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Still from a simulation of individual galaxies forming, starting at a time when the Universe was just a few million years old. Credit: Hopkins Research Group, Caltech

Astronomers can go their whole career without finding a new object in the sky. But for Lina Necib, a postdoctoral scholar in theoretical physics at Caltech, the discovery of a cluster of stars in the Milky Way, but not born of the Milky Way, came early—with a little help from supercomputers, the Gaia space observatory, and new deep learning methods.

Writing in Nature Astronomy this week, Necib and her collaborators describe Nyx, a vast new stellar stream in the vicinity of the Sun, that may provide the first indication that a dwarf galaxy had merged with the Milky Way disk. These stellar streams are thought to be globular clusters or dwarf galaxies that have been stretched out along its orbit by tidal forces before being completely disrupted.

The discovery of Nyx took a circuitous route, but one that reflects the multifaceted way astronomy and astrophysics are studied today. Necib studies the kinematics—or motions—of stars and dark matter in the Milky Way. "If there are any clumps of stars that are moving together in a particular fashion, that usually tells us that there is a reason that they're moving together."