Largest nebula as seen from Earth

As seen from Earth, the vast cloud of molecular hydrogen known as Barnard’s Loop is a faintly glowing arc only visible in long-exposure images. At an average distance of around 1,600 light years and measuring around 300 light years across, it contains the famous Orion and Horsehead nebulae. If it was visible to the naked eye Barnard’s Loop would fill most of the entire constellation of Orion in the night sky.