A star is a massive, luminous ball of ionized gas (or plasma) that form in giant, slowly rotating clouds of dust called stellar nurseries. A cloud’s gravity gradually causes it to condense into a sphere; as it collapses, this ball becomes hotter and denser, eventually forming a protostar. Under incredibly high pressure, atoms that normally repel each other begin to collide and combine in a process called nuclear fusion. In the case of a new star, molecules of hydrogen collide to form helium. When the protostar’s heat and pressure hit 1 million °C, fusion begins and the star ignites.
Eventually, a protostar’s expansion from fusion, and contraction from gravity balance out. This forms a dwarf star (like our sun), a star on the main sequence: the longest leg of a star’s lifespan, defined by hydrogen-burning at its core. As a star burns, it slowly replaces the hydrogen at its core with helium. Once this hydrogen core is depleted, the main sequence ends.
How long a star stays on the main sequence and what happens afterward depends on its mass. Medium sized stars like the sun spend an estimated 10 billion years in main sequence. After main sequence, a medium star’s outer layer cools and expands greatly, forming a red giant. In lower mass stars, the helium core remains inactive, and hydrogen-burning continues in an outer shell. But more massive medium stars eventually begin helium-burning, creating carbon at their core. Once the helium core is depleted, the star expands again, this time become brighter and bluer. Eventually, these giants can no longer perform fission and the outer shell is shed as a beautiful planetary nebula – at its center, a small, white dwarf. With no fuel to burn, white dwarves dim over time, eventually fizzling out as black dwarves.
High-mass stars – 10 to 100 times as massive as the sun – become supergiants post-main sequence. After burning all the helium at their core, they begin carbon-burning, then neon-burning, oxygen-burning, and silicon-burning. Eventually, the star’s core is converted into unreactive iron and core fusion ends. In time, the iron core too large for the star to support, and it suddenly collapses, creating a supernova explosion. In smaller massive stars, a neutron star – composed entirely on unreactive neutrons – remains. The largest stars create black holes when they die; their remnants are so massive, not even light can escape their gravitational pull.
Finally, low-mass stars – half the size of our sun or less, these stars – called red dwarves burn hydrogen very gradually, and spend up to hundreds of billions of years on the main sequence. Since the estimated age of the universe is 13.7 billion years, it’s assumed that not a single red dwarf has died since the universe began – and they won’t until it’s several times older. Though they get less attention then their bigger, brighter brothers, these little guys might just outshine time itself.