Earth & Sky

What are stars made of?

What are stars made of?

Stars are massive spheres of hot gas held together by gravity. Some stars are extremely massive – and some less massive – but most stars start out almost entirely made of hydrogen – with a smaller amount of helium.

Hydrogen and helium atoms are the lightest atoms – and they’re also the oldest material in the universe. Only the first stars were made from this pristine stuff of matter created in the Big Bang. Stars don’t stay purely hydrogen and helium – because stars are so hot inside that hydrogen nuclei fuse to form heavier, more complex elements.

This is fusion. Each fusion reaction results in a bit of leftover mass – which converts to light and heat. That’s how stars shine. Some of the first stars – formed early in the history of the universe – finished the first part of their evolution several billion years ago. And some of those stars exploded as supernovae – and spewed heavy elements like carbon, oxygen, and iron into space.

Now most stars – including our sun – still contain mostly hydrogen and helium. But the sun and other stars also contain the heavier, more complex elements – produced inside earlier generations of stars.

A star is a balance between gravity that wants to pull the star together and the pressure from the energy from the nuclear reactions that wants to push the star apart and sends heavier elements spewing back into interstellar gas and dust .

The outer layers of some stars are so empty that they can be described as red-hot vacuums. Other stars are so dense that a teaspoonful of the material composing the outer layers would weigh several tons.

As generations of stars come and go they cook up heavier elements in their cores – and then when they end their lives many of them go through stages of expelling material ‘enriched’ in elements heavier than helium explosively or gently back into space to mix with the gas already there which is mostly hanging out since the Big Bang.

Our thanks to the National Science Foundation.