The emission nebula NGC 2359, also known as the Thor's Helmet Nebula, which has a diameter of around 30 light years,, 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. 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.
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