Image of the Day: The Cosmic Bubble Mystery


This bubble-like emission nebula, known as Thor's Helmet, is approximately 30 light-years across and located in the Constellation Canis Major at a distance of about 15,000 light-years. We don't know what caused the bubbles and arcs in NGC 2359, but the main culprit is the massive Wolf-Rayet star located slightly left from the image center.

Wolf-Rayet stars are massive, highly luminous stars that continually shed material in a stellar wind — which commonly form bubbles in the interstellar medium. But the unusual structure of the NGC 2359 arcs indicate something more complex is going on. Experts wonder if the star is moving supersonically? Or, is there another energetic star in the vicinity? Only about 300 Wolf-Rayet stars are known to exist. NASA experts think that very massive stars become Wolf-Rayet stars just before they explode as supernova though no one has yet seen such a star explode.

The 200 Wolf-Rayet stars known in the Milky Way are considerably hotter and more luminous than our Sun. All extremely massive stars will eventually evolve though a Wolf-Rayet phase and generate bubbles because they continually eject their outer atmosphere as a stellar wind. This outgoing wind of particles typically carries away more than the mass of our Earth each year. The wind is caused by atmospheric particles absorbing outgoing starlight, although many details of this process are unknown.

Credit: NASA/Hubble

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