spaceSpace and Physics

The Farthest Stars In The Milky Way Might Come From Another Galaxy


Dr. Alfredo Carpineti

Senior Staff Writer & Space Correspondent

clockJan 11 2017, 19:06 UTC

Artist's impression of the Sagittarius Stream. David Martinez-Delgado (MPIA) & Gabriel Perez (IAC)

Harvard astronomers have discovered that some of the farthest stars in the Milky Way do not belong to our galaxy at all, but were actually stolen from one of the many small galaxies that orbit our corner of the universe.

The new discovery, which was accepted in the Astrophysical Journal and is available on arXiv, used dynamic simulations of the complex environment beyond the Milky Way’s disk (which is 100,000 light-years across) and data from the 11 farthest stars gravitationally bound to our galaxy.


The stars are located 300,000 light-years from Earth, and their position and velocities are consistent with the Sagittarius stream that extends for 1 million light-years. According to the simulations, five out of the 11 most distant stars in our galaxy actually belong to the stream of material named after the Sagittarius dwarf from which it originates.

"The starting speed and approach angle have a big effect on the orbit, just like the speed and angle of a missile launch affects its trajectory," co-author Professor Abraham Loeb said in a statement.

Knowing that, it was easy to work their origin back to the Sagittarius dwarf. The Sagittarius stream is believed to wrap around the Milky Way like a ribbon and it has at least two known branches. In a phenomenon known as galactic cannibalism, our galaxy is slowly but surely unraveling the Sagittarius dwarf.


Although the existence of the stream was predicted in the mid-1990s and evidence for it was discovered in 2002, we still know very little about the full structure of the stream.

"The star streams that have been mapped so far are like creeks compared to the giant river of stars we predict will be observed eventually," added lead author Marion Dierickx. "More interlopers from Sagittarius are out there just waiting to be found."

Future telescopes, like the Large Synoptic Survey Telescope, will spot more of these stars and help astronomers work out the full structure of the curious Sagittarius stream.

spaceSpace and Physics
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  • Milky Way,

  • stars,

  • Sagittarius Dwarf,

  • Sagittarius Stream