Stars are massive, luminous, spherical celestial bodies that serve as the fundamental building blocks of galaxies. They are composed mostly of plasma and radiate light and heat resulting from the fusion of hydrogen atoms in a tremendous nuclear process known as nuclear fusion. Stars live for billions of years and exhibit immense diversity in size, color, and brightness.
Star formation begins from giant gas clouds called "Nebulae." These nebulae collapse under gravity, increasing in density and temperature to form a "Protostar." When the protostar’s core temperature reaches a certain point, nuclear reactions begin, fusing hydrogen to form helium. This releases a massive amount of energy, resulting in a "Main Sequence Star" like the Sun.
Stars remain in this stage for billions of years before the hydrogen in their cores begins to run out. Low and medium-mass stars, like the Sun, transform into Red Giants, then shed their outer layers to form a Planetary Nebula, leaving their core behind as a White Dwarf. High-mass stars, however, explode at the end of their lives as Supernovae, turning into Neutron Stars or Black Holes.