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Thor's Helmet NGC 2359 in Canis Major

is a most unusual nebula. Located dead-center in the 'Viking helmet' is a type of bright star called a Wolf-Rayet, which are incredibly hot and luminous. The double bubble of blue rings surrounding the W-R are the result of interaction between the star's high-velocity stellar wind and the interstellar matter surrounding it. The famous Bubble Nebula in Cassiopaeia is a similar phenomenon.