In this blog post, we’ll trace the life cycle of a star—from its birth in a nebula to its final moments as it emits light through nuclear fusion—and explore how this process mirrors human life, drawing on scientific principles and philosophical reflections.
The matter in outer space composed of gas and dust is called a nebula. When a nebula becomes unstable due to internal changes or external impacts, it begins to contract, sometimes shrinking to less than one-tenth of its original size. As a nebula contracts, its mass remains constant but its volume decreases, causing its density to gradually increase; the density at the center, in particular, rises sharply. When the density at the center exceeds a certain threshold, nuclear fusion—the process by which protons fuse together—begins, releasing energy and generating light. This is starlight. This moment marks the birth of a star.
The term “star” is part of humanity’s universal cultural heritage and is used in various contexts, but in astronomy, it follows a more precise definition. In a broad sense, all celestial bodies visible in the sky may be called stars, but in a narrow sense, only stars like the Sun are classified as such. Planets like Earth or Mars, and satellites like the Moon, are not included. Ultimately, only celestial bodies that emit their own light through nuclear fusion can be considered true stars. When the temperature and density at the center of a nebula exceed a critical threshold, nuclear fusion begins, and from the moment light is generated as a result, that celestial body begins its life as a star.
Stars also have a life cycle. More precisely, they have a lifespan. A star’s lifespan is closely related to its mass at birth. Stars exist by generating light through nuclear fusion in their cores, and the duration of this nuclear fusion is the star’s lifespan. In other words, if the moment light is generated in a nebula marks a star’s birth, then the point at which it can no longer produce energy through nuclear fusion marks its death. Generally, larger stars appear to live longer because they contain more matter, but in reality, the opposite is true. Most of the matter that makes up a star is hydrogen, which consists of one proton and one electron. While a larger mass means the star contains more protons, the nuclear fusion occurring in its core is correspondingly faster and more intense, causing it to consume its fuel much more rapidly. Consequently, while more massive stars shine brighter, their lifespans are actually shorter. In other words, a star’s lifespan tends to be inversely proportional to its mass.
The lifespan of a star with a mass similar to the Sun is estimated to be about 10 billion years, during which it continuously produces light through nuclear fusion. In contrast, red dwarfs, which are much smaller than the Sun, undergo nuclear fusion at a very slow rate, so their lifespans are expected to reach 1 trillion years. Conversely, stars much heavier than the Sun consume energy very rapidly, and in some cases, their lifespans are as short as tens of millions to less than 100 million years. Once the stage of producing light through nuclear fusion ends, the star enters the final stage of its life. Stars like the Sun undergo a process of repeated expansion and contraction as their internal equilibrium breaks down during this final stage. This process is the star’s attempt to maintain its own equilibrium, but once it crosses a critical point, the star splits into an outer layer that expands and a core that contracts.
The expanded outer layers become a planetary nebula and disperse into space, while the core remains as a white dwarf with high density and temperature.
A planetary nebula can be described as the process of returning to the form of the nebula where the star was born, and it contains elements such as oxygen, nitrogen, and carbon that were created inside the star. These materials drift through space once again and become the raw materials for new stars. It is a cyclical process in which the death of one star leads to the birth of another. Meanwhile, although white dwarfs no longer undergo nuclear fusion, they emit energy and shine due to residual heat and gravitational contraction. This is a different form of “light” from that produced during the nuclear fusion stage and can be viewed as another phase in the star’s life.
When I contemplate the life cycle of a star, I can’t help but feel that it bears a strange resemblance to human life. As I grow older, I find myself increasingly aware of life’s end and often look back on the time that has passed. I sometimes wonder if I am now in the final stage of a star that once produced light through nuclear fusion. Though I have created “light” in my own way throughout my life, I’ve been feeling lately that my strength is gradually waning. Perhaps that is why, just as a star repeats cycles of expansion and contraction to maintain its balance, I, too, seem to be striving to find balance in my own life.
When I think of the final moments of a star like the Sun, I also find myself contemplating the direction of my own life moving forward. In its final moments, a star sends the matter it has created within itself back into the universe, leaving behind the foundation for new life. I, too, feel a desire to give back to society whatever I have accumulated throughout my life. If that can serve as even a small foundation for someone else’s life, it would be a deeply meaningful endeavor. Furthermore, just as a white dwarf expends its remaining energy to emit light, I wish to live out the time I have left by sharing everything I possess to the very end. Ultimately, just as a star passes through death to exist in another form, human life may not be an end in itself, but rather a journey that continues in a different way.
