Helmi stream
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.[1]
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.[2][3] 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.[2]
In 2024 a subdwarf of spectral type sdT4 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 +110±90 km/s.[4]
Extragalactic planet
[edit | edit source]The Helmi stream was briefly believed to be home to the first discovered planet purportedly of extragalactic origin, orbiting the star HIP 13044.[1] However, further analysis of radial velocity data failed to confirm the discovery.[5]
See also
[edit | edit source]References
[edit | edit source]- ^ a b Scientific American, "Extragalactic Expat: Newfound Exoplanet Likely Came from Another Galaxy", John Matson , 18 November 2010
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