STRUCTURE OF A SUPERGIANT

1 Red supergiant

A high-mass star that is nearing the end of its life and has long since exhausted the supplies of hydrogen fuel for fusion in its core.

2 Monster star

The largest red supergiants can grow to diameters larger than Jupiter’s orbit around the Sun.

3 Still burning

The star’s core keeps generating energy by fusion of heavier elements, growing denser over time.

4 Fusion shells

Meanwhile, nuclear fusion of lighter elements spreads out in a series of shells around the core.

5 Outer envelope

The huge amounts of energy coming from the core and its surrounding shells cause the star’s upper layers to balloon in size.

6 Cool surface

The star’s size gives it a huge surface area, so despite pumping out huge amounts of energy the surface stays cool and appears red.

7 Convection cells

Currents within the outer envelope form rising and sinking masses of hot and cool gas, giving the surface a blotchy appearance.

8 Iron core

Before the star dies, a core of solid iron begins to build up. Unlike lighter elements, iron fusion absorbs rather than releases energy, triggering the core’s collapse.

9 Heavier shells

Closer to the core, heavy elements continue to fuse into still heavier ones, allowing the supergiant to keep shining.

10 Helium fusion

A second shell of helium fusion follows the hydrogen shell out, creating heavy elements.

11 Hydrogen fusion shell

Changes in the star’s density and temperature allow hydrogen fusion to continue around the core after hydrogen in the centre has been exhausted.