Life Cycle of a Star

Introduction

Stars are the basic elements that make up galaxies and go through incredible changes throughout their lives. Stars begin as clouds of dust and gas, grow to shine brightly, and eventually die. Then they either quietly fade away or explode in a supernova.

In this article, we'll take a closer look at every part of a star's life, from its early days in a star-forming nursery to its final moments, which may leave behind a neutron star, black hole, or a simple white dwarf. Join us as we explore the life stories of stars, the powerful processes that energize the universe, and help create the building blocks necessary for life.

The Start of a Star's Life

Nebula: A star's life begins in a nebula, which is a giant cloud of dust and gas. Gravity pulls the nebula's particles together and creates dense regions.

Protostar: As the region becomes denser, it heats up and forms a protostar, which is a hot, glowing mass that is not yet a fully-fledged star.

Main Sequence: When a star's core gets hot enough, hydrogen turns into helium, releasing energy. This marks the birth of a main-sequence star. During this time, the star stays stable because the energy from fusion balances the gravity pulling inward. This phase is the longest part of a star's life, and it can last for billions of years, depending on the star's mass.

From this moment forward, the star's life can continue down two paths. It can become either an average star or a supergiant star.

The Life of an Average Star

If the star formed as an average star similar to our Sun, it might spend about 10 billion years in the main sequence phase because it burns much slower through its fuel. During this stable period, the star fuses hydrogen into helium in its core, maintaining a consistent light and heat output.

Our Sun, for example, is currently about 4.6 billion years into its main sequence phase, meaning it has roughly another 5.4 billion years before it exhausts its hydrogen fuel. After this phase, it will expand into a red giant and eventually shed its outer layers, leaving behind a dense white dwarf.

This long main sequence phase allows life to potentially develop and thrive on surrounding planets, as seen with Earth.

Red Giant

After the main sequence, average stars expand into red giants. This expansion happens because the star runs out of hydrogen in its core. The core gets smaller and hotter, which makes the hydrogen around it heat up and start burning.

Meanwhile, the star's outer layers grow bigger and cooler. Even though the outer surface cools down, the star expands and becomes much brighter overall.

At some point, the core becomes hot enough to start fusing helium into carbon and oxygen. This stage is shorter and less stable than the main sequence.

Planetary Nebula

Once a star burns through all its helium, its outer layers float away into space, creating a colorful, glowing gas cloud called a planetary nebula. This cloud briefly lights up the sky while the star's core, all that's left, shines through from the center.

The planetary nebula disperses over time, enriching the surrounding space with heavy elements and dust that can contribute to forming new stars and planets. This process highlights the cyclical nature of stellar evolution, where the death of one star can help birth new celestial bodies.

White Dwarf

The remaining core cools and becomes a white dwarf, a small, dense remnant of the star. It no longer undergoes nuclear fusion and slowly cools and fades over billions of years.

These white dwarfs are incredibly dense; a single teaspoon of white dwarf material would weigh tons on Earth. As they cool, they may eventually become black dwarfs, although the universe is not old enough for any black dwarfs to exist yet. 

The Life of a Massive Star

Massive stars burn through their hydrogen fuel much faster because their cores have higher pressures and temperatures. This makes their main sequence life much shorter than average-sized stars.

As a result, massive stars evolve quickly, transitioning through various phases before ending their lives in spectacular supernova explosions. Their rapid evolution also means they are crucial in enriching the interstellar medium with heavy elements essential for forming new stars and planets.

Betelgeuse as seen through a telescope. It is one of the most well-known red supergigant stars.

Red Supergiant

After millions of years spent in the main sequence, a massive star uses up all the hydrogen fuel in its core. As a result, the core shrinks, gets hotter, and starts burning helium and other heavier elements. This process makes the star swell up into a huge red supergiant. This stage doesn’t last as long as the main sequence, but it's important because it creates elements heavier than helium.

Betelgeuse, for instance, is currently in the later stages of its stellar evolution. It has already exhausted the hydrogen in its core and expanded into a red supergiant. Located about 700 light-years away in the constellation Orion, Betelgeuse has a radius roughly 1,400 times that of the Sun. Due to its massive size, Betelgeuse will likely end its life in a dramatic supernova explosion, a fate typical for stars of its kind.

Supernova

Once the core of a massive star begins to produce iron, it can't create energy anymore because iron uses up energy instead of releasing it. This causes the core to collapse under its own weight. The result is a supernova, a huge explosion that is so bright it can briefly shine brighter than an entire galaxy.

A Neutron Star or a Black Hole

The remnants of a supernova explosion depend on the star's original mass. If the leftover core is 1.4 to 3 times bigger than the Sun's mass, it becomes a neutron star, which is incredibly dense and made mostly of neutrons. Neutron stars are so dense that a sugar-cube-sized amount of their material would weigh about a billion tons on Earth.

If the core is more massive, it collapses into a black hole, where gravity is so strong that even light can't escape it. Black holes have such intense gravitational fields that they warp space and time around them, leading to fascinating phenomena such as time dilation and the event horizon, beyond which nothing can return.

These remnants are among the most extreme objects in the universe, providing valuable insights into the nature of gravity and the fundamental laws of physics.

Conclusion

1. The life cycle of a star begins in a stellar nebula where gravity condenses gas and dust into new stars.
2. Throughout their main sequence, stars burn hydrogen to produce light and heat, which sustains their luminosity and heat output during this long and stable period of their life cycle.
3. As the stars age, they expand into red giants or supergiants and eventually exhaust their fuel.
4. Lower-mass stars end their lives by shedding layers and forming white dwarfs. Meanwhile, massive stars explode in supernovae, leaving neutron stars or black holes behind.
5. The elements left behind after a star sheds its outer layers or explodes as a supernova distributes into space, fostering the creation of new stars and planets.

References

1. Bertulani, Carlos A. (2013). Nuclei in the Cosmos. World Scientific.
2. Laughlin, Gregory; Bodenheimer, Peter; Adams, Fred C. (1997). "The End of the Main Sequence". The Astrophysical Journal. 482 (1): 420–432.
3. Heber, U. (1991). Evolution of Stars: The Photospheric Abundance Connection: Proceedings of the 145th Symposium of the International Astronomical Union.
4. NASA - "Star Basics"
5. Britannica - "Star formation and evolution"