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The Helmi Stream is a stellar stream of the Milky Way galaxy. It started as a dwarf galaxy, now absorbed by the Milky Way as a stream. It was discovered in 1999, is formed of old stars deficient in heavy elements, and has a mass of 10 to 100 million solar masses. It was absorbed by the Milky Way some 6 to 9 billion years ago.Scientific American, "Extragalactic Expat: Newfound Exoplanet Likely Came from Another Galaxy", John Matson , 18 November 2010
The stream was named after Amina Helmi, who discovered this stellar stream after noticing this group of stars all moving at the same speed and in the same direction. The Helmi Stream discovery affirmed theories that the merging of galaxies played a significant role in creating the giant structures of the Milky Way galaxy.
In 2024 a subdwarf of T dwarf was identified as a possible member of the Helmi stream. The brown dwarf is called CWISE J155349.96+693355.2 has Vtan > 300 km/s and a poorly constrained radial velocity of .
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