Low-mass stars, like our Sun, become red giants when they exhaust the hydrogen fuel in their cores after billions of years. With no hydrogen left to sustain fusion, the core contracts, heating up and triggering hydrogen fusion in a surrounding shell. This causes the outer layers to expand dramatically, cooling and reddening the star’s surface, transforming it into a red giant. During this phase, the star swells up to 100 times its original size, becoming brighter but cooler, with temperatures dropping to around 3,000-5,000 K.
Once a low-mass star burns through its fuel, its outer layers are slowly and gently stripped away in what is called a planetary nebula, a bright shining shell of gas is lit up by the newly exposed core. The beauty of planetary nebulae, like the Cat's Eye Nebula, lies in their complex shapes, shaped by stellar winds and magnetic fields. Planetary nebulae do not last long; they dissipate in 10,000–20,000 years, yet they leave behind important materials like carbon and nitrogen in space.
This core from the planetary nebula is stripped of its outer layers and becomes a white dwarf—a small, dense, and extremely hot remnant composed mostly of carbon and oxygen. No longer undergoing fusion, the white dwarf cools and fades over billions of years, radiating its residual heat into space. The surrounding planetary nebula disperses within about 10,000–20,000 years. White dwarfs get cooler over billions of years and theoretically become black dwarfs. However, the universe is too young for any black dwarfs to exist yet.
High-mass stars end in a catastrophic explosion known as a supernova. The iron core grows in mass until it reaches critical mass (the Chandrasekhar limit, ~1.4 solar masses) and collapses in on itself because of the sheer amount of gravity. The collapse recoils and ejects the star's outer layers at speeds of up to 19,000 miles per second. One supernova can shine as bright as an entire galaxy for weeks and disperse heavy elements like iron and gold into space. What is left of the core depends on its mass.
When a massive star (8-20 times the Sun’s mass) exhausts its nuclear fuel, its core collapses under gravity, triggering a supernova explosion that expels its outer layers. The core, compressed to extreme densities, forms a neutron star—a compact object about 10-20 km in diameter, composed almost entirely of neutrons. The immense gravitational force overcomes electron degeneracy pressure, packing protons and electrons into neutrons, resulting in a star with a mass 1.4-3 times that of the Sun, often observed as a pulsar if it rotates rapidly and emits radiation beams.
For stars more massive than about 20 solar masses, the core collapse during a supernova is so extreme that it surpasses the neutron star limit. The core’s mass exceeds around 2-3 solar masses, where neutron pressure cannot resist gravity. The collapsing core forms a black hole, a region where gravity is so intense that not even light can escape. The supernova explosion may still occur, ejecting outer layers, but the core becomes a singularity, surrounded by an event horizon, marking the black hole’s boundary.
The death of a star is not the end but a new beginning. The material ejected by planetary nebulae and supernovas seeds space with heavier elements forged during these fusions. Supernovas are particularly crucial, producing elements beyond iron through rapid neutron capture. These elements, such as carbon, oxygen, iron, and more, are the building blocks of planets, atmospheres, and life itself. The dense gas and dust merge to create new nebulae, repeating the cycle. Our Solar System formed from the remnants of ancient stars, and the atoms in our bodies once existed within star cores. This universal recycling illustrates how everything in the universe is connected, from the smallest organisms to giant galaxies.