Milky Way, seen edge-on.

Seen edge on, the Milky Way’s middle sports a double bump. ESO/NASA/JPL-Caltech/M. Kornmesser/R. Hurt

Astronomy and astrophysics

How the Milky Way’s middle grew a peanut-shaped bulge

Model points to motion of the stars in the galactic disc.

A prominent model of how the Milky Way developed its central bulge has found support from the chemical profile of the Galaxy’s stars.

Observations indicate that from the side, the Milky Way’s centre has a profile resembling a peanut: two lobes connected by a narrow waist. According to the disc-formation theory of the bulge’s origin, the orbits of the stars in the Milky Way’s disc became unstable over time, elongating into a central bar of stars. As the Milky Way rotated, the bar did, too, flinging stars outward to form the lobes seen today. The theory was not entirely convincing, however, because it could not fully predict the known chemical profiles of stars in the bulge.

Francesca Fragkoudi at the Paris Observatory and her colleagues simulated the Galaxy’s evolution according to this theory. Their model represented the structure of the Galaxy more faithfully than previous simulations, and as a result, it better replicated the bulge’s chemistry.

The results make other potential explanations for the bulge less likely, the authors say.