Like a river that suddenly disappears underground, the Perseus Arm in the Milky Way breaks off in a region astronomers call the Giraffe Bay. Where the bright band of stars should continue, a dark 'void' is visible. A team of researchers decided to comb this patch of sky using data from the Gaia telescope and found 31 open clusters in the 'void'—families of stars born from the same cloud of gas and dust.
Each cluster is like a school of fish that gradually disperses. Eight of the found objects are so sparse that they can be considered remnants: the stars are already wandering off into the galaxy. The scientists measured the distances to the clusters (from 3 to 5 kiloparsecs, i.e., about 10–16 thousand light-years), their ages (from 20 million to a billion years), and the reddening of light — a quantity that shows how much dust lies between us and the stars. There is a lot of dust: up to 1.5 magnitudes of E(B-V), as if looking through a sooty glass.
The new clusters do not belong to the Perseus Arm. Their positions and motions point to the outer Norma Arm — another spiral branch that runs farther from the center. This means that the 'break' in the Perseus Arm may not be a real hole but a place where the arm simply hides behind dust or where stars are born less frequently. Perhaps the Milky Way is not a perfect spiral with clear arms but a 'patchy' galaxy where arms sometimes break into separate clumps.
The discovery increased the number of known clusters in this direction by 31% for distances of 3–4 kiloparsecs and by 12% for more distant ones. Now the map of the Giraffe Bay is more complete, and astronomers can better judge how our galaxy is structured. The river of stars has not disappeared: it just flows deeper, under a layer of dust.
A reddening of E(B-V) up to 1.5 means that dust dims blue light 4 times more than red light. If you looked at such a cluster with the naked eye, it would appear noticeably more yellow.