The Hoag Object appears as a nearly perfect ring of young blue stars surrounding a core of older yellow stars. The gap between the ring and the nucleus is really transparent, since another annular galaxy can be seen in the background at the 01:00 position (one o'clock). However, it is thought that it may contain star clusters too faint to be seen. Many of the details of these galaxies remain unclear, the main one being how they came to be formed. Annular galaxies are generally formed by the collision between a small galaxy and a larger disk-shaped galaxy. This collision produces a density wave that leads to the formation of the ring. However, there is no indication of a second galaxy, and in addition the core of the Hoag Object has a very slow speed relative to the ring, which seems to rule out this hypothesis. Alternatively, it has been suggested that the Hoag Object may be the product of an "extreme bar instability" that occurred a few billion years ago in a barred spiral galaxy, although some authors have objected that the core of the object is spherical and does not have a disk shape.
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By: Alejandro Sebastián Von Heguer, National University of Lomas de Zamora, Buenos Aires. Source: Schweizer et al., Astrophysical Journal, 1987.
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