On the outskirts of galaxy M83, also known as the Southern Pinwheel, a river of ancient stars drifts through the void. It stretches 81 kiloparsecs — roughly 264,000 light-years, two and a half times the diameter of our Milky Way. Astronomers resolved individual stars in this stream, red giants, using the Hyper Suprime-Cam on the Subaru Telescope and mapped their distribution.
The stream is extremely faint: its average surface brightness is about 31.8 magnitudes per square arcsecond. That's dimmer than the light of a single candle placed on the Moon. The stream is 9 kiloparsecs wide, with clumps and gaps along its length, much like a real river. Its stars are poor in heavy elements (metallicity around -1.23), indicating they are old and formed in a small galaxy, not a large spiral.
Comparison with the known northern stream in M83 suggests both are likely remnants of the same dwarf galaxy that M83 tore apart and absorbed. Moreover, the S-stream's shape echoes the distribution of neutral hydrogen in the outer disk. It seems the same merger stirred the gas at the galaxy's edge, sparking new star formation there — giving rise to an extended ultraviolet disk.
The S-stream is 81 kiloparsecs long. If such a stream extended from the Sun, it would reach nearly to the center of the Milky Way and back, all while remaining almost invisible.