The death of a star depends on its mass since smaller stars are usually not hot enough to fuse heavier elements such as helium and carbon compared to those with a higher mass.
How Do Small Mass Stars Die?
Over the course of its life, the lower-mass star consumes its hydrogen core and converts it to helium. Low-mass stars spend billions of years in their cores, fusing hydrogen into helium through a proton-proton chain. Despite their gravity, the proton-proton chain does not generate enough energy to support massive stars' amazing masses.
When their cores exhaust hydrogen, those stars fuse helium to carbon, much as Earth does. When a massive star runs out of hydrogen nuclei in its core, it can just switch directly to fusing helium - as I mentioned earlier, there is no step of degeneracy or flash of helium. Massive stars obtain their carbon fuel from ash which the helium melts down into the core center of the helium-fused core.
At first, a star such as our sun fused hydrogen (as now) to helium, thus producing energy, which eventually leaves the sun in sunlight. Sun-like stars would heat up enough, after burning off hydrogen, to fuse the helium back into carbon, but this is the end of the line for the Sun. If a star is larger than roughly five times our sun's mass, a supernova will occur as hydrogen burning stops, with the remaining matter condensing into a black hole.
When the newly formed star runs out of hydrogen, its core will start collapsing once more. If the star, now at the end of its life, is less than the Chandrasekhar limit -- 1.4 times as massive as our sun -- it will be a white dwarf; above this limit, it will be a neutron star. Any star that ends its life with its core ending at more than 1.44 Solar masses (after its outer layers are expelled in the planetesimal nebula) cannot form a white dwarf but ends up in a still stranger state.
Unlike sunlike stars, which have gently blown off their outer layers in planetary nebulas and contracted down into (carbon- and oxygen-rich) white dwarfs, or red dwarfs, which have never reached the point where they are burning up with helium, and just contracted to a (helium-based) white dwarf, the more massive stars are doomed to cataclysmic events. These lower-mass stars eventually become small, dim, compact objects known as white dwarfs, which are balls of inert carbon-and-oxygen balls about as big as the Earth. For midsize stars, the problem is that once a ball of oxygen and carbon forms at the core, there is not enough mass to fuse it to something heavier.
After the helium is gone, the masses in those stars are sufficient to fuse carbon into heavier elements like oxygen, neon, silicon, magnesium, sulfur, and iron. All stars' MS phase ends when hydrogen is exhausted from the star's core.
How Do Medium Mass Stars Die?
Medium-sized stars are those too large to finish out as white dwarfs, too small to turn into black holes, that live out their dying years as neutron stars. These lower-mass stars end as small, dim, compact objects known as white dwarfs, balls of inert carbon and oxygen about as large as the Earth.
Stars far more massive than the sun (tens or hundreds of times) burn helium into carbon once they have become red giants, like middle-mass stars, but they burn carbon atoms too, producing oxygen, nitrogen, and other heavy elements, up to iron. When their cores exhaust hydrogen, these stars fuse helium to carbon, as does Earth.
After millions to billions of years, depending on how massive the star is, stars exhaust hydrogen atoms for fusion, and stars begin dying. When medium-sized stars, such as the sun, run out of hydrogen, midsize star cores shrink and become hotter. When newly formed stars run out of hydrogen, their cores will shrink again.
Instead, its core will break up, leading to an uncontrolled fusion reaction, which blows apart the outer parts of the star into a supernova explosion and then collapses down into either a neutron star or a black hole. What is left after the supernova blast is a neutron star -- the star's core collapsed -- or if it has enough mass, a black hole. The remains of the core may form a neutron star or black hole, depending on the mass of the initial star.
After a supernova explosion, the remaining core becomes a neutron star. When a star goes supernova, its core implodes, becoming a neutron star or black hole, depending on the mass. Massive stars obtain carbon fuel from an ash fusion helix dumped in the center of a helium-fused core.
When a massive star runs out of hydrogen nuclei in the core, it can switch directly to helium fusion -- like I mentioned earlier, there is no step of degeneracy or flash of helium. All stars turn red giants, but what happens after depends greatly on their mass. Intermediate-mass stars, too small to be supernovae but with masses higher than our suns, finish the lives of the medium-mass stars as planetary nebulae (PNs), releasing, once again, not so violently, but picturesquely, intriguingly, a lot of material from their processes.
Eventually, when a Red Giant star's core cools, the remaining gases float outwards in space, creating a planetary nebula. The upper layers will expand and expel material, gathering around the dying star to form the planetary nebula. In the dying last gasp, a midsize star vomits its guts, creating a sparkling planetary nebula with wisps of thin gas and dust surrounding a now exposed carbon-and-oxygen core in the middle.
How Do Large Mass Stars Die?
Unlike sunlike stars, which softly blew out their outer layers into the planetesimal nebula and contracted down into (carbon-and-oxygen-rich) white dwarfs, or red dwarfs, which never reached the point of helium burning, but merely contracted to a (helium-based) white dwarf, the more massive stars are doomed for a cataclysmic event. When massive stars die, they are blown away in all their glory. Their immense size means there is sufficient gravitational pressure to fuse hydrogen, but also helium. Really massive stars burn through theirs rapidly but are also hot enough to fuse heavier elements like helium and carbon.
After the helium is gone, the masses of those stars are big enough to fuse carbon to heavier elements like oxygen, neon, silicon, magnesium, sulfur, and iron. When their cores exhaust hydrogen, these stars fuse the helium back into carbon, much like Earth does. After a higher-mass star uses its hydrogen fuel (turning it to helium) at the core, it fuses the helium with carbon.
In lower mass stars, once helium fusion has occurred, the core never gets hot enough, nor dense enough, to fuse more elements, and thus the star begins to die. For mid-mass stars, the problem is that once the ball of oxygen and carbon forms in the core, there is not enough mass around it to fuse it to something heavier. Only the huge stars have a high enough interior pressure to crush carbon, oxygen, and nitrogen nuclei together and extract power from them.
If the star is sufficiently massive, its core can become hot enough to sustain more exotic nuclear reactions, which consume helium and yield all manner of heavier elements all the way to iron. The overall process, and we will see it happening as a star gets larger and larger, is we will get heavier and heavier elements in the core. Like low-mass stars, where you find a carbon/oxygen core surrounded by a helium-fused shell surrounded by a hydrogen-fused shell, we would expect a core in massive stars to be built the same way but to involve a lot more layers, like in the following diagram.
The vast majority of stars will simply disappear into white dwarfs, whereas all stars of starting masses from about 8M to about 150M will form a degenerate iron core which will decay into a proto-neutron star. Average-sized stars (up to 1.4 times that of the Sun) will be the least affected by extinction. Stars heavier than eight times the mass of the Sun end their lives very abruptly.
One kind, called the core-collapse supernova, occurs during the final phase in the lives of massive stars at least eight times as massive as our sun. Stars similar to our sun, stars that grow up to about the size of Jupiter called red dwarfs, and supermassive stars, which are typically hundreds of times as massive as our Sun, undergo fusion reactions.
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