Heart Nebula

The Heart Nebula in Cassiopeia is a large emission nebula around 7,500 light-years away, and the heart shape is a cavity: the young open cluster Melotte 15 at its centre contains several O-type stars, among the hottest and most massive known, whose winds and ultraviolet radiation have blown the gas outward and ionised what is left. The bright ridges and dark pillars around the centre are the denser material that has resisted, and new stars are still forming at their tips. The glowing knot at the upper right, NGC 896, is part of the same complex. William Herschel found the nebula in 1787, and at about two and a half degrees across it covers five full Moons of sky.