There are two possible outcomes when a star uses up its helium or other elements, depending on its mass.
1. Low-mass stars (up to about 8 times the mass of our Sun):
Once a low-mass star exhausts its helium fuel in the core, it does not have sufficient mass or pressure to ignite fusion of heavier elements, like carbon or oxygen. The core then contracts due to gravity, while the outer layers of the star expand, forming a red giant. Eventually, the star sheds its outer layers, leaving behind a dense, hot core called a white dwarf. This core is composed mostly of carbon and oxygen, and it is supported against gravity by electron degeneracy pressure. Over time, the white dwarf cools down and becomes a dim, compact object, known as a black dwarf.
2. High-mass stars (more than about 8 times the mass of our Sun):
When a high-mass star consumes the helium in its core, fusion reactions to create heavier elements continue. The star undergoes multiple fusion stages wherein it fuses heavier and heavier elements, ultimately forming iron in its core. However, fusion of iron is not energetically favorable, so the core cannot sustain itself against gravity. The core collapses under its own weight very rapidly, leading to a cataclysmic event known as a supernova. During a supernova explosion, the outer layers of the star are ejected into space while the core collapses further. Depending on the mass of the core, it can result in the formation of either a neutron star or a black hole.
In summary, low-mass stars end their lives as white dwarfs, while high-mass stars end their lives in supernova explosions, forming either neutron stars or black holes.
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