Hoag's Object

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Hoag's Object is a non-typical galaxy of the type known as a ring galaxy. The appearance of this object has interested amateur astronomers as much as its uncommon structure has fascinated professionals. 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 8 billion stars.[3]



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.[2][a] 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 can be seen through Hoag's Object, between the nucleus and the outer ring of the galaxy, at the one o'clock position in the picture to the right.

History and formation

Even though Hoag's Object was clearly shown on the Palomar Star Survey, it was not included in either the Morphological Catalogue of Galaxies, the Catalogue of Galaxies and Clusters of Galaxies, or the Catalogue of Galactic Planetary Nebulae.[2]

In the initial announcement of his discovery, Art Hoag proposed the hypothesis that the visible ring was a product of gravitational lensing. This idea was later discarded because the nucleus and the ring have the same redshift, and because more advanced telescopes revealed the knotty structure of the ring, something that would not be visible if the ring were the product of gravitational lensing.[4]

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