How Elements Are Formed in Stars (stellar nucleosynthesis)

Elements are formed in stars through fusion processes that build up atomic nuclei step by step, increasing their atomic number. Small stars produce lighter elements, massive stars produce heavier ones, and supernovae form the heaviest. These processes explain how elements are formed across different environments: in the Big Bang, in stellar fusion, and in supernova nucleosynthesis.

Elements are formed in stars through a process called stellar nucleosynthesis. This involves nuclear fusion reactions that take place under extremely high temperatures and pressures in the cores of stars.

Formation of Light Elements

Before stars existed, the Big Bang created the first elements. This process is known as Big Bang nucleosynthesis. It occurred within the first few minutes of the universe’s existence and produced mainly:

• Hydrogen (H) – the simplest element
• Helium (He) – formed from the fusion of hydrogen
• Small amounts of Lithium (Li)

This explains how elements are formed in the Big Bang theory, but only the lightest ones.

The Big Bang created only the lightest elements—everything heavier formed inside stars or supernovae.

Stellar Nucleosynthesis: Formation of Heavier Elements in Stars

Once stars formed from hydrogen gas clouds, they began to produce heavier elements through nuclear fusion. This is how elements are formed in stars and how stars act as element factories.

Hydrogen Fusion (Main-Sequence Stars)

When stars form, hydrogen fusion begins in their cores. This is the main process powering stars like our Sun.

• In this stage, four hydrogen nuclei (protons) fuse to form one helium nucleus, releasing energy.
• This fusion occurs through two main pathways:
  • The proton-proton chain – dominant in smaller stars like the Sun; a step-by-step fusion of protons
  • The CNO cycle – common in larger stars; uses carbon, nitrogen, and oxygen as catalysts to speed up hydrogen fusion

Hydrogen fusion increases atomic number from 1 (hydrogen) to 2 (helium) and is the foundation of stellar element formation.

Helium Fusion (Older Stars)

When hydrogen is used up:

• Helium atoms fuse together in the triple-alpha process
• This forms carbon (C)

Helium fusion builds elements like carbon and oxygen, increasing atomic numbers and building the early periodic table.

Fusion of Heavier Elements

In more massive stars:

• Carbon fuses with helium to form oxygen (O)
• Further fusion reactions create neon (Ne), magnesium (Mg), silicon (Si), and sulfur (S)
• Eventually, iron (Fe) is formed

These fusion steps explain how elements are formed in stellar nucleosynthesis, building up the periodic table inside stars.

Larger stars act as forges for much of the periodic table, forming elements up to iron through a series of fusion steps.

Why Iron Is the Fusion Limit

Fusion up to iron releases energy, which supports the star against gravity. But fusing iron (Z = 26) or heavier elements uses energy, rather than producing it.

• Once a star builds an iron core, it can no longer support its own weight.
• This leads to core collapse in massive stars, setting the stage for a supernova.

Fusion ends at iron. Elements heavier than iron need a different process to form.

Supernova Nucleosynthesis

Elements heavier than iron cannot be formed by fusion in stars because it requires more energy than it produces. These elements are formed during supernova nucleosynthesis.

What Is Supernova Nucleosynthesis?

• When massive stars die, they explode in a supernova
• During this explosion, rapid neutron capture (r-process) creates heavy elements
• This includes gold (Au), platinum (Pt), uranium (U), and many others

This process explains how elements are formed in supernova and how are elements formed in supernova nucleosynthesis.

Role of Stars in Building Periodic Table

The periodic table reflects the elements formed through cosmic processes:

• Big Bang formed H, He, and Li
• Stars formed elements up to Fe
• Supernovae formed elements beyond Fe

This answers the question How elements are formed in the periodic table — it’s a result of multiple cosmic processes over billions of years.