2.3.1 Planetary nebula
The star keeps on burning until the core runs out of helium, then it collapses again.
The contraction heats up the core, and the temperature rises so much that helium burning starts in a shell around an inert core of carbon and oxygen. This leads to a further expansion and cooling of the star.
Helium shell burning is not a stable process, and leads to a series of pulses, where the shell alternately expands and contracts. Each pulse can last a few thousand years, and is marked by a variation in energy output of the star. The pulses can be so strong that they push off the outer layers of the star as a planetary nebula – beautiful shells of gas ejected from and heated by the dying star.
Planetary nebulae come in a wide variety of shapes and colours. The name planetary nebula is an unfortunate accident. When they were first discovered, they looked like giant gas planets, but since, it has been discovered that the only link they have to planet formation is that they produce elements that are needed for planet building.
Next, you’ll hear about supernovae.