By Emily Newton 
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When you look up at the night sky, you see thousands of stars. Even the Earth’s Sun is a yellow dwarf star in the middle of its life cycle. The real question is, how did they all get there? Here’s a closer look at the life cycle of a star, and how the size and mass of one of these stellar bodies affect its existence.
Contrary to popular media, stars don’t just pop up fully formed with a series of planets surrounding them. The process takes millions and sometimes billions of years, and it all starts with a cloud of interstellar gas.
Every star in the sky began its life as a nebula, a molecular cloud of gas and dust. These nebulae consist primarily of hydrogen and helium, with some other trace elements. The clouds themselves typically range from 1,000 to 10 million tons of the Sun’s mass and may span hundreds of light-years.
Over time, the cloud will start to spin, developing a center of gravity and pulling everything in the nebula to that point. The gravity continues to grow and strengthen until, at a pivotal moment, the pressure causes the nucleus of the hydrogen and helium molecules to collapse in a process called nuclear fusion.
Sometimes stars, called brown dwarfs, start to form but don’t have enough heat and pressure to trigger nuclear fission. They’re roughly 12 to 14 times the mass of Jupiter and 0.012 and 0.014 times the Sun’s mass, but from Earth, they’re only visible on infrared telescopes. Once fusion happens, a star is born — but what happens after that?
Before moving on to what happens to each type of star during its life, there is one crucial point to touch on. There is a direct relationship between the mass of a star and its longevity.
Massive stars might have more hydrogen, but they burn through it more quickly than smaller ones to sustain their large size. Small stars don’t have to burn as brightly, so they live longer.
This is all relative, since the average lifespan of a star spans billions of years. The Earth’s Sun is approximately 4.5 billion years old and likely has enough hydrogen to burn for another 5 billion years. How does this mass-to-lifespan ratio affect the different types of stars?
O- and B-class stars are some of the largest ones you will see in the night sky. You can break their lifespan down into five stages.
Stage one occurs just after the first fusion that gives birth to this new celestial body. Both helium and hydrogen exist within the star, but at the moment, it’s only burning the hydrogen. At this stage, it’s a main-sequence star, the most stable part of its life cycle. Approximately 90% of all stars in the universe are classified as main sequence, ranging from one-tenth of the Sun’s mass to up to 200 times more massive.
Once it runs out of hydrogen, the star enters stage two. Throughout millions or billions of years, the core loses its stability. Although helium is flammable, the star doesn’t burn it. Instead, this instability causes the helium to fuse into carbon, which blends into elements like iron, sulfur and neon. At this point, the core will also turn to iron, while the outer helium shell of the star starts to expand.
The third stage lasts approximately a million years and involves a series of nuclear reactions that form additional shells around the star’s iron core.
Stage four is the most explosive time in the life cycle of a star. At some point, the core will collapse in on itself and create a massive shockwave called a supernova. What’s left of the star will expand out in all directions, destroying anything in its path.
From this point, there are two different ways the high-mass star can enter stage five. If the remaining material is 1.5 to three times larger than the Sun, it will collapse back in on itself and become a neutron star. If it’s larger than that, what’s left of the star will become a black hole instead.
Low-mass stars aren’t necessarily small. Using the Sun as a size comparison, most low-mass stars are roughly 1.4 solar units, or 1.4 times the size of the Sun. While they may be larger, they are considerably lighter in weight than G-class stars like the Sun.
The beginning of a low-mass star’s life is similar to high- and medium-mass ones — it forms from a dust cloud, initiates nuclear fusion and burns as part of the main sequence for billions of years. Once these stars exhaust their hydrogen, the core begins to collapse, becoming hotter and denser over millions of years. Eventually, this core will reach a temperature of roughly 100 million degrees Kelvin, at which point the helium molecules begin to fuse into carbon. The exterior of the star darkens to red, becoming a red giant as it expands.
As this happens, a helium flash occurs, causing the star’s exterior to expand and cool the core slightly. It goes through this cycle a few times, heating and cooling as the outer shell expands and contracts. This is where it gets interesting.
Instead of exploding like a high-mass star, it eventually loses cohesion as the gravity can no longer contain its exterior layers. It becomes what is known as a planetary nebula.
Once that happens, all that’s left is the core of the star, which continues to burn as a white dwarf. As it runs out of fuel, it eventually darkens to a black dwarf.
Most people are pretty familiar with G-Class stars — the Sun is one of them. Right now, it’s a main-sequence star in the middle of its life cycle. It’s stable, aside from the occasional solar flare or coronal mass ejection, and provides the planet with the heat and light it needs to survive.
The fate of a medium-mass star like the Sun is similar to that of low-mass stars. It will start to expand into a red giant — and will likely swallow the Earth when that happens — and then eventually diffuse into a planetary nebula, leaving a white dwarf behind.
The lifespan of a low-mass star ultimately depends on its mass. However, it can last billions to trillions of years, far exceeding the Sun’s lifespan. This is because these stars burn hydrogen fuel very slowly.
A low-mass star — like the Sun — will become a white dwarf at the end of its lifespan. After shedding its outer layers, it will retain a dense core of carbon and oxygen. As it cools, it will turn cold and dark black.
A medium-mass star is 1.4 to 3.2 times the Sun’s mass, originating from a nebula. Gravity causes the material to collapse into a hot, dense protostar. When its core reaches the right temperature, it enters the main sequence stage, where it fuses hydrogen into helium for millions of years.
After it has exhausted its use of hydrogen, the star expands into a red giant and its core starts fusing heavier elements together. This will continue until it becomes primarily composed of iron, at which point the fusion will cease.
The core will eventually collapse under its own gravity, resulting in a massive supernova explosion. A neutron star will remain within the core, marking the star’s final stage of life.
High-mass stars follow a similar life cycle to that of low- and medium-mass stars but evolve much more rapidly. They start energy production through hydrogen fusion, primarily via the carbon-nitrogen-oxygen cycle. The core contracts and heats up as it consumes the hydrogen, resulting in the fusion of heavier elements for a structured shell layer of hydrogen, helium, carbon, neon, oxygen and silicon.
The fusion cycle ends once iron takes over the core, as it requires more energy than it releases. The core will then explode into a supernova, leaving behind a dense neutron star. If the star used to be exceptionally massive, it could form a black hole.
A neutron star is what remains of the star’s core after a supernova. As time passes, the neutron star becomes denser, shrinking to 12.5 miles in diameter, shorter than the distance of a half-marathon.
The gravitational pull of these stars is over 100 billion times the amount felt on Earth, with the ability to crush the human body flat within moments. The star’s remnants are so dense that just a teaspoon of the material would weigh over 1 billion tons. Currently, humans have been unable to get close enough for a sample.
While the Sun is middle-aged, in astronomical terms, you don’t have to worry about it becoming a red giant during your or your children’s lifetimes. The Earth will likely last another 5 billion years. By then, the population will likely be among the stars themselves, and the home planet will be nothing but a distant memory.
Featured Image Credit: NASA and ESA
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