Cygnus Loop Nebula

 The nebula lies about 1,500 light-years away, and is a supernova remnant, left over from a massive stellar explosion that occurred between 5,000 to 8,000 years ago.

During the middle of the 19th century, a star system known as Eta Carinae suddenly became the second-brightest star in the night sky, then gradually faded again. Known as the Great Eruption, this event released about 10 percent of the energy that would have been liberated if the star had gone supernova, and caused the star to shed approximately 10 Suns’ worth of mass. Yet somehow, Eta Carinae survives to this day. Understanding the behavior of Eta Carinae (which is estimated to still hold at least 100 times the mass of our Sun) will provide astronomers with knowledge of the end-stages of very massive stars, and allow them to distinguish between eruptions and supernova explosions.