The Life Cycle of Stars

Stars are born, live for millions or billions of years, and then die — and how a star ends its life depends entirely on how massive it was. Our own Sun will end quietly as a slowly cooling ember, but a star many times heavier will destroy itself in a colossal explosion that briefly outshines a whole galaxy and scatters the very atoms from which planets and living things are later made. Every element heavier than helium in your body was forged inside a star. This lesson traces the life cycle of a star, from its birth in a nebula, through its long stable middle age, to the very different fates of a Sun-like star and a much more massive one, and explains how stars manufacture the chemical elements.

Higher tier: The detailed life cycle of stars is Higher-tier content in OCR Gateway Science A. Foundation students should still know that the Sun is a star formed from a nebula.

By the end of this lesson you should be able to describe how a star forms from a nebula, explain what keeps a main-sequence star stable, describe the life cycles of a Sun-like star and a massive star, and explain how nuclear fusion in stars makes the chemical elements.

---

Star Formation: From Nebula to Main Sequence

Every star begins life in a nebula — a vast cloud of dust and gas (mostly hydrogen) drifting in space.

1. Nebula. Under the pull of its own gravity, the cloud is slowly pulled together, becoming denser.
2. Protostar. As the gas falls inwards it is compressed and heats up, forming a hot, dense ball called a protostar. The temperature at the centre climbs higher and higher.
3. Main-sequence star. When the core becomes hot and dense enough, nuclear fusion of hydrogen into helium begins. The energy released makes the star shine, and it becomes a stable main-sequence star — the long, stable middle phase of a star's life, where our Sun is now.

Exam Tip: The formation sequence is nebula → protostar → main-sequence star. Say that gravity pulls the cloud together and that the star becomes a main-sequence star when fusion of hydrogen into helium starts in the core.

---

What Keeps a Main-sequence Star Stable

A main-sequence star is stable because two opposing effects are balanced:

• Gravity pulls all the star's material inwards, trying to make it collapse.
• The energy released by nuclear fusion in the core creates an outward pressure (from the hot gas and radiation) pushing outwards.

While these two are balanced, the star neither collapses nor expands, and it remains stable for a very long time — billions of years for a star like the Sun. This balance lasts as long as there is plenty of hydrogen fuel in the core to fuse.

It is worth thinking about why this balance is so stable, and what eventually breaks it. As long as fusion continues in the core, the star produces the outward pressure it needs to hold itself up against gravity, and the two effects settle into a steady standoff that can last for billions of years. The Sun, for instance, has been on the main sequence for about 4.6 billion years and has roughly the same again still to go. But the balance depends entirely on a supply of fuel: fusion is only possible while there is hydrogen in the core to fuse. When the core hydrogen begins to run out, the outward pressure can no longer be maintained in the same way, the balance is upset, and the star is forced to change — leaving the main sequence and beginning the later stages of its life. So the length of a star's stable main-sequence phase is essentially set by how long its fuel lasts, and this in turn depends on its mass: surprisingly, the most massive stars, despite having the most fuel, burn through it fastest and so live the shortest lives, while low-mass stars are frugal and last far longer.

Exam Tip: A main-sequence star is stable because the inward pull of gravity is balanced by the outward pressure from fusion. Use the words "balanced" and name both forces — this exact balance is a common exam question.

---

The Life Cycle of a Sun-like Star

A star about the size of the Sun (a low-to-medium mass star) follows this path once its core hydrogen runs low:

1. Main sequence — stable hydrogen fusion (where the Sun is now).
2. Red giant — when the hydrogen in the core runs out, the core contracts and the outer layers swell and cool, so the star expands into a huge, cooler, red red giant. Fusion of helium into heavier elements takes place.
3. White dwarf — the star then sheds its outer layers (as a glowing shell of gas called a planetary nebula), leaving behind a hot, dense core called a white dwarf. Over an extremely long time this slowly cools and fades.

$\text{nebula} \rightarrow \text{protostar} \rightarrow \text{main sequence} \rightarrow \text{red giant} \rightarrow \text{white dwarf}$nebula→protostar→main sequence→red giant→white dwarf

Exam Tip: For a Sun-like star, the ending is: main sequence → red giant → white dwarf. There is no supernova for a low-mass star — that fate is reserved for much more massive stars.

---

The Life Cycle of a Massive Star

A star much more massive than the Sun lives a shorter, more dramatic life and ends explosively:

1. Main sequence — the massive star fuses hydrogen, but far more quickly than the Sun.
2. Red supergiant — when the core hydrogen runs out, the star swells into an enormous red supergiant, fusing heavier and heavier elements up to iron.
3. Supernova — the core suddenly collapses and the star explodes in a supernova, an explosion so bright it can briefly outshine an entire galaxy. The supernova scatters the elements the star has made out into space.
4. Neutron star or black hole — the collapsed core that remains becomes either an incredibly dense neutron star, or — if the original star was massive enough — a black hole, whose gravity is so strong that not even light can escape.

$\cdots \rightarrow \text{main sequence} \rightarrow \text{red supergiant} \rightarrow \text{supernova} \rightarrow \text{neutron star or black hole}$⋯→main sequence→red supergiant→supernova→neutron star or black hole

The mermaid diagram below compares the two fates side by side.