Galactic onion

The glowing object in this image is an elliptical galaxy called NGC 3923 . It is located over 90 million light-years away in the constellation of Hydra .

NGC 3923 is an example of a shell galaxy where the stars in its halo are arranged in layers.

Finding concentric shells of stars enclosing a galaxy is quite common and is observed in many elliptical galaxies. In fact, every tenth elliptical galaxy exhibits this onion-like structure, which has never been observed in spiral galaxies . The shell-like structures are thought to develop as a consequence of galactic cannibalism, when a larger galaxy ingests a smaller companion. As the two centres approach, they initially oscillate about a common centre, and this oscillation ripples outwards forming the shells of stars just as ripples on a pond spread when the surface is disturbed.

NGC 3923 has over twenty shells, with only a few of the outer ones visible in this image and its shells are much more subtle than those of other shell galaxies . The shells of this galaxy are also interestingly symmetrical, while other shell galaxies are more skewed.

A version of this image was entered into the Hubble’s Hidden Treasures image processing competition by contestant Judy Schmidt.