What Is the Life Cycle of a Star?

When observing the night sky, it’s easy to forget that every point of twinkling light is a star at some point in its life cycle. Between a star’s violent birth and even more violent death, it remains stable for up to tens of billions of years. Let’s look at these phases in more detail.

The Nebula Phase

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The energy generated during the collapse of material during the nebula phase feeds the newly-formed protostar, but it is not yet sufficiently hot for nuclear fusion to take place. The protostar continues to grow by pulling in more and more matter from the nebula from which it formed—a process called accretion—causing a protoplanetary disk to form around the protostar’s core. This leftover soup of dust, ice, and gases clump together to form planetesimals, which collide and merge to become fully-fledged planets.

If a star has a mass of more than 0.08 solar masses (one solar mass is equivalent to the mass of our sun), nuclear reactions begin, and nuclear reactions cement the body’s status as a star. Planets form around it, and the star enters the main sequence.

The Main Sequence

After millions of years of pressure building and temperatures rising during the protostar phase, the forming body faces an ultimatum. If it doesn’t ever reach 0.08 solar masses, it won’t generate sufficient temperature (10 million Kelvin) to initiate the nuclear fusion phase, resulting in a failed star called a brown dwarf that spends eternity cooling off and fading away.

On the other hand, if the protostar does reach 0.08 solar masses, the star’s core squeezes hydrogen atom nuclei together to form helium, an intensely powerful process that prevents the star from collapsing under its own gravity. This phase is like a spark that lights a fire—once it’s lit, it will stay lit until its energy runs out.

Stars spend around 90% of their lives in the main sequence phase, and most of the stars we see when we spend the night stargazing are main sequence stars. Our Sun (pictured above) is estimated to be roughly halfway through this stage.

The Death Phases

The mass of a star is crucial as it moves away from its main sequence phase, because it determines how it behaves as it nears its end of life. Regardless of its mass, however, what triggers the start of the end of a star’s life is its core running out of hydrogen, meaning less nuclear fusion can take place.

Nuclear fusion is crucial in a star, as it balances the gravitational forces that pull matter together. Since the nuclear fusion taking place is dwindling during a star’s death, gravity begins to win and starts to squeeze the star’s core.

Low-Mass Stars

Sun-like low-mass stars use up the final parts of their hydrogen supply relatively slowly, meaning the death process can take billions of years. This phase involves them becoming red giants, as fusion converts helium into carbon.

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When a white dwarf stops producing light, it becomes a black dwarf. However, no stars are yet to reach this phase, since the universe is not old enough. This means black dwarfs remain a conceptual stage in a star’s death.

High-Mass Stars

The process for high-mass stars (eight solar masses or more) is much quicker, as they convert their hydrogen fuel much more quickly—a process that takes only a few million years. When the massive star runs out of hydrogen in its core, it expands and cools to form a red supergiant.

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Finally, remnant stars with a mass of less than 2.2–2.9 solar masses—known as the Tolman–Oppenheimer–Volkoff limit—form dense neutron stars (illustrated below).

If their mass exceeds the Tolman–Oppenheimer–Volkoff limit, nothing can generate the forces required to oppose the immense gravitational pull. This indefinite gravitational collapse results in the formation of a black hole, a little-understood cosmic object whose gravity is so strong that not even light can escape.