IC 434
Template:Short description Template:Infobox nebula
IC 434 is a bright emission nebula in the equatorial constellation of Orion. It was discovered on February 1, 1786, by German-British astronomer William Herschel.<ref name=Ochsendorf_Tielens_2015/> The nebula is located at a distance of approximately Template:Cvt from the Sun and spans the interior of a neutral hydrogen shell with an angular size of Template:Val. At that distance, the dimensions correspond to a projected size of Template:Cvt.<ref name=Ochsendorf_Tielens_2015/>
This is an H II region that is being ionized by ultraviolet radiation from the nearby Sigma Orionis (σ Ori) multi-star system. An additional half dozen stars provide further illumination of IC 434. This H II region is one of the youngest to form around members of the Orion OB1 association of hot stars. The region has a radius of about Template:Cvt and a mass of around Template:Val The mass displaced by the advancing ionization front of IC 434 is estimated at Template:Val.<ref name=Bally_et_al_2018/> The nebula includes dust with an estimated mass of Template:Val.<ref name=Ochsendorf_Tielens_2015/>
IC 434 is surrounded by a shell of neutral hydrogen gas with the identifier GS206-17+13. The primary driving energy behind this expanding shell was likely supplied by the blue supergiant Epsilon Orionis.<ref name=Ochsendorf_Tielens_2015/> The Horsehead Nebula is a dark nebula that is silhouetted against the diffuse background of IC 434. It is protruding from the Orion B molecular cloud, which is part of the Orion molecular cloud complex.<ref name=Bally_et_al_2018/>