Hoag’s Object is a non-typical galaxy known as a ring galaxy. The galaxy is named after Arthur Allen Hoag who discovered it in 1950 and identified it as either a planetary nebula or a peculiar galaxy with eight billion stars. A nearly perfect ring of young hot blue stars circles the older yellow nucleus of this ring galaxy, it spans about 120,000 light years and is 600 million light-years away in the constellation Serpens (Snake), which is slightly larger than the Milky Way Galaxy; the diameter of the inner core of the galaxy is about 17,000 light-years, while the surrounding ring has an inner diameter of 75,000 light-years.
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 ring galaxy (SDSS J151713.93+213516.8) can be seen through Hoag’s Object, between the nucleus and the outer ring of the galaxy, at roughly the one o’clock position in the picture.
Many of the details of the galaxy remain a mystery, foremost of which is how it formed. So-called “classic” ring galaxies are generally formed by the collision of a small galaxy with a larger disk-shaped galaxy. This collision produces a density wave in the disk that leads to a characteristic ring-like appearance. However, there is no sign of any second galaxy that would have acted as the “bullet”, and the likely older core of Hoag’s Object has a very low velocity relative to the ring, making the typical formation hypothesis quite unlikely. Interestingly, a few galaxies share the primary characteristics of Hoag’s Object, including a bright detached ring of stars, but their centers are elongated or barred, and they may exhibit some spiral structure. While none match Hoag’s Object in symmetry, this handful of galaxies are known to some as Hoag-type galaxies. Noah Brosch and colleagues showed that the luminous ring lies at the inner edge of a much larger neutral hydrogen ring.
Credits: Hubble, NASA, Wikipedia.