Field guide · Stars

Giant and supergiant stars

Stars that have swollen to enormous size near the end of their lives. Put Betelgeuse where the Sun is and it would swallow Mars and reach most of the way to Jupiter.

Giants

10–100 times the Sun’s width

Supergiants

Up to 1,000 times and more

Marked in the class as

III (giant), I (supergiant)

A star spends most of its life steadily turning hydrogen into helium in its core. When that fuel runs low, the core shrinks and heats up, and the outer layers balloon outwards and cool. The star becomes a giant — cooler at its surface, so redder, but far larger and brighter overall.

Many of the sky’s most eye-catching orange and red stars are giants: Arcturus, Aldebaran, Pollux. The most massive stars become supergiants, like Betelgeuse and Antares, hundreds of times the Sun’s width.

What happens next depends on mass. A Sun-like giant sheds its layers as a planetary nebula. A supergiant ends as a supernova — Betelgeuse will, some time in the next hundred thousand years, and for weeks it will be visible in daylight.

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