When asking someone what they see in this image, the answers are as diverse as they are interesting: a dementor from Harry Potter, a woman’s head facing to the left, an elephant from behind, Sulley from Monsters Inc, a human heart, a deformed hand…the list goes on and on.
Unfortunately, the actual name of the nebula does not correspond to any of the objects that we see when we look at this image. However, this particular photograph was taken in the ultraviolet part of the electromagnetic spectrum, which is part of the reason that the name of the nebula does not match up with what we see when we look at it—the nebula looks vastly different if you look at images that were taken at different wavelengths.
This picture of the Cygnus Loop Nebula was taken by NASA's Galaxy Evolution Explorer in March of 2012. In the photo, we see the eerie tendrils of hot dust and gas glowing brightly as they move away from the supernova remnant, expanding at about 100 km (60 miles) per second. These radiant filaments of material were heated by the shockwave from the supernova, which is still spreading outward from the original explosion. There has been a bit of debate surrounding this object, and scientists aren’t entirely sure when this beast was created. Nonetheless, the nebula is believed to be about 1,500 light-years away, and the massive stellar explosion that created it is thought to have occurred between 5,000 and 8,000 years ago.
The original supernova would have been bright enough to see from Earth with the naked eye. Alas…that we were not around to see it…
Below is an image of the Cygnus Loop nebula supernova blast wave. The image shows the blast wave as it is overcoming dense clumps of gas. And as you can see, in spite of HST's high resolution, these points cannot be resolved. This means that the clumps of gas must be small enough to fit inside our solar system, making them relatively small structures by interstellar standards. As NASA notes, "the bluish ribbon of light stretching left to right across the picture might be a knot of gas ejected by the supernova; this interstellar "bullet" traveling over three million miles per hour (5 million kilometres) is just catching up with the shock front, which has slowed down by ploughing into interstellar material."