Star Lifecycle

Birth of a Star

Interstellar Clouds

Stars are formed from interstellar clouds - a massive cloud of gas and dust floating about in space. Gravity pulls this cloud closer and closer together, getting denser and denser, until...


...a protostar is formed. The temperature of a protostar is not yet hot enough to undergo nuclear fusion, but will still glow faintly just from the excitation of electrons - the same way that a heated metal bar will glow as well. Eventually, as it continues collapsing under the influence of gravity, the temperature reaches a critical threshold and the star becomes hot enough to support nuclear fusion. A chain reaction of helium being formed from isotopes of hydrogen kicks in, rapidly propagating through the core, and birthing...

Main Sequence Stars

a star at last! This star is known as a main sequence star - which simply means a star that fuses hydrogen into helium in its core. This happens through a process called the proton-proton chain, in which helium-4, the most abundant isotope of helium, is produced. This happens through the following 3 steps, generating excess heat and light at each step (as per E=mc2) which is then radiated from the star:

  1. Two 1H protons (hydrogen nuclei) fuse to produce a 2H deuteron (deuterium nucleus), plus a e+ positron and ve electron neutrino.
  2. A 2H deuteron from step 1 fuses with another 1H proton to form 3He helium-3 and release a γ gamma ray.
  3. At this point there are four possible branches, however the main branch involves two 3He helium-3 nuclei fusing, producing 4He helium-4 and two 1H protons (which can be reused in step 1).

This can be summarised by the following three nuclear equations:

  1. 1H+1H2H+e++ve
  2. 2H+1H3He+γ
  3. 3He+3He4He+1H+1H

And thus, the overall reaction can be condensed into the following monster equation:


Despite us only having 41H on the left side of the equation, it's necessary to have at least six 1H protons to kickstart the reaction, with two of the protons acting as a sort of "catalyst" before two protons are released again at the third step. However, obviously, there is no shortage of hydrogen nuclei in a main sequence star!

Stars emit much more electromagnetic radiation than protostars actively from the nuclear fusion reaction, which also prevents the star from collapsing in on itself thanks to a delicate balance of gravitational equilibrium. The vast amounts of energy released creates a force pushing outwards from the centre, opposing the gravitational force pulling the star inwards.

GravityNuclear Fusion