Stars die because they run out of fuel to maintain a balance of the star pressure and gravity, causing the star to die through cooling or a stellar explosion.
Larger stars burn through their hydrogen cores far faster than smaller, midsize stars. Larger stars burn through their fuel far faster than smaller stars, dying after several million years as they explode into supernovae. When giant stars die, they are extinct in full splendor. Their enormous size means that gravitational pressure is sufficient for not only melting hydrogen but helium as well.
Once the helium atoms are consumed, an average-mass star, like our sun, simply runs out of fuel. When hydrogen runs out in the core, those stars fuse helium to carbon, much as Earth does. As a star's hydrogen fuel is converted to helium and then to certain heavier elements, more heat is required to trigger a nuclear meltdown.
For stars that are about as massive as the sun, this load raises the temperature in its core until its helium is hot enough to fuse carbon. Once the helium is gone, the masses of these stars are large enough for the carbon to fuse into heavier elements like oxygen, neon, silicon, magnesium, sulfur, and iron. If a star is sufficiently massive, its core can get hot enough to sustain more exotic nuclear reactions, which consume the helium and yield all manner of heavier elements, all the way down to iron.
When the newly formed star runs out of hydrogen, its core will start collapsing once more. When the core of a dwarf star eventually uses up the hydrogen fuel needed to radiate outward, its outer atmosphere begins to collapse under its own weight. The gravitational pull from all of a star's mass attempts to pull the star to a small point, but the energy released from hydrogen fusion propels it outward, creating a delicate balance that may last millions, or even trillions, of years.
The Star stops producing energy and dies, but during these last stages, Stars shed their outer layers. The star begins slowly collapsing in on itself, and along the way, it attempts to exhaust all its resources to produce various materials. The collapse melts the helium layer over its hot core, creating enough heat to scald the red-giant star's outer gases with enough force that they expand past their capacity to contain themselves.
Before a star's inevitable collapse, nuclear reactions beyond the protostar's core force a dying star outward, which is what we call the Red Giant stage. Squeezed down by the unrelenting punch of gravity, the core helium would also be heated, expanding and cooling off the star's outer envelope, while the star would become luminous and turn into a giant. The smallest stars, known as red dwarfs, burn through their nuclear fuel so slowly they can survive for up to 100 billion years -- far older than the current Sun.
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