What is a red giant star?

A red giant star is a late stage in the life cycle of a star, reached after it has used up most of the hydrogen fuel in its core.

Here’s what happens in simple terms:
When hydrogen fusion slows down, the core contracts under gravity, while the outer layers expand significantly. As the star expands, its surface cools, giving it a reddish appearance and a much larger size than before.

Key features of a red giant:

• Very large in size compared to its earlier stage
• Cooler surface temperature, which makes it appear red
• High brightness due to its expanded surface area

A well-known example is Betelgeuse, a red giant visible in the Orion constellation.

Future perspective:
Our own Sun is expected to become a red giant in about 5 billion years, expanding so much that it may engulf nearby planets like Mercury and possibly Venus.

Concept link: The red giant phase is a crucial transition that leads to the final stages of a star’s evolution.