Question

What causes a low mass star to become a planetary nebula? What type of remnant will be left behind in this case? What element does not fuse that leads to the low mass star becoming a planetary nebula?

Answer 1

Low mass stars spend billions of years fusing hydrogen to helium in their cores via the proton-proton chain. Over its lifetime, a low mass star consumes its core hydrogen and converts it into helium. Eventually, nuclear fusion exhausts all the hydrogen in the star's core. When hydrogen fusion can no longer happen in the core gravity begins to collapse the core again Eventually the core becomes hotter and denser and reaches a temperature of 100 million K, and helium nuclei begin to fuse into carbon After about 100 million years, the star fuses all its core helium into carbon Eventually gravity can no longer contain the outer layers of the red giant and the star ejects these layers into space The remaining carbon core is still very hot and emits ultraviolet radiation that ionizes the gas in the expanding shell and makes it glow brightly. This glowing gas is called a planetary nebula.

As the outer layers drift away from the star, the core is left and shines brightly and is very hot (100,000°C+) - the core will turn into a white dwarf ie, the core of the star is left

carbon cannot fuse so the star can no longer hold its layers causing it to turn into a planetary nebula.