This fate is ensured by the fact that, unlike all of the previous stages of nuclear fusion that generated energy, iron requires energy to fuse into heavier elements. In a sense the core becomes a massive energy sink and as its mass nears the Chandrasekhar mass limit ($/sim1.

Low mass stars and high mass stars share similarities and differences. One of the similarities is they both start the same way, with a huge collection of gases, primarily hydrogen and helium. Another similarity would be the way they generate their energy, through a process known as nuclear fusion.

For low-mass stars (left hand side), after the helium has fused into carbon, the core collapses again. As the core collapses, the outer layers of the star are expelled. A planetary nebula is formed by the outer layers. The core remains as a white dwarf and eventually cools to become a black dwarf.

Death of low-mass stars Low-mass stars are those that end up as white dwarfs. Our Sun is an example of a low-mass star; Betelgeuse is an example of a high-mass star.

After the hydrogen in the star’s core is exhausted, the star can fuse helium to form progressively heavier elements, carbon and oxygen and so on, until iron and nickel are formed. Supernova explosions result when the cores of massive stars have exhausted their fuel supplies and burned everything into iron and nickel.

In a massive star, the weight of the outer layers is sufficient to force the carbon core to contract until it becomes hot enough to fuse carbon into oxygen, neon, and magnesium. This cycle of contraction, heating, and the ignition of another nuclear fuel repeats several more times.