What is a Stellar Stream?

A stellar stream is created when stars from one galaxy get pulled from their home galaxy by another. The stellar stream is a long thin filament of stars produced by the stretching action of tidal forces. Only about a dozen stellar streams have been named or studied in detail. Being a phenomena which only occurs on a galactic scale, most stellar streams are too faint and distant to study very thoroughly.

The most familiar stellar stream and one of the first to be confirmed as such is the Arcturus stream, located only 37 light-years away, containing the star Arcturus. The Arcturus stream is a remnant of a dwarf galaxy devoured long ago by the Milky Way. Over its lifetime, the Milky Way has probably consumed dozens or even hundreds of smaller dwarf galaxies, and continues to do so today. We even observe star clusters that seem to be remnants of the cores of devoured galaxies, such as the Omega Centauri star cluster. We know these are former galactic cores and not conventional open clusters because open clusters are made up of stars that form around the same time, whereas a galactic core contains stars of greatly varying ages.

One of the best-studied stellar streams is the Magellanic stream, a star bridge connecting together two of the nearest galaxies to the Milky Way, the Small Magellanic Cloud and the Large Magellanic Cloud. Because the Magellanic clouds are among the closest galaxies to our own, at only 150,000 light-years distant, we can observe individual stars in the "clouds" and their parallaxes, letting us made a 3D map of the galaxies and their intervening stellar stream.

Just like planets such as Saturn cause dust to form into rings around them, some galaxies rip apart others and form them into rings as well. One stellar stream in the form of a ring is the Monoceros Ring, created as the Milky Way swallows a dwarf galaxy, the Canis Major Dwarf Galaxy, about 100 times smaller than it.