When you look up at the night sky, you see thousands of stars. Our own sun is a yellow dwarf star in the middle of its life cycle. 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.
In The Beginning
Contrary to popular media, stars don’t just pop up fully formed with a series of planets surrounding them. That process takes millions or even billions of years, and it all starts with a cloud of interstellar gas.
Every star in the sky started life as a nebula, which is a cloud of gas and dust. These nebulae are primarily made up of hydrogen and helium, with some other trace elements. 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.
Stars that start to form but don’t have enough heat and pressure to trigger nuclear fission are known as brown dwarfs. They’re roughly twice the mass of Jupiter, but from Earth, they’re only visible on infrared telescopes.
Once fusion happens, a star is born — but what happens after that?
Life cycle of a Star
Before we move on to what happens to each type of star during its life, there is one crucial point we need 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 quicker 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 numbers in the billions of years. Our home star is 4.603 billion years old and probably has enough hydrogen to burn for another 5 billion. How does this mass-to-lifespan ratio affect the different types of stars?
O- and B-Class 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 known as a main sequence star, and this is likely the most stable part of its life cycle.
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 around a million years and includes a series of nuclear reactions that form more shells around the iron core of the star.
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 our 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.
How does this differ from small and medium-sized stars?
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K- and M-Class Stars
Low-mass stars aren’t necessarily small. Using our sun for size comparison, most low-mass stars are roughly 1.4 solar units — or 1.4 times the size of our sun. While they may be larger, the are considerably lighter in weight than G-class stars like our 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 run out of hydrogen the core begins to collapse, becoming hotter and denser as millions of years pass. Eventually, this core will reach a temperature of roughly 100 million degrees Kelvin, where 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. This forces the exterior of the star to expand and cools 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.
We’re all pretty familiar with G-Class stars — our 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 our planet with the heat and light it needs to survive.
The fate of a medium-mass star like our sun is similar to that of low-mass stars. It will start to expand into a red giant — and will likely swallow our planet when that happens — and then eventually diffuse into a planetary nebula, leaving a white dwarf behind.
The End of Life on Earth
While our 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. We’ll probably get another 5 billion years of life out of the old girl. By then we’ll likely be among the stars ourselves, and our home planet will be nothing but a distant memory.
Featured Image Credit: NASA and ESA