Free Astronomy Magazine November-December 2023

24 NOVEMBER-DECEMBER 2023 ASTRO PUBLISHING Stellar layers of a galactic onion by NOIRLab Josie Fenske M uch like humans, galaxies are shaped by the environment in which they form. While no two are exactly alike, they can be divided into three main types: spiral, elliptical and irregular. Of these types, elliptical galaxies are the largest and are thought to evolve out of galactic collisions and mergers between spirals. About one-tenth of ellip- tical galaxies are classified as shell galaxies, charac- terized by the concentric shells that make up their galactic halos. A striking example of this type of galaxy is NGC 3923, with its onion-like layers beautifully show- cased in this image taken with the DOE-built Dark Energy Camera (DECam) on the Víctor M. Blanco 4- meter Telescope at Cerro Tololo Inter-American Ob- servatory (CTIO), a Program of NSF’s NOIRLab. Located in the constellation Hydra, NGC 3923 is about 70 million light-years away from Earth and 150,000 light-years across, making it about 50% larger than our Milky Way. As is thought to be the story of all shell galaxies, the layered structure seen in NGC 3923 likely developed as a consequence of a merger with another, smaller spiral galaxy in the past. As they merged, the larger galaxy’s gravitational field slowly peeled off stars