You are viewing a free preview of this lesson.
Subscribe to unlock all 9 lessons in this course and every other course on LearningBro.
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.
Every star begins life in a nebula — a vast cloud of dust and gas (mostly hydrogen) drifting in space.
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.
A main-sequence star is stable because two opposing effects are balanced:
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.
A star about the size of the Sun (a low-to-medium mass star) follows this path once its core hydrogen runs low:
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.
A star much more massive than the Sun lives a shorter, more dramatic life and ends explosively:
⋯→main sequence→red supergiant→supernova→neutron star or black hole
The mermaid diagram below compares the two fates side by side.
Subscribe to continue reading
Get full access to this lesson and all 9 lessons in this course.