The Life Cycle Of Stars

Recent theories on the birth, life, and death of stars have led us to the model we’ll cover. The diagram below shows us the different paths that stars take in their life cycle.

Forming a Main Sequence Star

The life cycle of all stars begins with a cloud of dust and gas in outer space, which we call a nebula. The nebula is mostly made up of hydrogen.

Over time, gravity pulls the fragments of gas and dust together, forming a hot ball of gas called a protostar. As more particles collide and join the protostar, it gets larger in size, which increases the force of gravity. The increase in gravity attracts more dust and gas, but it also makes the protostar denser.

The particles inside start to collide more often and more frequently, which raises the temperature of the protostar. When the temperature gets high enough, hydrogen nuclei start fusing together to form helium nuclei. We call this process nuclear fusion.

Nuclear fusion releases a large amount of energy, and this constant release of energy keeps the core of the star hot.

The outward pressure, caused by the high temperature and the release of energy, is balanced by the force of gravity. This allows stars to remain stable for billions of years. At this point, we call the star a main sequence star.

  • Our Sun is currently in this stage

Forming a Red Giant

Eventually, the star will run out of hydrogen, which fuels the nuclear fusion reactions. If there is no hydrogen, then nuclear fusion can no longer take place in the star and there will be no more energy released. This means that the inward force of gravity will take over and contract the star into a relatively small ball.

However, it will eventually get so hot and dense that nuclear fusion starts to take place again. At this point, the star will also start to expand again. But this time, the nuclear fusion reactions will not just form helium; they will also form heavier elements. This includes elements up to iron (Fe) on the periodic table.

The life cycle of a star depends on its original size, which determines how much the star will expand.”

  • If it was an average-sized star (like our Sun), then it will form a red giant
  • If it was a massive star (larger than our Sun) then it will form a red supergiant

The massive stars have much shorter lifespans.

Path of a red giant

Planetary nebula

After the second stage of fusion, the red giant becomes unstable and ejects its outer layer of dust and gas, forming what we call a planetary nebula.

White dwarf

The ejection of the planetary nebula leaves behind a hot, dense, solid core. The remaining core is known as a white dwarf, as it gives off a lot of light, appearing white. The white dwarf is relatively small and does not host any nuclear fusion reactions.

black dwarf

Over time, the white dwarf cools down and gets darker, as it emits all of its energy. Eventually, it becomes a black dwarf. As it no longer has enough energy to emit light, it appears black.

Path of a super red giant

When a star becomes a super red giant, it shines brightly due to increased nuclear fusion. After many cycles of expansion and contraction, the star will collapse and explode into a supernova. This forms elements that are even heavier than iron, which are expelled across the universe.

The fate of the core of a supernova depends on its mass, leading to one of the two paths:

  • The star can condense into a very dense core, which we call a neutron star
  • The star can collapse on itself and form a black hole

Black holes

We call them ‘black holes’ because they are so dense that their gravity is able to pull in any light that gets close. This means that they appear invisible. Nothing can escape from a black hole, not even light.