Because helium in fusion reactions is not very stable, the core starts to grow and shrink very quickly. Strong stellar winds blow the gas and plasma in the outer layer of the star outwards. These gases form a cloud around the core of the star. As more and more of the gas moves away from the star, deeper and deeper layers at higher and higher temperatures are sent out. When the gas heats up to around 30,000 degrees kelvin, the gas starts to glow. The cloud has then become a planetary nebula.