Red Giant: Definition & Facts

Instructor: Katie Chamberlain

Katie has a PhD in Microbiology and has experience preparing online education content in Biology and Earth Science.

If you think Hagrid from 'Harry Potter' is big, then check out this lesson. Red giant stars give the word 'giant' a whole new meaning! You will also learn a thing or two about what makes a star tick on the inside.


A red giant is a star that is past its peak and has consumed its core's supply of hydrogen fuel. As a result, helium has built up in the core, hydrogen has fused in the outer shells, and the star has expanded into a giant red monstrosity.

Demonstration of the size difference when our sun becomes a red giant
Demonstration of the sun as a red giant

The Making of a Giant

All stars start the same way when gravity causes hydrogen to fuse into helium. A star fusing hydrogen is in the main sequence state where it will burn contently for various amounts of time, depending on its size. However, in medium and large stars the hydrogen will eventually deplete. Lack of hydrogen fusion means that the core can no longer counteract gravity, and the outer layers come crashing inward.

As the outer layers contract, the temperature and pressure in the core increases. The increase in temperature causes hydrogen fusion in the outer shells, which then expand. Depending on the size of the star, it will either swell into a red giant or supergiant (which is basically just a larger red giant). The outer layers far from the core burn cooler (3500-4500K), which causes the star to appear red.

In medium stars, the increased pressure and temperature is enough to cause the helium in the core to fuse into carbon. In large stars, helium fusion is just the tip of the iceberg, as many elements will undergo fusion due to the tremendous pressure in a large star's core. As increasingly heavy elements fuse in the core, concentric shells of different elements are formed.

Layers of elements inside a red supergiant star
Layers of elements inside a red supergiant star

Our sun, a medium star, will eventually swell to become a red giant. When this occurs, its diameter will engulf the orbits of Mercury, Venus, and Earth! However, you don't have to pack your bags just yet, because our sun likely won't turn into a red giant for about five billion more years.

The Fall of a Giant

In small and medium stars, the helium fusion reactions of the core will eventually cease. As the outer layers contract, they will be unable to cause carbon fusion (the next step after helium fusion). Large amounts of mass are then lost from the outer layers, giving rise to a planetary nebula. The now tiny core of the star will continue as a white dwarf star, which will cool over time into a black dwarf.

Planetary nebula captured by the Hubble telescope
Planetary nebula

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