The Monogem Ring is a superbubble, discovered in the soft X-ray energy band by ROSAT, that lies about 1,000 light-years away in the constellations Monoceros and Gemini. It is believed to be the highly evolved remains of a supernova that took place some 86,000 years. If it shone as brightly in visible light, it would illuminate a region of the night sky about 25° across.