Saturday, July 31, 2021

Thor's Helmet – NGC 2359

Thor's helmet is an emission nebula located in the southern constellation of Canis Major.  Its designation in the New General Catalog is NGC 2359. The nebula is 30 light-years across and is at a distance of 11,960 light-years from Earth.  It was called Thor's Helmet because its bubble shape and filaments resemble the helmet representations of the Norse god.

 The nebula is located about 8 degrees northeast of Sirius, the brightest star in the sky.  The central star of Thor's Helmet is the Wolf-Rayet WR 7 star (HD 56925, HIP 35 378).  WR 7 is a large, bright star located near the center of the helmet.  The star is believed to be about 280,000 times brighter than the Sun, 16 times more massive and 1.41 times larger.  It has a surface temperature of 112,000 K and is about 20 million years old.

  Wolf-Rayet stars are evolved stars, exceptionally hot and luminous in the last stage of their life, that are rapidly losing mass as a result of a strong high-speed stellar wind.  The WR 7's stellar wind speed is estimated to be 1,545 km/s.  The star will end its life as a bright supernova in the relatively near future.

 Thor's helmet was formed as a result of WR 7's expulsion from its outer layers.  The overall shape of the helmet is a result of the interaction of the expanding bubble of expelled material with giant molecular clouds in the area.  The ionizing radiation from WR 7 is responsible for the nebula's glow, visible at radio wavelengths to X-rays.

 Different parts of Thor's helmet are expanding at different rates, estimated at 10 km/s to ​​30 km/s or more.  The different rates of expansion place estimates of the nebula's age in the range between 78,500 and 236,000 years.

  Thor's helmet can be seen on 6-inch or smaller telescopes, but only appears as a diffuse stain on smaller instruments.  A 10-inch telescope will show some of the arcs that extend from the central region of the nebula.

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