Exotic cosmic objects known as ultra-luminous X-ray sources produce about 10 million times more energy than the sun. They're so radiant, in fact, that they appear to surpass a physical boundary called the Eddington limit, which puts a cap on how bright an object can be based on its mass. Ultra-luminous X-ray sources (ULXs, for short) regularly exceed this limit by 100 to 500 times, leaving scientists puzzled.