|Designation|| NGC 2359|
|Right ascension||07h 18m 30s|
|Declination||-13° 13′ 48″|
|Type of object||Emission nebula|
|Dimensions||8' x 8'|
|Magnitude||Apparent Mag: +11.45|
|Distance from Earth||11,960 ly|
Thor's helmet (NGC 2359, Sharpless 2-298) is an emission nebula in the constellation of Canis Major. Situated not far from the star Sirius, the nebula's bubble like shape along with its filaments resemble the Norse god Thor's helmet. At the centre of the nebula is a Wolf-Rayet star, a particularly large and luminous type of star. While the nebula can be made out small telescopes, less than 6 inch, it is best observed using instruments that are 10 inches or larger.
The nebula was the first Wolf-Rayet nebula to be observed, when Francis Pease investigated the nebula between 1917 and 1919. Using the Mt Wilson Observatory, his observations agreed with earlier observations made by John Herschel. Later in 1922, Edwin Hubble demonstrated the presence of nitrogen lines in its spectrum. In 1980, the nebula was studied by the WISE satellite (Wide-Infrared Survey Explorer) in the infrared.
Properties and Structure
The nebula is believed to be around 11,960 light years from Earth and it thought to be around 30 light years across. The radiation from the central star is causing the nebula to expand though this expansion is not uniform, with different areas expanding between 10 to 30 km/s. The intricate filaments in the nebula are thought to be due to its interaction with a nearby molecular cloud, while the central bubble is due to the pressure produced by its central star.
At the centre of the nebula is situated a Wolf-Rayet star. These types of star are huge, perhaps 10-80 solar masses and extremely bright, 200,000 more luminous than the Sun. They are often found in conjunction with bright emission nebulae like Thor's helmet. It is thought such nebulae may result from the ejection of some of the star's outer layers in a stellar wind. The star at the centre of Thor's helmet (WR 7, HD 56925, HIP 35 378) produces a stellar wind that travels at around 1,545 km/s. It has a mass of 16 solar masses and is 1.41 times larger (radius) than the Sun. It is 280,000 brighter than the Sun and it temperature of 112,000 Kelvin means most of the light it emits is in the ultraviolet portion of the electromagnetic spectrum. It is this ultraviolet radiation that causes the surrounding gas to glow. This glow is visible across the electromagnetic spectrum, from radio down to x-ray. The XMM-Newton satellite detected intense x-ray emissions from the central star.