Zw II 28 is a ring galaxy. Ring galaxies are thought to form when one galaxy passes through the disc of another, larger one. Because galaxies are mostly empty space, such “collisions” are not as destructive as one might suppose. The probability of two stars physically colliding is nearly zero. It’s the gravitational effects of the two galaxies that causes disruption, upsetting the balance both galaxies and causing the pair to redistribute to form a dense central core surrounded by bright stars. In the process, clouds of gas and dust collapse, triggering intense star formation in the outer ring and filling it with hot, young, blue stars.
Zw II 28’s sparkling pink and purple loop is not typical of a ring galaxy. It lacks a visible central companion, but recent observations using Hubble have shown that there may be a possible companion lurking just inside the ring, where the loop seems to double back on itself.
Image Credit: ESA / NASA