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Stars are enormous, glowing spheres of hot plasma, mostly made up of hydrogen and helium, held together by their own gravity.
In their cores, the pressure and temperature become so extreme that hydrogen nuclei fuse into helium—a process called nuclear fusion—which releases vast amounts of energy in the form of light and heat. Stars are born in huge, cold clouds of gas and dust known as nebulae. When a region in such a cloud collapses under gravity, it forms a dense, hot core called a protostar, which eventually ignites fusion and becomes a full-star. Over the majority of its lifetime—the “main sequence” phase—it remains stable, balancing the inward pull of gravity with outward pressure from fusion.
What happens when a star finally runs out of its hydrogen fuel depends largely on its mass. Smaller stars swell into red giants then shed their outer layers to form planetary nebulae, leaving behind dense white dwarfs. More massive stars undergo successive fusion of heavier elements, then may explode in spectacular supernovae, ultimately collapsing into neutron stars or black holes.