Stellar Evolution

Ayan Joshi

Every star visible in the sky has a story. Some stars burn for billions of years at a steady and calm output of energy. Other stars live fast and die young, but when they die, they end in violent explosions that can outshine other galaxies. They all begin in clouds of gas scattered throughout space and coalesce to form giant balls of light. It’s important to understand how and why stars change because without stars, the universe would be very different. No planets, no humans, no life in general. The study of stellar evolution is the study of how stars are born, how they live, and how they die, and why their lives matter to everything we know.
All stars are born in a nebula, a massive region in space that contains stellar matter, gas, and dust. All this matter, gas and dust are drawn together using Newtonian gravity shown in his Law of Gravity, and as more and more matter is pulled towards this object, it begins to rotate. At this point, these rotating objects will become young stellar objects, or YSOs. YSOs are also known as protostars. In the protostar phase, there is simply a slow, steady accumulation of dust and gas, increasing the mass and increasing the internal temperature of the YSO through the condensing. Although this process can be described in a few short sentences, it is imperative to understand that this process can last up to millions of years.
Because of centripetal force inflicted by the rotating star, matter begins to coalesce. After a period of time that lasts around 50 million years, the pressure of the protostar is so high and the temperature of the YSO reaches more than 10 million degrees Celcius, the first hydrogen atoms will be squeezed together through fusion and helium is formed. Nuclear fusion has been initiated and since a star is defined by nuclear fusion at its core, the protostar has evolved and a brand new star is born. Whenever a star is born, it is classified into a category of main-sequence stars, which have nuclear fusion of hydrogen into helium.
Main sequence stars come in all sizes and colors. For example, the Sun is a main sequence star, and it’s yellow and moderately sized. There can be extremely large main sequence stars and they can be different colors as well. One key rule to understand is that the larger the mass of a star, the more fusion it goes through and the quicker the star burns up and dies. So, on the contrary, if a star is really small, it has less nuclear fusion, and smaller mass stars can live for a long time. Another rule that is important, but also confusing is that the hotter the star, the bluer it gets, and the colder the star, the redder it gets. The reason why many confuse this is because as humans, the color red is commonly associated with heat and the color blue reminds one of the cold.
During this main sequence phase, the force and energy from the nuclear fusion at the core of a star is strong enough to resist the inward gravitational pull, which would lead to catastrophe. This concept is called hydrostatic equilibrium and is a big defining factor of main sequence stars. This phase of equilibrium can hold for hundreds of thousands of years all the way to a few billion years. What the star becomes next is a little more complicated. Stars are all about mass in the sense that mass determines a star’s future. From the end of this phase of equilibrium, there are two paths on which a star can proceed. The two categories are stars with less than eight solar masses, and stars with greater than eight solar masses. One solar mass is equal to the mass of the Sun, so the Sun is in the first category.
If the star has less than eight solar masses, the main sequence star will next turn into a red giant star. This occurs because all the hydrogen at the center of the star’s core will run out, causing the star to use hydrogen further from its core. When this happens, the star gradually expands, but cools as the color turns from a yellow to a dark red. These red giant stars can be hundreds of times larger than the original main sequence star. The hydrogen used for nuclear fusion will run out and be replaced with helium. The helium can be used for fusion and be turned into carbon. It is here in a star’s life where many heavy elements can be found due to nuclear fusion. After certain amounts of fusion, the star doesn’t have much to burn, causing the star to shrink against the force of gravity.
The final stage of a low mass (less than eight solar masses) star is when the star becomes a planetary nebula. When the star begins to shrink, it becomes very unstable and because of this, the outer layers of the star are gushed off into the space around it. Layer after layer, the gas drifts into the space around it leaving a star called a white dwarf. A white dwarf is a very small and incredibly dense star. The light that is seen from a white dwarf is leftover heat and energy from previous stages of stellar evolution. However, if a white dwarf is above 1.44 solar masses, a limit known as the Chandrasekhar limit, the white dwarf will collapse and condense into a black hole. Otherwise, the white dwarf will continue to cool off over the next billions of years until it turns into a black dwarf, a white dwarf that has cooled so much that it cannot project any light or emit any heat.
Talking back about further evolution of main sequence stars, if the star has greater than eight solar masses, it becomes a red supergiant. Red supergiants follow a similar pattern to red giants; both stars use hydrogen and helium for fusion. Supergiants then use carbon and keep on using heavier and heavier elements for nuclear fusion, until the star has a core of iron. Nuclear fusion halts at iron because iron is the most stable element, with a tight nucleus. Trying to fuse iron will only consume energy, not release it, as seen with lighter elements. Because the star has no force from the nuclear fusion, there is no resistance to the inward gravitational collapse.
With absolutely no force balancing the gravity, the star collapses in an event called a supernova. This is a core-collapse supernova, the most common type of supernova. These collapses can occur at up to 30,000 kilometers per second and once it explodes, there is abundant energy released into space and new elements are created. Only the core of the star is left, and it will turn into either a neutron star or black hole (more on this soon). There is another type of supernovae that is less common, but equally important: Type 1a supernovae. This type of supernova occurs in a binary system with a low-mass star (less than 8 solar masses) or a red giant and a white dwarf. Because of the white dwarf’s density, it has a very strong gravitational pull. The white dwarf, using gravity, pulls material from the other paired star, and it becomes larger and larger, in a process called accretion. The white dwarf will gain enough mass to cross the Chandrasekhar limit, and will explode as it collapses on itself. Nothing of the star is left behind, resulting in one of the brightest events ever visible in the entire universe.
After the core-collapse supernova, the core can either make neutron stars or black holes. If the star was massive, the core collapses and all the matter condenses into a small star that is around 25 km wide. It is the second densest object in the entire universe, after a black hole, of course. These neutron stars are called such because it crushes the star so much that protons and electrons squash together, leaving only neutrons. Pulsars are a type of neutron star that rapidly spin hundreds of times every second, and they release beams of radiation.
If before the core-collapse supernova, the star was incredibly massive (even larger than the star that made a neutron star), then the supernova will cause the extremely large star to condense into an extremely small, and dense object, called a black hole. The gravity of a black hole is so strong that not even light (travels at 300,000,000 m/s) can escape it. The black holes grow by pulling different gasses, dusts, or any other matter, and possibly even other black holes.
This is why every star is unique. From the beginning of a star’s life to the end, stellar objects change and grow incredibly. This story is what science has proved about how stars change, and this is the story of a star.


Works Cited
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