A ring-shaped nebula formed by an expanding shell of gas around an aging star.
- ‘The Boomerang Nebula is a young planetary nebula and the coldest object found in the Universe so far.’
- ‘To obtain direct clue of the presence of magnetic fields in planetary nebulae, astronomers decided to focus on the central stars, where the magnetic fields should have survived.’
- ‘Until recently, it was thought that shells around planetary nebulae were a rare phenomenon.’
- ‘Astronomers call such an object a planetary nebula, because its round shape resembles that of a planet when viewed with a small telescope.’
- ‘One way that astronomers classify planetary nebulae is by the number of axes that they contain.’