (Sgr dSph), also known as the Sagittarius Dwarf Elliptical Galaxy (Sgr dE or Sag DEG), is an elliptical loop-shaped satellite galaxy of the Milky Way. It contains four globular clusters in its main body, with the brightest of them—NGC 6715 (M54)—being known well before the discovery of the galaxy itself in 1994. Sgr dSph is roughly 10,000 light-years in diameter, and is currently about 70,000 light-years from Earth, travelling in a polar orbit (an orbit passing over the Milky Way's galactic poles) at a distance of about 50,000 light-years from the core of the Milky Way (about one third of the distance of the Large Magellanic Cloud). In its looping, spiraling path, it has passed through the plane of the Milky Way several times in the past. In 2018 the Gaia project of the European Space Agency showed that Sgr dSph had caused perturbations in a set of stars near the Milky Way's core, causing unexpected rippling movements of the stars triggered when it moved past the Milky Way between 300 and 900 million years ago.
Officially discovered in 1994, by Rodrigo Ibata, Mike Irwin, and Gerry Gilmore, Sgr dSph was immediately recognized as being the nearest known neighbor to the Milky Way at the time. (The disputed Canis Major Dwarf Galaxy, discovered in 2003, might be the actual nearest neighbor.) Although it is one of the closest companion galaxies to the Milky Way, the main parent cluster is on the opposite side of the Galactic Center from Earth, and consequently is very faint, although covering a large area of the sky. Sgr dSph appears to be an older galaxy, with little interstellar dust and composed largely of Population II stars, older and metal-poor, as compared to the Milky Way. No neutral hydrogen gas related to Sgr dSph has been found.
Further discoveries by astrophysics teams from both the University of Virginia and the University of Massachusetts Amherst, drawing upon the 2MASS Two-Micron All Sky Infrared Survey data, revealed the entire loop-shaped structure.
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