The Main Stages of the Life Cycle of a Star

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The life cycle of a star

Stars are an essential ingredient for life on the planets surrounding them. When do they form, become mature, get old, and die? How do these processes work? How can a dead star give birth to other stars? Let’s get started.

Before a Star Is Born

Where is a star formed? It’s in a nebula, or more specifically, a molecular cloud. When stars are formed, large molecular clouds split into pieces, and they further accrete so that they become notable objects. Their mass is determined by environmental factors, such as the mass of the fragment.

The mass will judge whether it is a rogue planet, a brown dwarf, or a star. While stars undergo nuclear fusion, planets don’t.

When the star starts gaining mass, it is a protostar. It is not dense enough to fuse hydrogen into helium but is already a notable object. As the material accrete, its gravitational pull increases, and its temperature increases. Finally, a main-sequence star is formed when nuclear fusion starts occurring in the star.

Main-Sequence Stars

A main-sequence star is kept in a balance where its size is consistent for millions or billions of years. While their gravity makes it collapse on itself, the resulting nuclear fusion from the pressure creates an outward force that perfectly cancels out the star’s gravity. In other words, a star is in a continuous fight between gravity and nuclear fusion, and if something goes out of order, the star either shrinks or expands.

A star is not a fireball. It’s more like a nuclear power plant. What’s happening in the core of a star is not combustion. It’s the nuclear fusion that results from the immense temperature inside the core. The particles move so fast that they merge when they collide.

Counter-intuitively, the least massive stars last longer than the most massive stars. That’s because the gravity of a low-mass star is not so strong, and the core is not crushed that hard. Therefore, they burn fuel at a slower speed, making them last longer. This is also why stars with lower mass are cooler than more massive stars.

Giant Stars

When a star gets old, it runs out of hydrogen. What happens now? The star shrinks because there is no fuel. However, the pressure and temperature in the core increase when it shrinks, which makes the core burn helium faster.

For a medium-sized star, its life ends here. The core cannot be dense enough to continue fusing elements. At that time, it gets rid of its outer layers, which leaves a white dwarf. It uses electron degeneracy pressure to prevent itself from collapsing further.

However, if the core of a star exceeds the Chandrasekhar limit, which is about 1.4 solar masses, the core will continue to collapse. Meanwhile, it becomes even larger than a regular giant star because the nuclear fusion is a lot quicker. When a star’s core is literally full of iron, it cannot fuse and generate an outward force anymore. Therefore, gravity will start compressing the star so much that the protons and electrons merge into neutrons.

The End of Their Lives

When a medium-sized star ends its life, it becomes a white dwarf surrounded by a planetary nebula that is formerly the outer parts of the star. Although white dwarfs are incredibly dense, they are not the densest thing in the Universe.

When a massive star dies, it explodes into a supernova. The outer layers of a star quickly head toward the star’s core and bounce outward by a wave of neutrinos created by the supernova. This can produce a neutron star where the collapse is stopped due to the pressure from the dense pack of neutrons. However, if the star is massive enough, nothing can stop its collapse, and it will become a black hole, where nothing can escape beyond the event horizon.

Creating Other Stars

After a star dies, the remnant of the star is a molecular cloud.That means it can start the star formation process all over again. And that’s how new stars form after old stars die. If this chain doesn’t exist, our Universe will only have one generation of stars, which means that we cannot exist.

Some of the essential elements in our bodies are created by supernovas. They are so heavy that they cannot be fused by stars. Only with more violent events like supernovas, elements substantially heavier than iron, such as molybdenum, can be created.

Conclusion

In this article, we’ve talked about a star’s birth, death, and how they provide material for other stars to form. A star’s glorious life starts in a nebula, progresses by fusing elements together, gets old by expanding, and dies when they leave most of their mass behind.

If we’ve missed any important steps, we will appreciate if you leave your opinion in the comments below. Moreover, if you want to learn more about these processes, you can visit the websites in the references below.

References and Credits

  1. Richard Brill. (1999, December 6). How is a star born? Retrieved June 15, 2021, from https://www.scientificamerican.com/article/how-is-a-star-born/
  2. NASA’s Science Mission Directorate. (n.d.). Stars. Retrieved June 15, 2021, from https://science.nasa.gov/astrophysics/focus-areas/how-do-stars-form-and-evolve
  3. Isabelle Dumé. (2005, November 16). How do stars form? – Physics World. Retrieved June 15, 2021, from https://physicsworld.com/a/how-do-stars-form/
  4. (n.d.). Stars. Retrieved June 16, 2021, from https://imagine.gsfc.nasa.gov/science/objects/stars1.html
  5. James Stein. (2012, January 19). The Chandrasekhar Limit: The Threshold That Makes Life Possible. Retrieved June 16, 2021, from https://www.pbs.org/wgbh/nova/article/the-chandrasekhar-limit-the-threshold-that-makes-life-possible/
  6. CrashCourse. (2015, September 11). High Mass Stars: Crash Course Astronomy #31. Retrieved June 16, 2021, from https://www.youtube.com/watch?v=PWx9DurgPn8

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