Heart and Soul nebulae. The Soul nebula (IC 1848) is at lower left, with the Heart nebula (IC 1805) at centre right. These are emission nebulae, huge clouds of gas and dust that glow as the hydrogen they contain is ionised by radiation from the hot young stars that have formed within them. The nebulae lie around 7000 light years from Earth in the constellation Cassiopeia. Different colours represent different ionised gases; sulphur is red, hydrogen is green and oxygen is blue.