Hoag’s Object is a galaxy with an central region and a bright outer ring, but lacks any intervening material.
No. 7: With This Ring, I Thee Puzzle.
In 1950, astronomer Arthur Hoag came upon a tiny, faint, 16th-magnitude ring surrounding a ball-like center, and reasonably assumed it was a planetary nebula — a nearby puff of gas expelled from a single old-aged star. He also proposed an alternative and far more exotic explanation that this was an “Einstein Ring” from a faraway quasar. In this scenario, the quasar’s light is distorted into a halo by space-warping caused by a massive foreground spherical galaxy that it seems to surround. But later spectroscopic studies rejected this because the golden central ball and the blue ring have exactly the same redshift, indicating a whopping rush-away speed of 7,916 miles (12,740 kilometers) per second, which proves they’re both located exactly the same distance from us.
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