A ring-shaped nebula formed by an expanding shell of gas round an ageing star.
- Astronomers call such an object a planetary nebula, because its round shape resembles that of a planet when viewed with a small telescope.
- The Boomerang Nebula is a young planetary nebula and the coldest object found in the Universe so far.
- Until recently, it was thought that shells around planetary nebulae were a rare phenomenon.
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