Question

How do nebula form stars? If all the hydrogen is already fused by the dead star. how are new stars are born with a fresh supply of hydrogen?

Answer 1

Stars don't consume all of the hydrogen they are made from.

Explanation:

Fusion reactions in stars only takes place in the core or the layers around the core where temperatures and pressures are high enough.

When a large star comes to the end of its life, its core contains heavy elements, mainly iron. The outer layers of the star are still hydrogen rich. Energy producing fusion reactions can no longer take place in the core and it collapses under gravity to form a neutron star or a black hole.

The core collapse causes a supernova explosion which blows away the hydrogen rich outer layers to form a nebula.

So, there is often still enough hydrogen left over to make a new star.