2.3 The death of a star
What happens, though, when a star runs out of fuel?
The Sun will gradually run out of fuel but we probably have about another 4.5 billion years’ worth of hydrogen still to go, so there’s no need to worry quite yet!
As the hydrogen gets used up, the rate of energy production will also decrease. This upsets the balance between the outward pressure of hot gas and the inward pressure of gravity that has kept the Sun stable. The core of the Sun will then begin to collapse under its own weight, and as it collapses, the core will begin to heat up.
As the core contracts, it heats up the layers above it, which eventually become hot enough for nuclear fusion to take place in the same way as it did in the core. At the same time, the core continues to contract and get hotter, eventually becoming hot enough for the fusion of helium nuclei into heavier elements.
Three helium nuclei fuse together to form a nucleus of carbon – this is called helium burning, and produces sufficient energy to stabilise the star against further collapse. Because the star has to consume its fuel much faster to prevent it collapsing, this part of its lifecycle is much shorter than the time spent on the main sequence.
How does the appearance of the star change as the different nuclear reactions take place? In a star burning helium in its core and hydrogen in a shell surrounding the core, the rate of energy production rises. At the same time, the radius of the star increases, so its surface area also increases. Although there is now a greater supply of energy, it isn’t sufficient to keep the outer layers of the star at their previous temperature, so the temperature of the photosphere decreases to below 4000 K. The star is now orange-red in colour, and has become a Red Giant.
This isn’t the end, though, for the star! The star loses hold of its outer layers and it becomes a planetary nebula. The remains are no longer burning helium but are still hot, this is known as a white dwarf. This is true for all stars that are born with less than about eight times the mass of the Sun.
You will find out more about planetary nebula in the next section.