How does a star become a red giant?

Quick Answer

When a star runs out of hydrogen in its core, the core contracts and the outer layers expand and cool, making the star much larger and redder. This happens to stars like the Sun late in life.

The Full Story

A main-sequence star like the Sun shines because its core fuses hydrogen into helium. When core hydrogen is exhausted, fusion slows in the center, gravity compresses the core, and temperatures rise in a shell around it. Energy from that shell causes the outer layers to swell enormously while the surface cools, producing a red giant. The Sun is expected to become a red giant in about 5 billion years and may expand far beyond Earth’s current orbit. After this stage, lower-mass stars shed their outer layers and leave behind white dwarfs.

Key Facts

1.The Sun should become a red giant in roughly 5 billion years

2.Red giants are cooler at the surface but far more luminous

3.This stage follows core hydrogen exhaustion

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