THE LIFE OF A STAR: THE END (BUT NOT REALLY)

THE LIFE OF A STAR: THE END (BUT NOT REALLY)

In our last chapter, we discussed the main-sequence stage of a star. In this chapter, we'll be discussing when the main-sequence stage ends, and what happens when it does.

        In order to live, stars are required to maintain a hydrostatic equilibrium - which is the balance between the gravitational force and the gas pressure produced from nuclear fusion within the core. If gravity were to be stronger than this pressure, the star would collapse. Likewise, if the pressure were to be stronger than gravity, the star would explode. It's the balance - the equilibrium - between these two forces which keeps a star stable. Stars contain hydrogen - their primary fuel for fusion - in their core, shell, and envelope. The heat and density in the core is the only area in a main-sequence star that has enough pressure to undergo fusion. However, what happens once hydrogen runs out in the core is where things start to get ... explosive.

        For this, we'll be having two discussions: what happens in low-mass stars, versus what happens in high-mass stars.

                                                                      ~ Low-Mass Stars ~

        Low-mass stars are classified as those less than 1.4 times the mass of the sun (NASA). While low-mass stars last a lot longer than their higher-mass counterparts, these stars will eventually have fused all of the hydrogens in their core. Because the core doesn't have enough pressure to fuse helium (as it takes more pressure and heat to fuse heavier elements than less), gas pressure stops and gravity causes the core to contract. This converts gravitational potential energy into thermal energy, which heats up the hydrogen shell until it is hot enough to begin fusing. It also produces extra energy, which overcomes gravity in small amounts and causes the star to swell up a bit. As it expands, the pressure lessens and it cools. The increased energy also causes an increase in luminosity. This is what is now called a Red Giant star (ATNF). 

        Red Giants grow a lot, averagely reaching sizes of 100 million to 1 billion kilometers in diameter, which is 100-1,000 times larger than the sun. The growth of the star causes energy to be more spread out, and so cools it down to only around 3,000 degrees Celsius (still though, pretty hot). Because energy correlates with heat, and the red part of the electromagnetic spectrum is less energized, the stars glow a reddish color. Hence, the name Red Giant. Due to the current size of the sun, we can conclude that it will eventually become a Red Giant. This could be a big problem (literally), as the sun will grow so large that it will either consume Earth or become so close that it would be too hot to live. However, this won't be happening for around 5 billion years, so there's nothing immediately to worry about (Space.com).

        As more hydrogen is fused within the shell of the Red Giant, the produced helium falls down into the core. The increased mass leads to increased pressure, which leads to increased heat. Once the temperature in the core reaches 100 K (at which point the helium produced has enough energy to overcome repulsive forces), helium begins to fuse. This process is called the Triple Alpha Process (as the helium being fused are actually alpha particles, helium-4 nuclei), where three of the helium particles combine to form carbon-12, and sometimes a fourth fuses along to form oxygen-16. Both processes release a gamma-ray photon. In low-mass stars, the Triple Alpha Process spreads so quickly that the entire helium ore is fusing in mere minutes or hours. This is, accurately called, the Helium Flash. 

        After millions of years, the helium in the core will run out. Now the core is made entirely of the products of helium fusion: carbon and oxygen nuclei. As the fusion stops, gas pressure shrinks, and gravity causes the star to contract yet again. The temperature needed to fuse carbon and oxygen is even higher, as heavier elements require more energy to fuse (because, with more protons, there's more Coulombic Repulsion). However, this temperature cannot be reached, because the gravity acting on the core is not strong enough to create enough heat. The core can burn no longer.

        The helium shell of the star begins to fuse, as gravity IS strong enough to do that. The extra energy and gas pressure created causes the star to expand even more so now. The helium shell is not dense enough to cause one single helium flash, so small flashes occur every 10,000-100,000 years (due to the energy released, this is called a thermal pulse). Radiation pressure blows away most of the outer layer of the star, which gravity is not strong enough to contain. The carbon-rich molecules form a cloud of dust which expands and cools, re-emitting light from the star at a longer wavelength (ATNF).

        But what happens after the shell is fused? We'll get back to that in Chapter 7, where we'll discuss White Dwarfs and Planetary Nebulae.

        It's also important to note that not every low-mass star needs to become a Red Giant. Stars that are smaller than half the mass of the Sun (like, Red Dwarfs) are fully convective, meaning that the surface, envelope, shell, and core of star materials all mix. Because of this mixing, there is no helium buildup in the core. This means that there is not enough pressure to fuse the helium in fully convective stars, and so they skip the contraction and expansion phases of Red Giant Stars. Instead, with no gas pressure to counteract gravity, the star collapses in on itself and forms a White Dwarf (Cosmos).

                                                                     ~ High-Mass Stars ~

         High-mass stars are classified as those more than 1.4 times the mass of the sun (NASA). High-Mass Stars, as opposed to their Low-Mass counterparts, use up their hydrogen fast, and as such have much shorter lives. Just like Low-Mass Stars, they'll eventually run out of hydrogen in both their core and their shell, and this will cause the star to contract. Their density and pressure will become so strong that the core becomes extremely hot, and helium fusion starts quickly (there is no helium flash because the process of fusion will begin slowly, rather than in "a flash"). The release of energy will cause it to expand and cool into a Red Supergiant, and will also begin the fusion of the helium shell. 

        Once all of the helium is gone, leaving carbon and oxygen nuclei, the star contracts yet again. The mass (and the gravity squeezing it into a very small space with a very large density) of a high-mass star will be enough to generate the temperatures needed for carbon fusion. This produces sodium, neon, and magnesium. The neon can also fuse with helium (whose nuclei is released in the neon fusion) to create magnesium. Once the core runs out of neon, oxygen fuses. This process keeps going, creating heavier and heavier elements, until it stops at iron. At this point, the supergiant star resembles an onion. It is layered: with the heavier elements being deeper within the star, and the lighter elements closer to the surface (ATNF).

        But what happens after the star finally gets to iron? We'll get back to that in Chapters 8, 9, and 10 - where we'll discuss Supernovae, Neutron Stars, and Black Holes.

        We’re nearing the end of our star’s life, and now it’s time to look into the many ways it can go out.

        If our first five chapters were all about life, these next five will be all about death.

First -  Chapter 1: An Introduction

Previous -  Chapter 5: A Day in the Life

Next - Chapter 7: What Goes Around, Comes Around

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