NGC 2139 is a bright planetary nebula located in the constellation of Monoceros. It is approximately 3,000 light-years away from Earth and is known for its distinct round shape and vibrant colors, which are produced by the ionization of gas surrounding a dying star. The nebula was discovered by William Herschel in 1786. It is often studied by astronomers to understand the processes involved in stellar evolution, particularly how stars like our Sun shed their outer layers and create such nebulae as they reach the end of their life cycles.