HD 56925

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NGC 2359, Thor's Helmet (core).jpg
The popular name for the nebula surrounding HD 56925 is the Thor's Helmet Nebula.

The emission nebula NGC 2359, also known as the Thor's Helmet Nebula, which has a diameter of around 30 light years,[1], has been produced by an extremely hot Wolf-Rayet star, visible at its center. This star, designated HD 56925, has a surface temperature of 112,000 degrees Kelvin.[2] It is also highly unstable, ejecting stellar material into the interstellar medium at speeds approaching 7.2 million kilometers per hour. Even though it is a massive star of around 10 solar masses, it is losing the equivalent of the mass of the Sun every thousand years. With this level of mass loss, Wolf-Rayet stars like HD 56925 are unable to exist in this stage of their life for long, and are therefore rarely observed: only about 550 such stars are known in the Milky Way. Material from the star has been ejected in an even, spherical manner, producing a bubble of material. This bubble has been further shaped by interactions with the surrounding interstellar medium. HD 56925 is unusual because it lies at the edge of a dense, warm molecular cloud, and the asymmetrical shape of the outer parts of the surrounding nebula is due to bow shocks, produced when fast stellar winds hit denser, static material.