A river of stars (not a cluster, but a distinctly elongated, glowing ribbon) crosses the outskirts of a distant galaxy. Such a stream was spotted in data from the first survey by the Vera Rubin Observatory. It stretches along the minor axis of the galaxy LEDA 751050, like a scarf streaming from the galactic disk.
The stream is enormous: 130 by 15 kiloparsecs. For comparison, the Milky Way's diameter is about 30 kiloparsecs. So this stellar river is four times longer than our galaxy. Its stellar mass is roughly 600 million solar masses, and its surface brightness is so low that each square arcsecond shines like a single light bulb in a vast stadium.
This stream is nearly a twin of the Giant Stellar Stream in Andromeda, which astronomers have studied for two decades. Spotting it at a distance of a billion light-years is like picking out a single strand of hair on a person's head from lunar orbit. The discovery shows that once the Rubin Observatory accumulates its full dataset, it will be able to find thousands of such faint structures and use them to reconstruct the collision history of galaxies.
The stream is so faint that its brightness is 40 million times lower than the darkest sky the human eye can see without a telescope.