Melotte 15 is a cluster of stars located in a much larger nebula region known as the Heart Nebula. Discovered by William Herschel in 1787, it is formed by stellar winds and lit up by the bright stars in the center. Several of these stars are 50 times the mass of the sun. The tree-shaped structure at the center is nine light years from top to bottom, and 7500 light years from Earth.