NGC 2139 is a bright open star cluster located in the constellation of Monoceros. It is approximately 3,500 light-years away from Earth and contains several hundred stars, many of which are young and hot. Discovered by William Herschel in 1784, NGC 2139 is notable for its relatively high concentration of stars within a small area. The cluster is a part of the Milky Way galaxy and is often studied for insights into stellar formation and evolution.