Characteristics
A nearly perfect ring of young hot blue stars circles the older yellow nucleus of this ring galaxy ~600 million light-years away in the constellation Serpens. The diameter of the 6″ (seconds of arc) inner core of the galaxy is about 17±0.7 kly (5.3±0.2 kpc) while the surrounding ring has an inner 28″ diameter of 75±3 kly (24.8±1.1 kpc) and an outer 45″ diameter of 121±4 kly (39.9±1.7 kpc), which is slightly larger than the Milky Way Galaxy. The gap separating the two stellar populations may contain some star clusters that are almost too faint to see. As rare as this type of galaxy is, another more distant currently unnamed ring galaxy can be seen through Hoag's Object, between the nucleus and the outer ring of the galaxy, at roughly the 12:05 position in the picture to the right.
Read more about this topic: Hoag's Object